diff --git a/app/prototype_v4_1/data/questions.yaml b/app/prototype_v4_1/data/questions.yaml
index 981d7db2..552f3f7b 100644
--- a/app/prototype_v4_1/data/questions.yaml
+++ b/app/prototype_v4_1/data/questions.yaml
@@ -162,7 +162,7 @@ questions:
       valueKey: metric
       label: What is your height in centimetres?
       suffix: cm
-      inputmode: numeric
+      inputmode: decimal
       classes: nhsuk-input--width-4
     switchUnits:
       text: Switch to feet and inches
@@ -171,17 +171,17 @@ questions:
     validation:
       required: true
       type: number
-      min: 100
-      max: 250
+      min: 139.7
+      max: 243.8
     errors:
       required:
         text: Enter your height in centimetres
       invalid:
         text: Enter your height in centimetres using numbers
       min:
-        text: Height in centimetres must be 100 or more
+        text: Height in centimetres must be 139.7cm or more
       max:
-        text: Height in centimetres must be 250 or fewer
+        text: Height in centimetres must be 243.8cm or fewer
 
   - id: height-imperial
     type: text_group
@@ -220,13 +220,21 @@ questions:
         - answerKey: feet
           required: true
           type: number
-          min: 3
-          max: 8
+          integer: true
+          min: 0
         - answerKey: inches
           required: true
           type: number
+          integer: true
           min: 0
           max: 11
+      total:
+        min: 55
+        max: 96
+        items:
+          - answerKey: feet
+            multiplier: 12
+          - answerKey: inches
     errors:
       items:
         feet:
@@ -236,11 +244,11 @@ questions:
           invalid:
             text: Enter your height in feet using numbers
             href: "#height-imperial-feet"
-          min:
-            text: Height in feet must be 3 or more
+          integer:
+            text: Height in feet must be a whole number
             href: "#height-imperial-feet"
-          max:
-            text: Height in feet must be 8 or fewer
+          min:
+            text: Height in feet must be 0ft or more
             href: "#height-imperial-feet"
         inches:
           required:
@@ -249,12 +257,22 @@ questions:
           invalid:
             text: Enter your height in inches using numbers
             href: "#height-imperial-inches"
+          integer:
+            text: Height in inches must be a whole number
+            href: "#height-imperial-inches"
           min:
-            text: Height in inches must be 0 or more
+            text: Height in inches must be 0in or more
             href: "#height-imperial-inches"
           max:
-            text: Height in inches must be 11 or fewer
+            text: Height in inches must be 11in or fewer
             href: "#height-imperial-inches"
+      total:
+        min:
+          text: Height must be between 4 feet 7 inches and 8 feet
+          href: "#height-imperial-feet"
+        max:
+          text: Height must be between 4 feet 7 inches and 8 feet
+          href: "#height-imperial-feet"
 
   - id: weight-metric
     type: text
@@ -272,7 +290,7 @@ questions:
       valueKey: metric
       label: What is your weight in kilograms?
       suffix: kg
-      inputmode: numeric
+      inputmode: decimal
       classes: nhsuk-input--width-4
     switchUnits:
       text: Switch to stones and pounds
@@ -281,17 +299,20 @@ questions:
     validation:
       required: true
       type: number
-      min: 30
-      max: 250
+      decimalPlaces: 2
+      min: 25.4
+      max: 317.5
     errors:
       required:
         text: Enter your weight in kilograms
       invalid:
         text: Enter your weight in kilograms using numbers
+      decimalPlaces:
+        text: Weight in kilograms must have 2 decimal places or fewer
       min:
-        text: Weight in kilograms must be 30 or more
+        text: Weight in kilograms must be 25.4kg or more
       max:
-        text: Weight in kilograms must be 250 or fewer
+        text: Weight in kilograms must be 317.5kg or fewer
 
   - id: weight-imperial
     type: text_group
@@ -330,13 +351,21 @@ questions:
         - answerKey: stones
           required: true
           type: number
-          min: 4
-          max: 40
+          integer: true
+          min: 0
         - answerKey: pounds
           required: true
           type: number
+          integer: true
           min: 0
           max: 13
+      total:
+        min: 56
+        max: 700
+        items:
+          - answerKey: stones
+            multiplier: 14
+          - answerKey: pounds
     errors:
       items:
         stones:
@@ -346,11 +375,11 @@ questions:
           invalid:
             text: Enter your weight in stones using numbers
             href: "#weight-imperial-stones"
-          min:
-            text: Weight in stones must be 4 or more
+          integer:
+            text: Weight in stones must be a whole number
             href: "#weight-imperial-stones"
-          max:
-            text: Weight in stones must be 40 or fewer
+          min:
+            text: Weight in stones must be 0st or more
             href: "#weight-imperial-stones"
         pounds:
           required:
@@ -359,12 +388,22 @@ questions:
           invalid:
             text: Enter your weight in pounds using numbers
             href: "#weight-imperial-pounds"
+          integer:
+            text: Weight in pounds must be a whole number
+            href: "#weight-imperial-pounds"
           min:
-            text: Weight in pounds must be 0 or more
+            text: Weight in pounds must be 0lb or more
             href: "#weight-imperial-pounds"
           max:
-            text: Weight in pounds must be 13 or fewer
+            text: Weight in pounds must be 13lb or fewer
             href: "#weight-imperial-pounds"
+      total:
+        min:
+          text: Weight must be between 4 stone and 50 stone
+          href: "#weight-imperial-stones"
+        max:
+          text: Weight must be between 4 stone and 50 stone
+          href: "#weight-imperial-stones"
 
   - id: gender
     type: single
diff --git a/app/prototype_v4_1/docs/error-messages.md b/app/prototype_v4_1/docs/error-messages.md
new file mode 100644
index 00000000..e88bebd7
--- /dev/null
+++ b/app/prototype_v4_1/docs/error-messages.md
@@ -0,0 +1,533 @@
+# Error messages
+
+This document summarises validation logic and error messages for `prototype_v4_1`.
+
+Sources:
+
+- `data/questions.yaml` for base question content, validation rules and error text
+- `lib/question-validator.js` for shared validation behaviour
+- `lib/tobacco-flow.js` for runtime tobacco question overrides
+- `controllers/question.js` for route branching and answer-clearing logic
+
+## Shared validation logic
+
+- Blank values are `undefined`, `null`, an empty string after trimming, or an empty array.
+- `required` errors stop further validation for that question or field.
+- Date validation only accepts real calendar dates. If no date part is supplied, the `required` message is used; if any date part is supplied but the date is not real, the `invalid` message is used.
+- Number validation rejects non-finite numbers, then checks whole-number, decimal-place, minimum and maximum rules in that order.
+- Grouped text inputs validate each item first. Total minimum and maximum checks only run when the grouped values are present and numerically valid.
+- Conditional reveal validation only runs for the selected trigger value. If the conditional value is blank, the conditional `required` message is used; number rules then use the question-level number error messages unless a runtime override replaces them.
+- Error links default to `#${question.input.id || question.id}`. Date, grouped and conditional inputs usually set explicit `href` values.
+
+## Flow logic
+
+- `phone-questionnaire`: `yes` goes to `phone-questionnaire-exit`; `no` continues to `smoker`.
+- `smoker`: `no` or `yes_fewer_than_100` goes to `not-eligible-for-screening`; current or previous smokers continue to `date-of-birth`.
+- `date-of-birth`: valid dates outside the eligible scan age range go to `not-eligible-for-scan`; eligible users continue to `face-to-face-appointment`.
+- `face-to-face-appointment`: `yes` goes to `book-appointment`; `no` continues to height and weight.
+- `height-*` and `weight-*`: metric and imperial pages are alternatives. Submitting one unit clears the other unit answer.
+- `cancer-diagnosis-relatives`: `yes` continues to `cancer-diagnosis-relatives-age`; `no` clears that answer and continues to smoking history.
+- `age-stopped-smoking`: only shown when `smoker` is `yes_previous`; otherwise the answer is cleared and the flow continues to `periods-stopped-smoking`.
+- `periods-stopped-smoking`: if `no`, `yearsStoppedSmoking` is cleared.
+- `smoking-type`: `none` goes to `smoking-type-exit`; otherwise the selected tobacco types create the tobacco sub-flow in tobacco.yaml order. Former smokers skip each type-specific `smoking-status` question.
+- Non-shisha tobacco types ask `smoking-frequency`, `smoking-quantity`, then `smoking-change`. Shisha asks `smoking-setting`, then asks `smoking-frequency` and `smoking-quantity` for each selected setting.
+- `smoking-change`: selected `greater` and `fewer` options create corresponding changed-smoking follow-up steps. Unselected changed-smoking answer groups are cleared.
+- `smoking-quantity` and `smoking-quantity-change`: if `another_amount` is not selected, the related `smokingQuantityOther` answer is cleared.
+
+## Runtime tobacco messages
+
+- Tobacco sub-flow pages use `getSmokingContentQuestionOverrides()` before validation. The validator sees the runtime heading, input name, selected value and tobacco-specific variant.
+- `smoking-status`, `smoking-frequency`, `smoking-setting`, `smoking-quantity`, `smoking-change`, `smoking-frequency-change`, `smoking-quantity-change`, and `smoking-years-change` can replace their base `required` text with contextual text generated from the runtime heading.
+- For headings beginning `How often`, the required message is `Select ...`; for `How much`, `How many`, or `How long`, the message is `Enter ...` for text inputs and `Select ...` for single-choice inputs; yes/no headings become `Select whether ...`.
+- For numeric tobacco quantity text inputs, non-rolling-tobacco and non-shisha types require whole numbers. The integer message is generated as `[answer phrase] must be a whole number`.
+- For shisha `another_amount`, the conditional input messages are `Enter the number of hours`, `Number of hours must be a number`, `Number of hours must be 0.5 or more`, and `Number of hours must be [maxHours] or fewer`. `maxHours` is 24 for daily/non-shisha, 168 weekly, 744 monthly, and 8760 yearly.
+- `smoking-quantity-change` adds a comparison error when a `greater` answer is not greater than the original amount, or a `fewer` answer is not fewer than the original amount after normalising by frequency. The message is generated as `[quantity phrase] must be more/fewer/less than [original quantity] [original frequency]`.
+
+## Question messages
+
+### accept-terms
+
+Question: Terms of use
+
+Type: `multiple`
+
+Answer key: `acceptTerms`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Confirm that you have read and agree to the terms of use | `#accept-terms` |
+
+### phone-questionnaire
+
+Question: Confirm if you have completed a lung cancer risk questionnaire by phone
+
+Type: `single`
+
+Answer key: `phoneQuestionnaire`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have previously completed a lung cancer risk questionnaire by phone | `#phone-questionnaire` |
+
+### smoker
+
+Question: Tobacco smoking
+
+Type: `single`
+
+Answer key: `smoker`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever smoked tobacco | `#smoker` |
+
+### date-of-birth
+
+Question: What is your date of birth?
+
+Type: `date`
+
+Answer key: `dateOfBirth`
+
+Validation: required; type: date
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter your date of birth | `#dateOfBirth-day` |
+| `invalid` | Enter a real date of birth | `#dateOfBirth-day` |
+
+### face-to-face-appointment
+
+Question: Check if you need a face-to-face appointment
+
+Type: `single`
+
+Answer key: `faceToFaceAppointment`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you need to leave the online service and ask for a face-to-face appointment | `#face-to-face-appointment` |
+
+### height-metric
+
+Question: Your height
+
+Type: `text`
+
+Answer key: `height`
+
+Validation: required; type: number; minimum 139.7; maximum 243.8
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter your height in centimetres | `#height-metric` |
+| `invalid` | Enter your height in centimetres using numbers | `#height-metric` |
+| `min` | Height in centimetres must be 139.7cm or more | `#height-metric` |
+| `max` | Height in centimetres must be 243.8cm or fewer | `#height-metric` |
+
+### height-imperial
+
+Question: Your height
+
+Type: `text_group`
+
+Answer key: `height`
+
+Validation: grouped input items: feet (required, number, whole number, min 0); inches (required, number, whole number, min 0, max 11); group total: min 55, max 96
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `feet.required` | Enter your height in feet | `#height-imperial-feet` |
+| `feet.invalid` | Enter your height in feet using numbers | `#height-imperial-feet` |
+| `feet.integer` | Height in feet must be a whole number | `#height-imperial-feet` |
+| `feet.min` | Height in feet must be 0ft or more | `#height-imperial-feet` |
+| `inches.required` | Enter your height in inches | `#height-imperial-inches` |
+| `inches.invalid` | Enter your height in inches using numbers | `#height-imperial-inches` |
+| `inches.integer` | Height in inches must be a whole number | `#height-imperial-inches` |
+| `inches.min` | Height in inches must be 0in or more | `#height-imperial-inches` |
+| `inches.max` | Height in inches must be 11in or fewer | `#height-imperial-inches` |
+| `total.min` | Height must be between 4 feet 7 inches and 8 feet | `#height-imperial-feet` |
+| `total.max` | Height must be between 4 feet 7 inches and 8 feet | `#height-imperial-feet` |
+
+### weight-metric
+
+Question: Your weight
+
+Type: `text`
+
+Answer key: `weight`
+
+Validation: required; type: number; maximum 2 decimal places; minimum 25.4; maximum 317.5
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter your weight in kilograms | `#weight-metric` |
+| `invalid` | Enter your weight in kilograms using numbers | `#weight-metric` |
+| `decimalPlaces` | Weight in kilograms must have 2 decimal places or fewer | `#weight-metric` |
+| `min` | Weight in kilograms must be 25.4kg or more | `#weight-metric` |
+| `max` | Weight in kilograms must be 317.5kg or fewer | `#weight-metric` |
+
+### weight-imperial
+
+Question: Your weight
+
+Type: `text_group`
+
+Answer key: `weight`
+
+Validation: grouped input items: stones (required, number, whole number, min 0); pounds (required, number, whole number, min 0, max 13); group total: min 56, max 700
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `stones.required` | Enter your weight in stones | `#weight-imperial-stones` |
+| `stones.invalid` | Enter your weight in stones using numbers | `#weight-imperial-stones` |
+| `stones.integer` | Weight in stones must be a whole number | `#weight-imperial-stones` |
+| `stones.min` | Weight in stones must be 0st or more | `#weight-imperial-stones` |
+| `pounds.required` | Enter your weight in pounds | `#weight-imperial-pounds` |
+| `pounds.invalid` | Enter your weight in pounds using numbers | `#weight-imperial-pounds` |
+| `pounds.integer` | Weight in pounds must be a whole number | `#weight-imperial-pounds` |
+| `pounds.min` | Weight in pounds must be 0lb or more | `#weight-imperial-pounds` |
+| `pounds.max` | Weight in pounds must be 13lb or fewer | `#weight-imperial-pounds` |
+| `total.min` | Weight must be between 4 stone and 50 stone | `#weight-imperial-stones` |
+| `total.max` | Weight must be between 4 stone and 50 stone | `#weight-imperial-stones` |
+
+### gender
+
+Question: Your gender identity
+
+Type: `single`
+
+Answer key: `gender`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select which option best describes you | `#gender` |
+
+### sex
+
+Question: Your sex at birth
+
+Type: `single`
+
+Answer key: `sex`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select your sex at birth | `#sex` |
+
+### ethnicity
+
+Question: Your ethnic background
+
+Type: `single`
+
+Answer key: `ethnicity`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select your ethnic background | `#ethnicity` |
+
+### education
+
+Question: Your education
+
+Type: `single`
+
+Answer key: `education`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select the highest level of education you have completed | `#education` |
+
+### respiratory-conditions
+
+Question: Have you ever been diagnosed with any of the following respiratory conditions?
+
+Type: `multiple`
+
+Answer key: `respiratoryConditions`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select if you have ever been diagnosed with any respiratory conditions | `#respiratory-conditions` |
+
+### asbestos-at-work
+
+Question: Tell us if you might have been exposed to asbestos at work
+
+Type: `single`
+
+Answer key: `asbestosAtWork`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever worked in a job where you might have been exposed to asbestos | `#asbestos-at-work` |
+
+### asbestos-at-home
+
+Question: Tell us if you ever lived with anyone who worked with asbestos
+
+Type: `single`
+
+Answer key: `asbestosAtHome`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever lived with anyone who worked with asbestos | `#asbestos-at-home` |
+
+### cancer-diagnosis
+
+Question: Tell us if you have ever been diagnosed with cancer
+
+Type: `single`
+
+Answer key: `cancerDiagnosis`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever been diagnosed with cancer | `#cancer-diagnosis` |
+
+### cancer-diagnosis-relatives
+
+Question: Tell us if your parents, siblings or children have ever been diagnosed with lung cancer
+
+Type: `single`
+
+Answer key: `cancerDiagnosisRelatives`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether any of your parents, siblings or children have ever been diagnosed with lung cancer | `#cancer-diagnosis-relatives` |
+
+### cancer-diagnosis-relatives-age
+
+Question: If your relatives were under 60 when they were diagnosed
+
+Type: `single`
+
+Answer key: `cancerDiagnosisRelativesAge`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether any of your relatives were younger than 60 when they were diagnosed with lung cancer | `#cancer-diagnosis-relatives-age` |
+
+### age-started-smoking
+
+Question: How old were you when you started smoking?
+
+Type: `text`
+
+Answer key: `ageStartedSmoking`
+
+Validation: required; type: number; minimum 1; maximum 120
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter the age you started smoking | `#age-started-smoking` |
+| `invalid` | Enter the age you started smoking using numbers | `#age-started-smoking` |
+| `min` | Age you started smoking must be 1 or older | `#age-started-smoking` |
+| `max` | Age you started smoking must be 120 or younger | `#age-started-smoking` |
+
+### age-stopped-smoking
+
+Question: How old were you when you stopped smoking?
+
+Type: `text`
+
+Answer key: `ageStoppedSmoking`
+
+Validation: required; type: number; minimum 1; maximum 120
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter the age you stopped smoking | `#age-stopped-smoking` |
+| `invalid` | Enter the age you stopped smoking using numbers | `#age-stopped-smoking` |
+| `min` | Age you stopped smoking must be 1 or older | `#age-stopped-smoking` |
+| `max` | Age you stopped smoking must be 120 or younger | `#age-stopped-smoking` |
+
+### periods-stopped-smoking
+
+Question: Periods when you stopped smoking
+
+Type: `single`
+
+Answer key: `periodsStoppedSmoking`
+
+Validation: required; conditional: yes (required, number, min 1, max 80)
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever stopped smoking for periods of 1 year or longer | `#periods-stopped-smoking` |
+| `invalid` | Total number of years you stopped smoking must be a number | `#years-stopped-smoking` |
+| `min` | Total number of years you stopped smoking must be 1 or more | `#years-stopped-smoking` |
+| `max` | Total number of years you stopped smoking must be 80 or fewer | `#years-stopped-smoking` |
+| `conditional.yes.required` | Enter the total number of years you stopped smoking | `#years-stopped-smoking` |
+
+### smoking-type
+
+Question: The type of tobacco you smoke or used to smoke
+
+Type: `multiple`
+
+Answer key: `smokingType`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select the types of tobacco you have smoked | `#smoking-type` |
+
+Variants: `previous`. Variants can change type, options, hints or validation before the same validator runs.
+
+### smoking-status
+
+Question: Do you currently smoke?
+
+Type: `single`
+
+Answer key: `smokingStatus`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you currently smoke this type of tobacco | `#smoking-status` |
+
+### smoking-frequency
+
+Question: How often do you smoke?
+
+Type: `single`
+
+Answer key: `smokingFrequency`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select how often you smoke this type of tobacco | `#smoking-frequency` |
+
+### smoking-quantity
+
+Question: How much do you smoke?
+
+Type: `text`
+
+Answer key: `smokingQuantity`
+
+Validation: required; type: number; minimum 1; maximum 200
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter how much you smoke | `#smoking-quantity` |
+| `invalid` | Enter how much you smoke using numbers | `#smoking-quantity` |
+| `min` | Amount smoked must be 1 or more | `#smoking-quantity` |
+| `max` | Amount smoked must be 200 or fewer | `#smoking-quantity` |
+
+Variants: `rolling_tobacco`, `shisha`. Variants can change type, options, hints or validation before the same validator runs.
+
+### smoking-setting
+
+Question: Do you usually smoke shisha in a group or on your own?
+
+Type: `multiple`
+
+Answer key: `smokingSetting`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you usually smoke shisha in a group or on your own | `#smoking-setting` |
+
+### smoking-change
+
+Question: Has the amount you normally smoke changed over time?
+
+Type: `multiple`
+
+Answer key: `smokingChange`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether the amount you normally smoke has changed over time | `#smoking-change` |
+
+### smoking-frequency-change
+
+Question: How often did you smoke?
+
+Type: `single`
+
+Answer key: `smokingFrequencyChange`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select how often you smoked this type of tobacco | `#smoking-frequency-change` |
+
+### smoking-quantity-change
+
+Question: How much did you smoke?
+
+Type: `text`
+
+Answer key: `smokingQuantityChange`
+
+Validation: required; type: number; minimum 1; maximum 200
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter how much you smoked | `#smoking-quantity-change` |
+| `invalid` | Enter how much you smoked using numbers | `#smoking-quantity-change` |
+| `min` | Amount smoked must be 1 or more | `#smoking-quantity-change` |
+| `max` | Amount smoked must be 200 or fewer | `#smoking-quantity-change` |
+
+Variants: `rolling_tobacco`. Variants can change type, options, hints or validation before the same validator runs.
+
+### smoking-years-change
+
+Question: How many years did you smoke this amount?
+
+Type: `text`
+
+Answer key: `smokingYearsChange`
+
+Validation: required; type: number; minimum 1; maximum 80
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter how many years you smoked this amount | `#smoking-years-change` |
+| `invalid` | Enter how many years using numbers | `#smoking-years-change` |
+| `min` | Number of years must be 1 or more | `#smoking-years-change` |
+| `max` | Number of years must be 80 or fewer | `#smoking-years-change` |
diff --git a/app/prototype_v4_1/docs/question-schema.md b/app/prototype_v4_1/docs/question-schema.md
index 85db6b46..513e5736 100644
--- a/app/prototype_v4_1/docs/question-schema.md
+++ b/app/prototype_v4_1/docs/question-schema.md
@@ -314,7 +314,10 @@ Supported validation fields are:
 | `type: date` | Requires day, month and year to form a real date. |
 | `min` | Minimum numeric value. |
 | `max` | Maximum numeric value. |
+| `integer` | Requires a numeric answer to be a whole number. |
+| `decimalPlaces` | Maximum decimal places for a numeric answer. |
 | `items` | Per-field validation for `text_group`. |
+| `total` | Combined numeric validation for `text_group` fields. |
 | `conditional` | Validation for conditional reveal inputs. |
 
 For numeric questions, provide all relevant messages:
diff --git a/app/prototype_v4_1/lib/question-validator.js b/app/prototype_v4_1/lib/question-validator.js
index 35d5ecf7..ef162476 100644
--- a/app/prototype_v4_1/lib/question-validator.js
+++ b/app/prototype_v4_1/lib/question-validator.js
@@ -12,8 +12,11 @@ const { getQuestion } = require('./questions')
  * @property {string} [type] - Type-specific validation, for example number or date.
  * @property {number} [min] - Minimum numeric value.
  * @property {number} [max] - Maximum numeric value.
+ * @property {boolean} [integer] - Whether a numeric value must be a whole number.
+ * @property {number} [decimalPlaces] - Maximum number of decimal places allowed.
  * @property {Object} [conditional] - Conditional reveal validation rules.
  * @property {Object[]} [items] - Validation rules for grouped inputs.
+ * @property {Object} [total] - Total validation rules for grouped numeric inputs.
  */
 
 /**
@@ -191,6 +194,30 @@ const mergeQuestion = (question, overrides = {}) => {
   }
 }
 
+/**
+ * Check whether a submitted numeric value is written as a whole number.
+ *
+ * @param {*} value - Submitted value.
+ * @returns {boolean} True when the submitted value is an integer string.
+ */
+const isIntegerValue = (value) => {
+  return /^-?\d+$/.test(String(value).trim())
+}
+
+/**
+ * Check whether a submitted numeric value has no more than the allowed number
+ * of decimal places.
+ *
+ * @param {*} value - Submitted value.
+ * @param {number} decimalPlaces - Maximum decimal places allowed.
+ * @returns {boolean} True when the value has an allowed decimal precision.
+ */
+const hasMaximumDecimalPlaces = (value, decimalPlaces) => {
+  const match = String(value).trim().match(/^-?\d+(?:\.(\d+))?$/)
+
+  return Boolean(match) && (!match[1] || match[1].length <= decimalPlaces)
+}
+
 /**
  * Validate a numeric value against invalid, min and max rules.
  *
@@ -203,11 +230,24 @@ const mergeQuestion = (question, overrides = {}) => {
 const validateNumber = (value, validation, errorsConfig, errors, defaultHref) => {
   const number = Number(value)
 
-  if (Number.isNaN(number)) {
+  if (Number.isNaN(number) || !Number.isFinite(number)) {
     errors.push(makeError(errorsConfig.invalid, defaultHref))
     return
   }
 
+  if (validation.integer && !isIntegerValue(value)) {
+    errors.push(makeError(errorsConfig.integer || errorsConfig.invalid, defaultHref))
+    return
+  }
+
+  if (
+    validation.decimalPlaces !== undefined &&
+    !hasMaximumDecimalPlaces(value, validation.decimalPlaces)
+  ) {
+    errors.push(makeError(errorsConfig.decimalPlaces || errorsConfig.invalid, defaultHref))
+    return
+  }
+
   if (validation.min !== undefined && number < validation.min) {
     errors.push(makeError(errorsConfig.min, defaultHref))
   }
@@ -217,6 +257,61 @@ const validateNumber = (value, validation, errorsConfig, errors, defaultHref) =>
   }
 }
 
+/**
+ * Validate the total value of grouped numeric inputs.
+ *
+ * @param {Object} groupValue - Submitted group values keyed by answer key.
+ * @param {Object} question - Normalised text_group question config.
+ * @param {ValidationError[]} errors - Mutable error collection.
+ */
+const validateGroupTotal = (groupValue, question, errors) => {
+  const totalValidation = question.validation?.total
+
+  if (!totalValidation) {
+    return
+  }
+
+  const total = (totalValidation.items || []).reduce((sum, item) => {
+    const multiplier = item.multiplier || 1
+
+    return sum + (Number(groupValue[item.answerKey]) * multiplier)
+  }, 0)
+  const totalErrors = question.errors?.total || {}
+  const defaultHref = `#${question.input?.items?.[0]?.id || question.id}`
+
+  if (totalValidation.min !== undefined && total < totalValidation.min) {
+    errors.push(makeError(totalErrors.min, defaultHref))
+  }
+
+  if (totalValidation.max !== undefined && total > totalValidation.max) {
+    errors.push(makeError(totalErrors.max, defaultHref))
+  }
+}
+
+/**
+ * Order grouped input errors by the visual input order.
+ *
+ * @param {ValidationError[]} errors - Validation errors for a text_group.
+ * @param {Object} question - Normalised text_group question config.
+ * @returns {ValidationError[]} Errors ordered by grouped input position.
+ */
+const orderGroupErrors = (errors, question) => {
+  const inputOrder = (question.input?.items || []).reduce((map, item, index) => {
+    map[`#${item.id}`] = index
+    return map
+  }, {})
+
+  return errors
+    .map((error, index) => ({ error, index }))
+    .sort((a, b) => {
+      const aOrder = inputOrder[a.error.href] ?? Number.MAX_SAFE_INTEGER
+      const bOrder = inputOrder[b.error.href] ?? Number.MAX_SAFE_INTEGER
+
+      return aOrder - bOrder || a.index - b.index
+    })
+    .map(({ error }) => error)
+}
+
 /**
  * Validate grouped inputs, such as imperial height or weight fields.
  *
@@ -227,6 +322,7 @@ const validateNumber = (value, validation, errorsConfig, errors, defaultHref) =>
 const validateInputGroup = (answers, question) => {
   const errors = []
   const groupValue = answers[question.answerKey]?.[question.input.valueKey] || {}
+  let canValidateTotal = Boolean(question.validation?.total)
 
   ;(question.validation?.items || []).forEach((itemValidation) => {
     const value = groupValue[itemValidation.answerKey]
@@ -235,15 +331,30 @@ const validateInputGroup = (answers, question) => {
 
     if (itemValidation.required && isBlank(value)) {
       errors.push(makeError(itemErrors.required, defaultHref))
+      canValidateTotal = false
       return
     }
 
     if (itemValidation.type === 'number' && !isBlank(value)) {
+      const number = Number(value)
+
+      if (
+        Number.isNaN(number) ||
+        !Number.isFinite(number) ||
+        (itemValidation.integer && !isIntegerValue(value))
+      ) {
+        canValidateTotal = false
+      }
+
       validateNumber(value, itemValidation, itemErrors, errors, defaultHref)
     }
   })
 
-  return errors
+  if (canValidateTotal) {
+    validateGroupTotal(groupValue, question, errors)
+  }
+
+  return orderGroupErrors(errors, question)
 }
 
 /**
diff --git a/app/prototype_v4_1/lib/tobacco-flow.js b/app/prototype_v4_1/lib/tobacco-flow.js
index 490b9b3a..7786f4f2 100644
--- a/app/prototype_v4_1/lib/tobacco-flow.js
+++ b/app/prototype_v4_1/lib/tobacco-flow.js
@@ -302,6 +302,119 @@ const getSmokingComparisonQuantity = (type, answer) => {
   return getSmokingQuantity(type, answer)
 }
 
+const rollingTobaccoQuantityValues = {
+  less_than_10: 10,
+  '10_to_30': 20,
+  '31_to_50': 40.5,
+  '51_to_75': 63,
+  '76_to_100': 88,
+  more_than_100: 100
+}
+
+const smokingFrequencyRateMultipliers = {
+  daily: 7,
+  weekly: 1,
+  monthly: 0.25,
+  yearly: 1 / 52
+}
+
+/**
+ * Check whether a value can be compared as a submitted numeric answer.
+ *
+ * @param {*} value - Submitted value.
+ * @returns {boolean} True when value is numeric.
+ */
+const isComparableNumber = (value) => {
+  if (value === undefined || value === null || String(value).trim() === '') {
+    return false
+  }
+
+  const number = Number(value)
+
+  return Number.isFinite(number)
+}
+
+/**
+ * Get a normalised quantity value for cross-frequency comparisons.
+ *
+ * @param {string} type - Tobacco type key.
+ * @param {*} quantity - Submitted quantity answer.
+ * @param {string} frequency - Submitted frequency answer.
+ * @returns {number|undefined} Normalised quantity, when comparable.
+ */
+const getSmokingQuantityComparisonRate = (type, quantity, frequency) => {
+  const multiplier = smokingFrequencyRateMultipliers[frequency]
+
+  if (!multiplier) {
+    return undefined
+  }
+
+  if (type === 'rolling_tobacco') {
+    const value = rollingTobaccoQuantityValues[quantity]
+
+    return value ? value * multiplier : undefined
+  }
+
+  return isComparableNumber(quantity) ? Number(quantity) * multiplier : undefined
+}
+
+/**
+ * Build the left side of a changed-quantity comparison error.
+ *
+ * @param {string} type - Tobacco type key.
+ * @returns {string} Error phrase.
+ */
+const getSmokingQuantityChangeErrorPhrase = (type) => {
+  const heading = removeQuestionMark(getSmokingTypeHeadings(type, true).quantityHeading || '')
+    .replace(/ in a normal (day|week|month|year)$/, '')
+
+  return upperFirst(lowerFirst(heading)
+    .replace(/^(how (?:much|many) .+?) did you normally smoke$/, '$1 you normally smoked')
+    .replace(/^(how (?:much|many) .+?) did you smoke$/, '$1 you normally smoked'))
+}
+
+/**
+ * Check whether a changed-smoking quantity contradicts the selected change direction.
+ *
+ * @param {Object} params - Comparison inputs.
+ * @returns {Object|undefined} Validation error when the quantities contradict.
+ */
+const getSmokingQuantityChangeComparisonError = ({
+  page,
+  step,
+  answer,
+  changeAnswer,
+  href
+}) => {
+  if (page !== 'smoking-quantity-change' || !step.change || !answer.smokingQuantity || !answer.smokingFrequency || !changeAnswer.quantity || !changeAnswer.frequency) {
+    return undefined
+  }
+
+  const currentRate = getSmokingQuantityComparisonRate(step.type, answer.smokingQuantity, answer.smokingFrequency)
+  const changedRate = getSmokingQuantityComparisonRate(step.type, changeAnswer.quantity, changeAnswer.frequency)
+
+  if (currentRate === undefined || changedRate === undefined) {
+    return undefined
+  }
+
+  const contradictsChange = step.change === 'greater'
+    ? changedRate <= currentRate
+    : changedRate >= currentRate
+
+  if (!contradictsChange) {
+    return undefined
+  }
+
+  const comparisonText = step.type === 'rolling_tobacco' && step.change === 'fewer'
+    ? 'less'
+    : smokingChangeTypes[step.change]?.label
+
+  return {
+    text: `${getSmokingQuantityChangeErrorPhrase(step.type)} must be ${comparisonText} than ${[getSmokingComparisonQuantity(step.type, answer.smokingQuantity), getSmokingFrequencyComparisonPeriod(answer.smokingFrequency)].filter(Boolean).join(' ')}`,
+    href
+  }
+}
+
 const smokingFrequencyPeriods = {
   daily: 'a day',
   weekly: 'a week',
@@ -309,6 +422,20 @@ const smokingFrequencyPeriods = {
   yearly: 'a year'
 }
 
+const smokingFrequencyComparisonPeriods = {
+  daily: 'per day',
+  weekly: 'per week',
+  monthly: 'per month',
+  yearly: 'per year'
+}
+
+const smokingFrequencyMaxHours = {
+  daily: 24,
+  weekly: 168,
+  monthly: 744,
+  yearly: 8760
+}
+
 /**
  * Convert a smoking frequency value into a period phrase.
  *
@@ -319,6 +446,31 @@ const getSmokingFrequencyPeriod = (frequency) => {
   return smokingFrequencyPeriods[frequency] || ''
 }
 
+/**
+ * Convert a smoking frequency value into a comparison phrase.
+ *
+ * @param {string} frequency - Frequency value.
+ * @returns {string} Comparison phrase, for example `per day`.
+ */
+const getSmokingFrequencyComparisonPeriod = (frequency) => {
+  return smokingFrequencyComparisonPeriods[frequency] || ''
+}
+
+/**
+ * Get the maximum number of hours allowed for an "another amount" shisha answer.
+ *
+ * @param {string} type - Tobacco type key.
+ * @param {string} frequency - Selected smoking frequency.
+ * @returns {number} Maximum number of hours.
+ */
+const getSmokingQuantityOtherMaxHours = (type, frequency) => {
+  if (type !== 'shisha') {
+    return 24
+  }
+
+  return smokingFrequencyMaxHours[frequency] || 24
+}
+
 /**
  * Replace the default "normal day/week/month/year" phrase in a heading.
  *
@@ -666,6 +818,16 @@ const getQuestionItemsWithLabels = (id, labels = {}, hintOverrides = {}) => {
   })
 }
 
+/**
+ * Check whether a smoking quantity must be a whole number.
+ *
+ * @param {string} type - Tobacco type key.
+ * @returns {boolean} True when decimal quantities should be rejected.
+ */
+const requiresWholeNumberSmokingQuantity = (type) => {
+  return !['rolling_tobacco', 'shisha'].includes(type)
+}
+
 /**
  * Build runtime overrides for a quantity question.
  *
@@ -680,6 +842,7 @@ const getSmokingQuantityQuestionOverrides = ({
   name,
   value,
   conditionalValue,
+  frequency,
   smokingType
 }) => {
   const question = getQuestion(page)
@@ -689,6 +852,15 @@ const getSmokingQuantityQuestionOverrides = ({
   const hint = hasHintOverride ? variantInput.hint : question.input.hint
   const questionType = variant.type || question.type
   const conditionalHref = '#smoking-quantity-other'
+  const maxHours = getSmokingQuantityOtherMaxHours(step.type, frequency)
+  const validation = {
+    ...variant.validation
+  }
+
+  if (requiresWholeNumberSmokingQuantity(step.type)) {
+    validation.integer = true
+  }
+
   const items = variant.options
     ? variant.options.map((option) => {
       const item = toComponentItem(option)
@@ -726,13 +898,13 @@ const getSmokingQuantityQuestionOverrides = ({
       suffix: questionType === 'text' ? smokingType.suffix : undefined
     },
     validation: {
-      ...variant.validation,
+      ...validation,
       conditional: {
         another_amount: {
           required: true,
           type: 'number',
           min: 0.5,
-          max: 24,
+          max: maxHours,
           answerKey: 'smokingQuantityOther',
           value: conditionalValue,
           href: conditionalHref
@@ -752,12 +924,16 @@ const getSmokingQuantityQuestionOverrides = ({
         text: 'Number of hours must be a number',
         href: conditionalHref
       },
+      integer: {
+        text: `${upperFirst(getAnswerPhraseFromHeading(heading))} must be a whole number`,
+        href: `#${question.input.id}`
+      },
       min: {
         text: 'Number of hours must be 0.5 or more',
         href: conditionalHref
       },
       max: {
-        text: 'Number of hours must be 24 or fewer',
+        text: `Number of hours must be ${maxHours} or fewer`,
         href: conditionalHref
       }
     },
@@ -772,6 +948,10 @@ const lowerFirst = (value = '') => {
   return value ? `${value.charAt(0).toLowerCase()}${value.slice(1)}` : ''
 }
 
+const upperFirst = (value = '') => {
+  return value ? `${value.charAt(0).toUpperCase()}${value.slice(1)}` : ''
+}
+
 /**
  * Convert a question heading into an error-message answer phrase.
  *
@@ -785,9 +965,10 @@ const getAnswerPhraseFromHeading = (heading = '') => {
     .replace(/^how long did you smoke /, 'how long you smoked ')
     .replace(/^how long do you currently smoke /, 'how long you currently smoke ')
     .replace(/^how long do you smoke /, 'how long you smoke ')
-    .replace(/^(how (?:much|many|long) .+?) did you smoke /, '$1 you smoked ')
-    .replace(/^(how (?:much|many|long) .+?) do you currently smoke /, '$1 you currently smoke ')
-    .replace(/^(how (?:much|many|long) .+?) do you smoke /, '$1 you smoke ')
+    .replace(/^(how (?:much|many|long) .+?) did you normally smoke(?= |$)/, '$1 you normally smoked')
+    .replace(/^(how (?:much|many|long) .+?) did you smoke(?= |$)/, '$1 you smoked')
+    .replace(/^(how (?:much|many|long) .+?) do you currently smoke(?= |$)/, '$1 you currently smoke')
+    .replace(/^(how (?:much|many|long) .+?) do you smoke(?= |$)/, '$1 you smoke')
 }
 
 /**
@@ -933,6 +1114,7 @@ const getSmokingContentQuestionOverrides = ({
         : `answers[${step.type}][smokingQuantity]`,
       value: isSettingSpecific ? settingAnswer.smokingQuantity : answer.smokingQuantity,
       conditionalValue: isSettingSpecific ? settingAnswer.smokingQuantityOther : answer.smokingQuantityOther,
+      frequency: isSettingSpecific ? settingAnswer.smokingFrequency : answer.smokingFrequency,
       smokingType
     })
   }
@@ -974,6 +1156,7 @@ const getSmokingContentQuestionOverrides = ({
       name: `answers[${step.type}][${smokingChange.answerKey}][quantity]`,
       value: changeAnswer.quantity,
       conditionalValue: changeAnswer.smokingQuantityOther,
+      frequency: answer.smokingFrequency,
       smokingType
     })
   }
@@ -1107,10 +1290,23 @@ const validateSmokingTypeQuestion = (req, page, step) => {
     }
   }
 
-  return validateQuestion(req.session.data.answers, page, {
+  const validationErrors = validateQuestion(req.session.data.answers, page, {
     ...overrides,
     errors
   })
+  const comparisonError = getSmokingQuantityChangeComparisonError({
+    page,
+    step,
+    answer,
+    changeAnswer,
+    href: `#${errorHref}`
+  })
+
+  if (comparisonError && !validationErrors.some((error) => error.href === comparisonError.href)) {
+    validationErrors.push(comparisonError)
+  }
+
+  return validationErrors
 }
 
 module.exports = {
diff --git a/app/prototype_v4_2/data/questions.yaml b/app/prototype_v4_2/data/questions.yaml
index 9f278b67..779e114d 100644
--- a/app/prototype_v4_2/data/questions.yaml
+++ b/app/prototype_v4_2/data/questions.yaml
@@ -108,7 +108,7 @@ questions:
       label: What is your height in centimetres?
       valueKey: metric
       suffix: cm
-      inputmode: numeric
+      inputmode: decimal
       classes: nhsuk-input--width-4
     switchUnits:
       text: Switch to feet and inches
@@ -117,17 +117,17 @@ questions:
     validation:
       required: true
       type: number
-      min: 100
-      max: 250
+      min: 139.7
+      max: 243.8
     errors:
       required:
         text: Enter your height in centimetres
       invalid:
         text: Enter your height in centimetres using numbers
       min:
-        text: Height in centimetres must be 100 or more
+        text: Height in centimetres must be 139.7cm or more
       max:
-        text: Height in centimetres must be 250 or fewer
+        text: Height in centimetres must be 243.8cm or fewer
   - id: height-imperial
     type: text_group
     answerKey: height
@@ -158,13 +158,21 @@ questions:
         - answerKey: feet
           required: true
           type: number
-          min: 3
-          max: 8
+          integer: true
+          min: 0
         - answerKey: inches
           required: true
           type: number
+          integer: true
           min: 0
           max: 11
+      total:
+        min: 55
+        max: 96
+        items:
+          - answerKey: feet
+            multiplier: 12
+          - answerKey: inches
     errors:
       items:
         feet:
@@ -174,11 +182,11 @@ questions:
           invalid:
             text: Enter your height in feet using numbers
             href: "#height-imperial-feet"
-          min:
-            text: Height in feet must be 3 or more
+          integer:
+            text: Height in feet must be a whole number
             href: "#height-imperial-feet"
-          max:
-            text: Height in feet must be 8 or fewer
+          min:
+            text: Height in feet must be 0ft or more
             href: "#height-imperial-feet"
         inches:
           required:
@@ -187,12 +195,22 @@ questions:
           invalid:
             text: Enter your height in inches using numbers
             href: "#height-imperial-inches"
+          integer:
+            text: Height in inches must be a whole number
+            href: "#height-imperial-inches"
           min:
-            text: Height in inches must be 0 or more
+            text: Height in inches must be 0in or more
             href: "#height-imperial-inches"
           max:
-            text: Height in inches must be 11 or fewer
+            text: Height in inches must be 11in or fewer
             href: "#height-imperial-inches"
+      total:
+        min:
+          text: Height must be between 4 feet 7 inches and 8 feet
+          href: "#height-imperial-feet"
+        max:
+          text: Height must be between 4 feet 7 inches and 8 feet
+          href: "#height-imperial-feet"
   - id: weight-metric
     type: text
     answerKey: weight
@@ -202,7 +220,7 @@ questions:
       valueKey: metric
       label: What is your weight in kilograms?
       suffix: kg
-      inputmode: numeric
+      inputmode: decimal
       classes: nhsuk-input--width-4
     switchUnits:
       text: Switch to stones and pounds
@@ -211,17 +229,20 @@ questions:
     validation:
       required: true
       type: number
-      min: 30
-      max: 250
+      decimalPlaces: 2
+      min: 25.4
+      max: 317.5
     errors:
       required:
         text: Enter your weight in kilograms
       invalid:
         text: Enter your weight in kilograms using numbers
+      decimalPlaces:
+        text: Weight in kilograms must have 2 decimal places or fewer
       min:
-        text: Weight in kilograms must be 30 or more
+        text: Weight in kilograms must be 25.4kg or more
       max:
-        text: Weight in kilograms must be 250 or fewer
+        text: Weight in kilograms must be 317.5kg or fewer
   - id: weight-imperial
     type: text_group
     answerKey: weight
@@ -252,13 +273,21 @@ questions:
         - answerKey: stones
           required: true
           type: number
-          min: 4
-          max: 40
+          integer: true
+          min: 0
         - answerKey: pounds
           required: true
           type: number
+          integer: true
           min: 0
           max: 13
+      total:
+        min: 56
+        max: 700
+        items:
+          - answerKey: stones
+            multiplier: 14
+          - answerKey: pounds
     errors:
       items:
         stones:
@@ -268,11 +297,11 @@ questions:
           invalid:
             text: Enter your weight in stones using numbers
             href: "#weight-imperial-stones"
-          min:
-            text: Weight in stones must be 4 or more
+          integer:
+            text: Weight in stones must be a whole number
             href: "#weight-imperial-stones"
-          max:
-            text: Weight in stones must be 40 or fewer
+          min:
+            text: Weight in stones must be 0st or more
             href: "#weight-imperial-stones"
         pounds:
           required:
@@ -281,12 +310,22 @@ questions:
           invalid:
             text: Enter your weight in pounds using numbers
             href: "#weight-imperial-pounds"
+          integer:
+            text: Weight in pounds must be a whole number
+            href: "#weight-imperial-pounds"
           min:
-            text: Weight in pounds must be 0 or more
+            text: Weight in pounds must be 0lb or more
             href: "#weight-imperial-pounds"
           max:
-            text: Weight in pounds must be 13 or fewer
+            text: Weight in pounds must be 13lb or fewer
             href: "#weight-imperial-pounds"
+      total:
+        min:
+          text: Weight must be between 4 stone and 50 stone
+          href: "#weight-imperial-stones"
+        max:
+          text: Weight must be between 4 stone and 50 stone
+          href: "#weight-imperial-stones"
   - id: gender
     type: single
     answerKey: gender
diff --git a/app/prototype_v4_2/docs/error-messages.md b/app/prototype_v4_2/docs/error-messages.md
new file mode 100644
index 00000000..8f8e08be
--- /dev/null
+++ b/app/prototype_v4_2/docs/error-messages.md
@@ -0,0 +1,609 @@
+# Error messages
+
+This document summarises validation logic and error messages for `prototype_v4_2`.
+
+Sources:
+
+- `data/questions.yaml` for base question content, validation rules and error text
+- `data/pages.yaml` for grouped-page composition and conditional question display
+- `lib/question-validator.js` for shared validation behaviour
+- `lib/tobacco-flow.js` for runtime tobacco question overrides
+- `controllers/question.js` for route branching and answer-clearing logic
+
+## Shared validation logic
+
+- Blank values are `undefined`, `null`, an empty string after trimming, or an empty array.
+- `required` errors stop further validation for that question or field.
+- Date validation only accepts real calendar dates. If no date part is supplied, the `required` message is used; if any date part is supplied but the date is not real, the `invalid` message is used.
+- Number validation rejects non-finite numbers, then checks whole-number, decimal-place, minimum and maximum rules in that order.
+- Grouped text inputs validate each item first. Total minimum and maximum checks only run when the grouped values are present and numerically valid.
+- Conditional reveal validation only runs for the selected trigger value. If the conditional value is blank, the conditional `required` message is used; number rules then use the question-level number error messages unless a runtime override replaces them.
+- Error links default to `#${question.input.id || question.id}`. Date, grouped and conditional inputs usually set explicit `href` values.
+
+## Flow logic
+
+- `phone-questionnaire`: `yes` goes to `phone-questionnaire-exit`; `no` continues to `smoker`.
+- `smoker`: `no` or `yes_fewer_than_100` goes to `not-eligible-for-screening`; current or previous smokers continue to `date-of-birth`.
+- `date-of-birth`: valid dates outside the eligible scan age range go to `not-eligible-for-scan`; eligible users continue to `face-to-face-appointment`.
+- `face-to-face-appointment`: `yes` goes to `book-appointment`; `no` continues to height and weight.
+- `height-*` and `weight-*`: metric and imperial pages are alternatives. Submitting one unit clears the other unit answer.
+- `asbestos`: grouped page that validates `asbestos-at-work` and `asbestos-at-home` together.
+- `cancer-diagnosis-relatives`: `yes` continues to `cancer-diagnosis-relatives-age`; `no` clears that answer and goes to `check-your-answers`.
+- `smoking-duration`: grouped page. `age-stopped-smoking` is visible only when `smoker` is `yes_previous` or type-specific `smoking-status` is `no`; otherwise `ageStoppedSmoking` is cleared. If `periods-stopped-smoking` is `no`, `yearsStoppedSmoking` is cleared.
+- `smoking-type`: `none` goes to `smoking-type-exit`; otherwise selected tobacco types create the tobacco sub-flow in tobacco.yaml order. Former smokers skip each type-specific `smoking-status` question.
+- `tobacco-smoking`: grouped tobacco page that validates `smoking-frequency` and `smoking-quantity` together for the active tobacco type.
+- `smoking-change`: selected `greater` and `fewer` options create corresponding changed-smoking follow-up steps. Unselected changed-smoking answer groups are cleared.
+- `tobacco-smoking-change`: grouped tobacco page that validates `smoking-frequency-change`, `smoking-quantity-change`, and `smoking-years-change` together for the active tobacco type and change direction.
+- `smoking-quantity` and `smoking-quantity-change`: if `another_amount` is not selected, the related `smokingQuantityOther` answer is cleared.
+
+## Page composition
+
+- `accept-terms`: `accept-terms`
+- `phone-questionnaire`: `phone-questionnaire`
+- `smoker`: `smoker`
+- `date-of-birth`: `date-of-birth`
+- `face-to-face-appointment`: `face-to-face-appointment`
+- `height-metric`: `height-metric`
+- `height-imperial`: `height-imperial`
+- `weight-metric`: `weight-metric`
+- `weight-imperial`: `weight-imperial`
+- `gender`: `gender`
+- `sex`: `sex`
+- `ethnicity`: `ethnicity`
+- `education`: `education`
+- `respiratory-conditions`: `respiratory-conditions`
+- `asbestos`: `asbestos-at-work`, `asbestos-at-home`
+- `cancer-diagnosis`: `cancer-diagnosis`
+- `cancer-diagnosis-relatives`: `cancer-diagnosis-relatives`
+- `cancer-diagnosis-relatives-age`: `cancer-diagnosis-relatives-age`
+- `smoking-duration`: `age-started-smoking`, `age-stopped-smoking` (shown when any configured condition matches), `periods-stopped-smoking`
+- `smoking-type`: `smoking-type`
+- `smoking-status`: `smoking-status`
+- `tobacco-smoking`: `smoking-frequency`, `smoking-quantity`
+- `tobacco-smoking-change`: `smoking-frequency-change`, `smoking-quantity-change`, `smoking-years-change`
+- `smoking-change`: `smoking-change`
+
+## Runtime tobacco messages
+
+- Tobacco sub-flow pages use `getSmokingContentQuestionOverrides()` before validation. The validator sees the runtime heading, input name, selected value and tobacco-specific variant.
+- `smoking-status`, `smoking-frequency`, `smoking-quantity`, `smoking-change`, `smoking-frequency-change`, `smoking-quantity-change`, and `smoking-years-change` can replace their base `required` text with contextual text generated from the runtime heading.
+- For headings beginning `How often`, the required message is `Select ...`; for `How much`, `How many`, or `How long`, the message is `Enter ...` for text inputs and `Select ...` for single-choice inputs; yes/no headings become `Select whether ...`.
+- For numeric tobacco quantity text inputs, non-rolling-tobacco and non-shisha types require whole numbers. The integer message is generated as `[answer phrase] must be a whole number`.
+- For shisha `another_amount`, the conditional input messages are `Enter the number of hours`, `Number of hours must be a number`, `Number of hours must be 0.5 or more`, and `Number of hours must be [maxHours] or fewer`. `maxHours` is 24 for daily/non-shisha, 168 weekly, 744 monthly, and 8760 yearly.
+- `smoking-quantity-change` adds a comparison error when a `greater` answer is not greater than the original amount, or a `fewer` answer is not fewer than the original amount after normalising by frequency. The message is generated as `[quantity phrase] must be more/fewer/less than [original quantity] [original frequency]`.
+
+## Question messages
+
+### accept-terms
+
+Question: Confirm you agree to the terms of use
+
+Type: `multiple`
+
+Answer key: `acceptTerms`
+
+Page: `accept-terms`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Confirm that you have read and agree to the terms of use | `#accept-terms` |
+
+### phone-questionnaire
+
+Question: Have you previously completed a lung cancer risk questionnaire by phone in the last 12 months?
+
+Type: `single`
+
+Answer key: `phoneQuestionnaire`
+
+Page: `phone-questionnaire`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have previously completed a lung cancer risk questionnaire by phone | `#phone-questionnaire` |
+
+### smoker
+
+Question: Have you ever smoked tobacco?
+
+Type: `single`
+
+Answer key: `smoker`
+
+Page: `smoker`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever smoked tobacco | `#smoker` |
+
+### date-of-birth
+
+Question: What is your date of birth?
+
+Type: `date`
+
+Answer key: `dateOfBirth`
+
+Page: `date-of-birth`
+
+Validation: required; type: date
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter your date of birth | `#dateOfBirth-day` |
+| `invalid` | Enter a real date of birth | `#dateOfBirth-day` |
+
+### face-to-face-appointment
+
+Question: Do you need to leave the online service and ask for a face-to-face appointment?
+
+Type: `single`
+
+Answer key: `faceToFaceAppointment`
+
+Page: `face-to-face-appointment`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you need to leave the online service and ask for a face-to-face appointment | `#face-to-face-appointment` |
+
+### height-metric
+
+Question: What is your height in centimetres?
+
+Type: `text`
+
+Answer key: `height`
+
+Page: `height-metric`
+
+Validation: required; type: number; minimum 139.7; maximum 243.8
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter your height in centimetres | `#height-metric` |
+| `invalid` | Enter your height in centimetres using numbers | `#height-metric` |
+| `min` | Height in centimetres must be 139.7cm or more | `#height-metric` |
+| `max` | Height in centimetres must be 243.8cm or fewer | `#height-metric` |
+
+### height-imperial
+
+Question: What is your height in feet and inches?
+
+Type: `text_group`
+
+Answer key: `height`
+
+Page: `height-imperial`
+
+Validation: grouped input items: feet (required, number, whole number, min 0); inches (required, number, whole number, min 0, max 11); group total: min 55, max 96
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `feet.required` | Enter your height in feet | `#height-imperial-feet` |
+| `feet.invalid` | Enter your height in feet using numbers | `#height-imperial-feet` |
+| `feet.integer` | Height in feet must be a whole number | `#height-imperial-feet` |
+| `feet.min` | Height in feet must be 0ft or more | `#height-imperial-feet` |
+| `inches.required` | Enter your height in inches | `#height-imperial-inches` |
+| `inches.invalid` | Enter your height in inches using numbers | `#height-imperial-inches` |
+| `inches.integer` | Height in inches must be a whole number | `#height-imperial-inches` |
+| `inches.min` | Height in inches must be 0in or more | `#height-imperial-inches` |
+| `inches.max` | Height in inches must be 11in or fewer | `#height-imperial-inches` |
+| `total.min` | Height must be between 4 feet 7 inches and 8 feet | `#height-imperial-feet` |
+| `total.max` | Height must be between 4 feet 7 inches and 8 feet | `#height-imperial-feet` |
+
+### weight-metric
+
+Question: What is your weight in kilograms?
+
+Type: `text`
+
+Answer key: `weight`
+
+Page: `weight-metric`
+
+Validation: required; type: number; maximum 2 decimal places; minimum 25.4; maximum 317.5
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter your weight in kilograms | `#weight-metric` |
+| `invalid` | Enter your weight in kilograms using numbers | `#weight-metric` |
+| `decimalPlaces` | Weight in kilograms must have 2 decimal places or fewer | `#weight-metric` |
+| `min` | Weight in kilograms must be 25.4kg or more | `#weight-metric` |
+| `max` | Weight in kilograms must be 317.5kg or fewer | `#weight-metric` |
+
+### weight-imperial
+
+Question: What is your weight in stones and pounds?
+
+Type: `text_group`
+
+Answer key: `weight`
+
+Page: `weight-imperial`
+
+Validation: grouped input items: stones (required, number, whole number, min 0); pounds (required, number, whole number, min 0, max 13); group total: min 56, max 700
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `stones.required` | Enter your weight in stones | `#weight-imperial-stones` |
+| `stones.invalid` | Enter your weight in stones using numbers | `#weight-imperial-stones` |
+| `stones.integer` | Weight in stones must be a whole number | `#weight-imperial-stones` |
+| `stones.min` | Weight in stones must be 0st or more | `#weight-imperial-stones` |
+| `pounds.required` | Enter your weight in pounds | `#weight-imperial-pounds` |
+| `pounds.invalid` | Enter your weight in pounds using numbers | `#weight-imperial-pounds` |
+| `pounds.integer` | Weight in pounds must be a whole number | `#weight-imperial-pounds` |
+| `pounds.min` | Weight in pounds must be 0lb or more | `#weight-imperial-pounds` |
+| `pounds.max` | Weight in pounds must be 13lb or fewer | `#weight-imperial-pounds` |
+| `total.min` | Weight must be between 4 stone and 50 stone | `#weight-imperial-stones` |
+| `total.max` | Weight must be between 4 stone and 50 stone | `#weight-imperial-stones` |
+
+### gender
+
+Question: Which of these best describes your gender identity?
+
+Type: `single`
+
+Answer key: `gender`
+
+Page: `gender`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select which option best describes your gender identity | `#gender` |
+
+### sex
+
+Question: What was your sex at birth?
+
+Type: `single`
+
+Answer key: `sex`
+
+Page: `sex`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select your sex at birth | `#sex` |
+
+### ethnicity
+
+Question: What is your ethnic background?
+
+Type: `single`
+
+Answer key: `ethnicity`
+
+Page: `ethnicity`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select your ethnic background | `#ethnicity` |
+
+### education
+
+Question: What is the highest level of education you have completed?
+
+Type: `single`
+
+Answer key: `education`
+
+Page: `education`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select the highest level of education you have completed | `#education` |
+
+### respiratory-conditions
+
+Question: Have you ever been diagnosed with any of the following respiratory conditions?
+
+Type: `multiple`
+
+Answer key: `respiratoryConditions`
+
+Page: `respiratory-conditions`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select if you have ever been diagnosed with any respiratory conditions | `#respiratory-conditions` |
+
+### asbestos-at-work
+
+Question: Have you ever worked in a job where you might have been exposed to asbestos?
+
+Type: `single`
+
+Answer key: `asbestosAtWork`
+
+Page: `asbestos`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever worked in a job where you might have been exposed to asbestos | `#asbestos-at-work` |
+
+### asbestos-at-home
+
+Question: Have you ever lived with anyone who worked with asbestos?
+
+Type: `single`
+
+Answer key: `asbestosAtHome`
+
+Page: `asbestos`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever lived with anyone who worked with asbestos | `#asbestos-at-home` |
+
+### cancer-diagnosis
+
+Question: Have you ever been diagnosed with cancer?
+
+Type: `single`
+
+Answer key: `cancerDiagnosis`
+
+Page: `cancer-diagnosis`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever been diagnosed with cancer | `#cancer-diagnosis` |
+
+### cancer-diagnosis-relatives
+
+Question: Have any of your parents, siblings or children ever been diagnosed with lung cancer?
+
+Type: `single`
+
+Answer key: `cancerDiagnosisRelatives`
+
+Page: `cancer-diagnosis-relatives`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether any of your parents, siblings or children have ever been diagnosed with lung cancer | `#cancer-diagnosis-relatives` |
+
+### cancer-diagnosis-relatives-age
+
+Question: Were any of your relatives younger than 60 years old when they were diagnosed with lung cancer?
+
+Type: `single`
+
+Answer key: `cancerDiagnosisRelativesAge`
+
+Page: `cancer-diagnosis-relatives-age`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether any of your relatives were younger than 60 when they were diagnosed with lung cancer | `#cancer-diagnosis-relatives-age` |
+
+### age-started-smoking
+
+Question: How old were you when you started smoking?
+
+Type: `text`
+
+Answer key: `ageStartedSmoking`
+
+Page: `smoking-duration`
+
+Validation: required; type: number; minimum 1; maximum 120
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter the age you started smoking | `#age-started-smoking` |
+| `invalid` | Enter the age you started smoking using numbers | `#age-started-smoking` |
+| `min` | Age you started smoking must be 1 or older | `#age-started-smoking` |
+| `max` | Age you started smoking must be 120 or younger | `#age-started-smoking` |
+
+### age-stopped-smoking
+
+Question: How old were you when you quit smoking?
+
+Type: `text`
+
+Answer key: `ageStoppedSmoking`
+
+Page: `smoking-duration`
+
+Validation: required; type: number; minimum 1; maximum 120
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter the age you quit smoking | `#age-stopped-smoking` |
+| `invalid` | Enter the age you quit smoking using numbers | `#age-stopped-smoking` |
+| `min` | Age you quit smoking must be 1 or older | `#age-stopped-smoking` |
+| `max` | Age you quit smoking must be 120 or younger | `#age-stopped-smoking` |
+
+### periods-stopped-smoking
+
+Question: Have you ever stopped smoking for periods of 1 year or longer?
+
+Type: `single`
+
+Answer key: `periodsStoppedSmoking`
+
+Page: `smoking-duration`
+
+Validation: required; conditional: yes (required, number, min 1, max 80)
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever stopped smoking for periods of 1 year or longer | `#periods-stopped-smoking` |
+| `invalid` | Total number of years you stopped smoking must be a number | `#years-stopped-smoking` |
+| `min` | Total number of years you stopped smoking must be 1 or more | `#years-stopped-smoking` |
+| `max` | Total number of years you stopped smoking must be 80 or fewer | `#years-stopped-smoking` |
+| `conditional.yes.required` | Enter the total number of years you stopped smoking | `#years-stopped-smoking` |
+
+### smoking-type
+
+Question: Have you ever smoked any of the following types of tobacco?
+
+Type: `multiple`
+
+Answer key: `smokingType`
+
+Page: `smoking-type`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select the types of tobacco you have smoked | `#smoking-type` |
+
+Variants: `previous`. Variants can change type, options, hints or validation before the same validator runs.
+
+### smoking-status
+
+Question: Do you currently smoke?
+
+Type: `single`
+
+Answer key: `smokingStatus`
+
+Page: `smoking-status`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you currently smoke this type of tobacco | `#smoking-status` |
+
+### smoking-frequency
+
+Question: How often do you smoke?
+
+Type: `single`
+
+Answer key: `smokingFrequency`
+
+Page: `tobacco-smoking`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select how often you smoke this type of tobacco | `#smoking-frequency` |
+
+### smoking-quantity
+
+Question: How much do you smoke?
+
+Type: `text`
+
+Answer key: `smokingQuantity`
+
+Page: `tobacco-smoking`
+
+Validation: required; type: number; minimum 1; maximum 200
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter how much you smoke | `#smoking-quantity` |
+| `invalid` | Enter how much you smoke using numbers | `#smoking-quantity` |
+| `min` | Amount smoked must be 1 or more | `#smoking-quantity` |
+| `max` | Amount smoked must be 200 or fewer | `#smoking-quantity` |
+
+Variants: `rolling_tobacco`, `shisha`. Variants can change type, options, hints or validation before the same validator runs.
+
+### smoking-change
+
+Question: Has the amount you normally smoke changed over time?
+
+Type: `multiple`
+
+Answer key: `smokingChange`
+
+Page: `smoking-change`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether the amount you normally smoke has changed over time | `#smoking-change` |
+
+### smoking-frequency-change
+
+Question: How often did you smoke?
+
+Type: `single`
+
+Answer key: `smokingFrequencyChange`
+
+Page: `tobacco-smoking-change`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select how often you smoked this type of tobacco | `#smoking-frequency-change` |
+
+### smoking-quantity-change
+
+Question: How much did you smoke?
+
+Type: `text`
+
+Answer key: `smokingQuantityChange`
+
+Page: `tobacco-smoking-change`
+
+Validation: required; type: number; minimum 1; maximum 200
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter how much you smoked | `#smoking-quantity-change` |
+| `invalid` | Enter how much you smoked using numbers | `#smoking-quantity-change` |
+| `min` | Amount smoked must be 1 or more | `#smoking-quantity-change` |
+| `max` | Amount smoked must be 200 or fewer | `#smoking-quantity-change` |
+
+Variants: `rolling_tobacco`. Variants can change type, options, hints or validation before the same validator runs.
+
+### smoking-years-change
+
+Question: How many years did you smoke this amount?
+
+Type: `text`
+
+Answer key: `smokingYearsChange`
+
+Page: `tobacco-smoking-change`
+
+Validation: required; type: number; minimum 1; maximum 80
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter how many years you smoked this amount | `#smoking-years-change` |
+| `invalid` | Enter how many years using numbers | `#smoking-years-change` |
+| `min` | Number of years must be 1 or more | `#smoking-years-change` |
+| `max` | Number of years must be 80 or fewer | `#smoking-years-change` |
+
diff --git a/app/prototype_v4_2/docs/question-schema.md b/app/prototype_v4_2/docs/question-schema.md
index ac82bc78..d080daab 100644
--- a/app/prototype_v4_2/docs/question-schema.md
+++ b/app/prototype_v4_2/docs/question-schema.md
@@ -391,7 +391,10 @@ Supported validation fields are:
 | `type: date` | Requires day, month and year to form a real date. |
 | `min` | Minimum numeric value. |
 | `max` | Maximum numeric value. |
+| `integer` | Requires a numeric answer to be a whole number. |
+| `decimalPlaces` | Maximum decimal places for a numeric answer. |
 | `items` | Per-field validation for `text_group`. |
+| `total` | Combined numeric validation for `text_group` fields. |
 | `conditional` | Validation for conditional reveal inputs. |
 
 For numeric questions, provide all relevant messages:
diff --git a/app/prototype_v4_2/lib/question-validator.js b/app/prototype_v4_2/lib/question-validator.js
index db2953f0..9b970ee2 100644
--- a/app/prototype_v4_2/lib/question-validator.js
+++ b/app/prototype_v4_2/lib/question-validator.js
@@ -12,8 +12,11 @@ const { getQuestion } = require('./questions')
  * @property {string} [type] - Type-specific validation, for example number or date.
  * @property {number} [min] - Minimum numeric value.
  * @property {number} [max] - Maximum numeric value.
+ * @property {boolean} [integer] - Whether a numeric value must be a whole number.
+ * @property {number} [decimalPlaces] - Maximum number of decimal places allowed.
  * @property {Object} [conditional] - Conditional reveal validation rules.
  * @property {Object[]} [items] - Validation rules for grouped inputs.
+ * @property {Object} [total] - Total validation rules for grouped numeric inputs.
  */
 
 /**
@@ -191,6 +194,30 @@ const mergeQuestion = (question, overrides = {}) => {
   }
 }
 
+/**
+ * Check whether a submitted numeric value is written as a whole number.
+ *
+ * @param {*} value - Submitted value.
+ * @returns {boolean} True when the submitted value is an integer string.
+ */
+const isIntegerValue = (value) => {
+  return /^-?\d+$/.test(String(value).trim())
+}
+
+/**
+ * Check whether a submitted numeric value has no more than the allowed number
+ * of decimal places.
+ *
+ * @param {*} value - Submitted value.
+ * @param {number} decimalPlaces - Maximum decimal places allowed.
+ * @returns {boolean} True when the value has an allowed decimal precision.
+ */
+const hasMaximumDecimalPlaces = (value, decimalPlaces) => {
+  const match = String(value).trim().match(/^-?\d+(?:\.(\d+))?$/)
+
+  return Boolean(match) && (!match[1] || match[1].length <= decimalPlaces)
+}
+
 /**
  * Validate a numeric value against invalid, min and max rules.
  *
@@ -203,11 +230,24 @@ const mergeQuestion = (question, overrides = {}) => {
 const validateNumber = (value, validation, errorsConfig, errors, defaultHref) => {
   const number = Number(value)
 
-  if (Number.isNaN(number)) {
+  if (Number.isNaN(number) || !Number.isFinite(number)) {
     errors.push(makeError(errorsConfig.invalid, defaultHref))
     return
   }
 
+  if (validation.integer && !isIntegerValue(value)) {
+    errors.push(makeError(errorsConfig.integer || errorsConfig.invalid, defaultHref))
+    return
+  }
+
+  if (
+    validation.decimalPlaces !== undefined &&
+    !hasMaximumDecimalPlaces(value, validation.decimalPlaces)
+  ) {
+    errors.push(makeError(errorsConfig.decimalPlaces || errorsConfig.invalid, defaultHref))
+    return
+  }
+
   if (validation.min !== undefined && number < validation.min) {
     errors.push(makeError(errorsConfig.min, defaultHref))
   }
@@ -217,6 +257,61 @@ const validateNumber = (value, validation, errorsConfig, errors, defaultHref) =>
   }
 }
 
+/**
+ * Validate the total value of grouped numeric inputs.
+ *
+ * @param {Object} groupValue - Submitted group values keyed by answer key.
+ * @param {Object} question - Normalised text_group question config.
+ * @param {ValidationError[]} errors - Mutable error collection.
+ */
+const validateGroupTotal = (groupValue, question, errors) => {
+  const totalValidation = question.validation?.total
+
+  if (!totalValidation) {
+    return
+  }
+
+  const total = (totalValidation.items || []).reduce((sum, item) => {
+    const multiplier = item.multiplier || 1
+
+    return sum + (Number(groupValue[item.answerKey]) * multiplier)
+  }, 0)
+  const totalErrors = question.errors?.total || {}
+  const defaultHref = `#${question.input?.items?.[0]?.id || question.id}`
+
+  if (totalValidation.min !== undefined && total < totalValidation.min) {
+    errors.push(makeError(totalErrors.min, defaultHref))
+  }
+
+  if (totalValidation.max !== undefined && total > totalValidation.max) {
+    errors.push(makeError(totalErrors.max, defaultHref))
+  }
+}
+
+/**
+ * Order grouped input errors by the visual input order.
+ *
+ * @param {ValidationError[]} errors - Validation errors for a text_group.
+ * @param {Object} question - Normalised text_group question config.
+ * @returns {ValidationError[]} Errors ordered by grouped input position.
+ */
+const orderGroupErrors = (errors, question) => {
+  const inputOrder = (question.input?.items || []).reduce((map, item, index) => {
+    map[`#${item.id}`] = index
+    return map
+  }, {})
+
+  return errors
+    .map((error, index) => ({ error, index }))
+    .sort((a, b) => {
+      const aOrder = inputOrder[a.error.href] ?? Number.MAX_SAFE_INTEGER
+      const bOrder = inputOrder[b.error.href] ?? Number.MAX_SAFE_INTEGER
+
+      return aOrder - bOrder || a.index - b.index
+    })
+    .map(({ error }) => error)
+}
+
 /**
  * Validate grouped inputs, such as imperial height or weight fields.
  *
@@ -227,6 +322,7 @@ const validateNumber = (value, validation, errorsConfig, errors, defaultHref) =>
 const validateInputGroup = (answers, question) => {
   const errors = []
   const groupValue = answers[question.answerKey]?.[question.input.valueKey] || {}
+  let canValidateTotal = Boolean(question.validation?.total)
 
   ;(question.validation?.items || []).forEach((itemValidation) => {
     const value = groupValue[itemValidation.answerKey]
@@ -235,15 +331,30 @@ const validateInputGroup = (answers, question) => {
 
     if (itemValidation.required && isBlank(value)) {
       errors.push(makeError(itemErrors.required, defaultHref))
+      canValidateTotal = false
       return
     }
 
     if (itemValidation.type === 'number' && !isBlank(value)) {
+      const number = Number(value)
+
+      if (
+        Number.isNaN(number) ||
+        !Number.isFinite(number) ||
+        (itemValidation.integer && !isIntegerValue(value))
+      ) {
+        canValidateTotal = false
+      }
+
       validateNumber(value, itemValidation, itemErrors, errors, defaultHref)
     }
   })
 
-  return errors
+  if (canValidateTotal) {
+    validateGroupTotal(groupValue, question, errors)
+  }
+
+  return orderGroupErrors(errors, question)
 }
 
 /**
diff --git a/app/prototype_v4_2/lib/tobacco-flow.js b/app/prototype_v4_2/lib/tobacco-flow.js
index e96f1a96..074bca58 100644
--- a/app/prototype_v4_2/lib/tobacco-flow.js
+++ b/app/prototype_v4_2/lib/tobacco-flow.js
@@ -267,6 +267,119 @@ const getSmokingComparisonQuantity = (type, answer) => {
   return getSmokingQuantity(type, answer)
 }
 
+const rollingTobaccoQuantityValues = {
+  less_than_10: 10,
+  '10_to_30': 20,
+  '31_to_50': 40.5,
+  '51_to_75': 63,
+  '76_to_100': 88,
+  more_than_100: 100
+}
+
+const smokingFrequencyRateMultipliers = {
+  daily: 7,
+  weekly: 1,
+  monthly: 0.25,
+  yearly: 1 / 52
+}
+
+/**
+ * Check whether a value can be compared as a submitted numeric answer.
+ *
+ * @param {*} value - Submitted value.
+ * @returns {boolean} True when value is numeric.
+ */
+const isComparableNumber = (value) => {
+  if (value === undefined || value === null || String(value).trim() === '') {
+    return false
+  }
+
+  const number = Number(value)
+
+  return Number.isFinite(number)
+}
+
+/**
+ * Get a normalised quantity value for cross-frequency comparisons.
+ *
+ * @param {string} type - Tobacco type key.
+ * @param {*} quantity - Submitted quantity answer.
+ * @param {string} frequency - Submitted frequency answer.
+ * @returns {number|undefined} Normalised quantity, when comparable.
+ */
+const getSmokingQuantityComparisonRate = (type, quantity, frequency) => {
+  const multiplier = smokingFrequencyRateMultipliers[frequency]
+
+  if (!multiplier) {
+    return undefined
+  }
+
+  if (type === 'rolling_tobacco') {
+    const value = rollingTobaccoQuantityValues[quantity]
+
+    return value ? value * multiplier : undefined
+  }
+
+  return isComparableNumber(quantity) ? Number(quantity) * multiplier : undefined
+}
+
+/**
+ * Build the left side of a changed-quantity comparison error.
+ *
+ * @param {string} type - Tobacco type key.
+ * @returns {string} Error phrase.
+ */
+const getSmokingQuantityChangeErrorPhrase = (type) => {
+  const heading = removeQuestionMark(getSmokingTypeHeadings(type, true).quantityHeading || '')
+    .replace(/ in a normal (day|week|month|year)$/, '')
+
+  return upperFirst(lowerFirst(heading)
+    .replace(/^(how (?:much|many) .+?) did you normally smoke$/, '$1 you normally smoked')
+    .replace(/^(how (?:much|many) .+?) did you smoke$/, '$1 you normally smoked'))
+}
+
+/**
+ * Check whether a changed-smoking quantity contradicts the selected change direction.
+ *
+ * @param {Object} params - Comparison inputs.
+ * @returns {Object|undefined} Validation error when the quantities contradict.
+ */
+const getSmokingQuantityChangeComparisonError = ({
+  page,
+  step,
+  answer,
+  changeAnswer,
+  href
+}) => {
+  if (page !== 'smoking-quantity-change' || !step.change || !answer.smokingQuantity || !answer.smokingFrequency || !changeAnswer.quantity || !changeAnswer.frequency) {
+    return undefined
+  }
+
+  const currentRate = getSmokingQuantityComparisonRate(step.type, answer.smokingQuantity, answer.smokingFrequency)
+  const changedRate = getSmokingQuantityComparisonRate(step.type, changeAnswer.quantity, changeAnswer.frequency)
+
+  if (currentRate === undefined || changedRate === undefined) {
+    return undefined
+  }
+
+  const contradictsChange = step.change === 'greater'
+    ? changedRate <= currentRate
+    : changedRate >= currentRate
+
+  if (!contradictsChange) {
+    return undefined
+  }
+
+  const comparisonText = step.type === 'rolling_tobacco' && step.change === 'fewer'
+    ? 'less'
+    : smokingChangeTypes[step.change]?.label
+
+  return {
+    text: `${getSmokingQuantityChangeErrorPhrase(step.type)} must be ${comparisonText} than ${[getSmokingComparisonQuantity(step.type, answer.smokingQuantity), getSmokingFrequencyComparisonPeriod(answer.smokingFrequency)].filter(Boolean).join(' ')}`,
+    href
+  }
+}
+
 const smokingFrequencyPeriods = {
   daily: 'a day',
   weekly: 'a week',
@@ -274,6 +387,20 @@ const smokingFrequencyPeriods = {
   yearly: 'a year'
 }
 
+const smokingFrequencyComparisonPeriods = {
+  daily: 'per day',
+  weekly: 'per week',
+  monthly: 'per month',
+  yearly: 'per year'
+}
+
+const smokingFrequencyMaxHours = {
+  daily: 24,
+  weekly: 168,
+  monthly: 744,
+  yearly: 8760
+}
+
 /**
  * Convert a smoking frequency value into a period phrase.
  *
@@ -284,6 +411,31 @@ const getSmokingFrequencyPeriod = (frequency) => {
   return smokingFrequencyPeriods[frequency] || ''
 }
 
+/**
+ * Convert a smoking frequency value into a comparison phrase.
+ *
+ * @param {string} frequency - Frequency value.
+ * @returns {string} Comparison phrase, for example `per day`.
+ */
+const getSmokingFrequencyComparisonPeriod = (frequency) => {
+  return smokingFrequencyComparisonPeriods[frequency] || ''
+}
+
+/**
+ * Get the maximum number of hours allowed for an "another amount" shisha answer.
+ *
+ * @param {string} type - Tobacco type key.
+ * @param {string} frequency - Selected smoking frequency.
+ * @returns {number} Maximum number of hours.
+ */
+const getSmokingQuantityOtherMaxHours = (type, frequency) => {
+  if (type !== 'shisha') {
+    return 24
+  }
+
+  return smokingFrequencyMaxHours[frequency] || 24
+}
+
 /**
  * Replace the default "normal day/week/month/year" phrase in a heading.
  *
@@ -537,13 +689,7 @@ const getSmokingChangeHeading = (page, type, change, changeAnswer = {}, answer =
   }
 
   if (page === 'smoking-years-change') {
-    const quantity = getSmokingQuantity(type, changeAnswer.quantity)
-
-    if (!quantity) {
-      return getQuestion('smoking-years-change').input.label
-    }
-
-    return `How many years did you smoke ${[quantity, getSmokingFrequencyPeriod(answer.smokingFrequency)].filter(Boolean).join(' ')}?`
+    return getQuestion('smoking-years-change').input.label
   }
 
   return ''
@@ -632,6 +778,16 @@ const getQuestionItemsWithLabels = (id, labels = {}, hintOverrides = {}) => {
   })
 }
 
+/**
+ * Check whether a smoking quantity must be a whole number.
+ *
+ * @param {string} type - Tobacco type key.
+ * @returns {boolean} True when decimal quantities should be rejected.
+ */
+const requiresWholeNumberSmokingQuantity = (type) => {
+  return !['rolling_tobacco', 'shisha'].includes(type)
+}
+
 /**
  * Build runtime overrides for a quantity question.
  *
@@ -646,6 +802,7 @@ const getSmokingQuantityQuestionOverrides = ({
   name,
   value,
   conditionalValue,
+  frequency,
   smokingType
 }) => {
   const question = getQuestion(page)
@@ -655,6 +812,15 @@ const getSmokingQuantityQuestionOverrides = ({
   const hint = hasHintOverride ? variantInput.hint : question.input.hint
   const questionType = variant.type || question.type
   const conditionalHref = '#smoking-quantity-other'
+  const maxHours = getSmokingQuantityOtherMaxHours(step.type, frequency)
+  const validation = {
+    ...variant.validation
+  }
+
+  if (requiresWholeNumberSmokingQuantity(step.type)) {
+    validation.integer = true
+  }
+
   const items = variant.options
     ? variant.options.map((option) => {
       const item = toComponentItem(option)
@@ -690,13 +856,13 @@ const getSmokingQuantityQuestionOverrides = ({
       suffix: questionType === 'text' ? smokingType.suffix : undefined
     },
     validation: {
-      ...variant.validation,
+      ...validation,
       conditional: {
         another_amount: {
           required: true,
           type: 'number',
           min: 0.5,
-          max: 24,
+          max: maxHours,
           answerKey: 'smokingQuantityOther',
           value: conditionalValue,
           href: conditionalHref
@@ -716,12 +882,16 @@ const getSmokingQuantityQuestionOverrides = ({
         text: 'Number of hours must be a number',
         href: conditionalHref
       },
+      integer: {
+        text: `${upperFirst(getAnswerPhraseFromHeading(heading))} must be a whole number`,
+        href: `#${question.input.id}`
+      },
       min: {
         text: 'Number of hours must be 0.5 or more',
         href: conditionalHref
       },
       max: {
-        text: 'Number of hours must be 24 or fewer',
+        text: `Number of hours must be ${maxHours} or fewer`,
         href: conditionalHref
       }
     },
@@ -753,10 +923,10 @@ const getAnswerPhraseFromHeading = (heading = '') => {
     .replace(/^how long did you smoke /, 'how long you smoked ')
     .replace(/^how long do you currently smoke /, 'how long you currently smoke ')
     .replace(/^how long do you smoke /, 'how long you smoke ')
-    .replace(/^(how (?:much|many|long) .+?) did you normally smoke/, '$1 you normally smoked')
-    .replace(/^(how (?:much|many|long) .+?) did you smoke /, '$1 you smoked ')
-    .replace(/^(how (?:much|many|long) .+?) do you currently smoke /, '$1 you currently smoke ')
-    .replace(/^(how (?:much|many|long) .+?) do you smoke /, '$1 you smoke ')
+    .replace(/^(how (?:much|many|long) .+?) did you normally smoke(?= |$)/, '$1 you normally smoked')
+    .replace(/^(how (?:much|many|long) .+?) did you smoke(?= |$)/, '$1 you smoked')
+    .replace(/^(how (?:much|many|long) .+?) do you currently smoke(?= |$)/, '$1 you currently smoke')
+    .replace(/^(how (?:much|many|long) .+?) do you smoke(?= |$)/, '$1 you smoke')
 }
 
 /**
@@ -878,6 +1048,7 @@ const getSmokingContentQuestionOverrides = ({
       name: `answers[${step.type}][smokingQuantity]`,
       value: answer.smokingQuantity,
       conditionalValue: answer.smokingQuantityOther,
+      frequency: answer.smokingFrequency,
       smokingType
     })
   }
@@ -919,6 +1090,7 @@ const getSmokingContentQuestionOverrides = ({
       name: `answers[${step.type}][${smokingChange.answerKey}][quantity]`,
       value: changeAnswer.quantity,
       conditionalValue: changeAnswer.smokingQuantityOther,
+      frequency: answer.smokingFrequency,
       smokingType
     })
   }
@@ -1120,10 +1292,23 @@ const validateSmokingTypeQuestion = (req, page, step) => {
     }
   }
 
-  return validateQuestion(req.session.data.answers, page, {
+  const validationErrors = validateQuestion(req.session.data.answers, page, {
     ...overrides,
     errors
   })
+  const comparisonError = getSmokingQuantityChangeComparisonError({
+    page,
+    step,
+    answer: context.answer,
+    changeAnswer: context.changeAnswer,
+    href: `#${errorHref}`
+  })
+
+  if (comparisonError && !validationErrors.some((error) => error.href === comparisonError.href)) {
+    validationErrors.push(comparisonError)
+  }
+
+  return validationErrors
 }
 
 /**
diff --git a/app/prototype_v4_3/data/questions.yaml b/app/prototype_v4_3/data/questions.yaml
index 581d4235..5318e502 100644
--- a/app/prototype_v4_3/data/questions.yaml
+++ b/app/prototype_v4_3/data/questions.yaml
@@ -108,7 +108,7 @@ questions:
       label: What is your height in centimetres?
       valueKey: metric
       suffix: cm
-      inputmode: numeric
+      inputmode: decimal
       classes: nhsuk-input--width-4
     switchUnits:
       text: Switch to feet and inches
@@ -117,17 +117,17 @@ questions:
     validation:
       required: true
       type: number
-      min: 100
-      max: 250
+      min: 139.7
+      max: 243.8
     errors:
       required:
         text: Enter your height in centimetres
       invalid:
         text: Enter your height in centimetres using numbers
       min:
-        text: Height in centimetres must be 100 or more
+        text: Height in centimetres must be 139.7cm or more
       max:
-        text: Height in centimetres must be 250 or fewer
+        text: Height in centimetres must be 243.8cm or fewer
   - id: height-imperial
     type: text_group
     answerKey: height
@@ -158,13 +158,21 @@ questions:
         - answerKey: feet
           required: true
           type: number
-          min: 3
-          max: 8
+          integer: true
+          min: 0
         - answerKey: inches
           required: true
           type: number
+          integer: true
           min: 0
           max: 11
+      total:
+        min: 55
+        max: 96
+        items:
+          - answerKey: feet
+            multiplier: 12
+          - answerKey: inches
     errors:
       items:
         feet:
@@ -174,11 +182,11 @@ questions:
           invalid:
             text: Enter your height in feet using numbers
             href: "#height-imperial-feet"
-          min:
-            text: Height in feet must be 3 or more
+          integer:
+            text: Height in feet must be a whole number
             href: "#height-imperial-feet"
-          max:
-            text: Height in feet must be 8 or fewer
+          min:
+            text: Height in feet must be 0ft or more
             href: "#height-imperial-feet"
         inches:
           required:
@@ -187,12 +195,22 @@ questions:
           invalid:
             text: Enter your height in inches using numbers
             href: "#height-imperial-inches"
+          integer:
+            text: Height in inches must be a whole number
+            href: "#height-imperial-inches"
           min:
-            text: Height in inches must be 0 or more
+            text: Height in inches must be 0in or more
             href: "#height-imperial-inches"
           max:
-            text: Height in inches must be 11 or fewer
+            text: Height in inches must be 11in or fewer
             href: "#height-imperial-inches"
+      total:
+        min:
+          text: Height must be between 4 feet 7 inches and 8 feet
+          href: "#height-imperial-feet"
+        max:
+          text: Height must be between 4 feet 7 inches and 8 feet
+          href: "#height-imperial-feet"
   - id: weight-metric
     type: text
     answerKey: weight
@@ -202,7 +220,7 @@ questions:
       valueKey: metric
       label: What is your weight in kilograms?
       suffix: kg
-      inputmode: numeric
+      inputmode: decimal
       classes: nhsuk-input--width-4
     switchUnits:
       text: Switch to stones and pounds
@@ -211,17 +229,20 @@ questions:
     validation:
       required: true
       type: number
-      min: 30
-      max: 250
+      decimalPlaces: 2
+      min: 25.4
+      max: 317.5
     errors:
       required:
         text: Enter your weight in kilograms
       invalid:
         text: Enter your weight in kilograms using numbers
+      decimalPlaces:
+        text: Weight in kilograms must have 2 decimal places or fewer
       min:
-        text: Weight in kilograms must be 30 or more
+        text: Weight in kilograms must be 25.4kg or more
       max:
-        text: Weight in kilograms must be 250 or fewer
+        text: Weight in kilograms must be 317.5kg or fewer
   - id: weight-imperial
     type: text_group
     answerKey: weight
@@ -252,13 +273,21 @@ questions:
         - answerKey: stones
           required: true
           type: number
-          min: 4
-          max: 40
+          integer: true
+          min: 0
         - answerKey: pounds
           required: true
           type: number
+          integer: true
           min: 0
           max: 13
+      total:
+        min: 56
+        max: 700
+        items:
+          - answerKey: stones
+            multiplier: 14
+          - answerKey: pounds
     errors:
       items:
         stones:
@@ -268,11 +297,11 @@ questions:
           invalid:
             text: Enter your weight in stones using numbers
             href: "#weight-imperial-stones"
-          min:
-            text: Weight in stones must be 4 or more
+          integer:
+            text: Weight in stones must be a whole number
             href: "#weight-imperial-stones"
-          max:
-            text: Weight in stones must be 40 or fewer
+          min:
+            text: Weight in stones must be 0st or more
             href: "#weight-imperial-stones"
         pounds:
           required:
@@ -281,12 +310,22 @@ questions:
           invalid:
             text: Enter your weight in pounds using numbers
             href: "#weight-imperial-pounds"
+          integer:
+            text: Weight in pounds must be a whole number
+            href: "#weight-imperial-pounds"
           min:
-            text: Weight in pounds must be 0 or more
+            text: Weight in pounds must be 0lb or more
             href: "#weight-imperial-pounds"
           max:
-            text: Weight in pounds must be 13 or fewer
+            text: Weight in pounds must be 13lb or fewer
             href: "#weight-imperial-pounds"
+      total:
+        min:
+          text: Weight must be between 4 stone and 50 stone
+          href: "#weight-imperial-stones"
+        max:
+          text: Weight must be between 4 stone and 50 stone
+          href: "#weight-imperial-stones"
   - id: gender
     type: single
     answerKey: gender
diff --git a/app/prototype_v4_3/docs/error-messages.md b/app/prototype_v4_3/docs/error-messages.md
new file mode 100644
index 00000000..df970ddf
--- /dev/null
+++ b/app/prototype_v4_3/docs/error-messages.md
@@ -0,0 +1,627 @@
+# Error messages
+
+This document summarises validation logic and error messages for `prototype_v4_3`.
+
+Sources:
+
+- `data/questions.yaml` for base question content, validation rules and error text
+- `data/pages.yaml` for grouped-page composition and conditional question display
+- `lib/question-validator.js` for shared validation behaviour
+- `lib/tobacco-flow.js` for runtime tobacco question overrides
+- `controllers/question.js` for route branching and answer-clearing logic
+
+## Shared validation logic
+
+- Blank values are `undefined`, `null`, an empty string after trimming, or an empty array.
+- `required` errors stop further validation for that question or field.
+- Date validation only accepts real calendar dates. If no date part is supplied, the `required` message is used; if any date part is supplied but the date is not real, the `invalid` message is used.
+- Number validation rejects non-finite numbers, then checks whole-number, decimal-place, minimum and maximum rules in that order.
+- Grouped text inputs validate each item first. Total minimum and maximum checks only run when the grouped values are present and numerically valid.
+- Conditional reveal validation only runs for the selected trigger value. If the conditional value is blank, the conditional `required` message is used; number rules then use the question-level number error messages unless a runtime override replaces them.
+- Error links default to `#${question.input.id || question.id}`. Date, grouped and conditional inputs usually set explicit `href` values.
+
+## Flow logic
+
+- `phone-questionnaire`: `yes` goes to `phone-questionnaire-exit`; `no` continues to `smoking-type`.
+- `smoking-type`: `none` goes to `smoking-type-exit`. If multiple tobacco types are selected, the flow asks `smoking-status-current`; if one type is selected, it asks that type-specific `smoking-status`.
+- `smoking-status-current`: only shown when more than one tobacco type was selected; it records which selected types are currently smoked and then continues to `date-of-birth`.
+- `smoking-status`: if the answer is `less_than_lifetime_threshold`, the flow goes to `not-eligible-for-screening`; otherwise it continues through that tobacco type sub-flow.
+- `date-of-birth`: valid dates outside the eligible scan age range go to `not-eligible-for-scan`; eligible users continue to `face-to-face-appointment`.
+- `face-to-face-appointment`: `yes` goes to `book-appointment`; `no` continues to height and weight.
+- `height-*` and `weight-*`: metric and imperial pages are alternatives. Submitting one unit clears the other unit answer.
+- `asbestos`: grouped page that validates `asbestos-at-work` and `asbestos-at-home` together.
+- `cancer-diagnosis-relatives`: `yes` continues to `cancer-diagnosis-relatives-age`; `no` clears that answer and goes to `check-your-answers`.
+- `smoking-duration`: rendered inside the active tobacco sub-flow. It validates `age-started-smoking`, conditionally `age-stopped-smoking`, and `periods-stopped-smoking` for the active tobacco type. Current tobacco types clear `ageStoppedSmoking`; `periods-stopped-smoking` of `no` clears `yearsStoppedSmoking`.
+- `tobacco-smoking`: grouped tobacco page that validates `smoking-frequency` and `smoking-quantity` together for the active tobacco type.
+- `smoking-change`: selected `greater` and `fewer` options create corresponding changed-smoking follow-up steps. Unselected changed-smoking answer groups are cleared.
+- `tobacco-smoking-change`: grouped tobacco page that validates `smoking-frequency-change`, `smoking-quantity-change`, and `smoking-years-change` together for the active tobacco type and change direction.
+- `smoking-quantity` and `smoking-quantity-change`: if `another_amount` is not selected, the related `smokingQuantityOther` answer is cleared.
+
+## Page composition
+
+- `accept-terms`: `accept-terms`
+- `phone-questionnaire`: `phone-questionnaire`
+- `smoker`: `smoker`
+- `date-of-birth`: `date-of-birth`
+- `face-to-face-appointment`: `face-to-face-appointment`
+- `height-metric`: `height-metric`
+- `height-imperial`: `height-imperial`
+- `weight-metric`: `weight-metric`
+- `weight-imperial`: `weight-imperial`
+- `gender`: `gender`
+- `sex`: `sex`
+- `ethnicity`: `ethnicity`
+- `education`: `education`
+- `respiratory-conditions`: `respiratory-conditions`
+- `asbestos`: `asbestos-at-work`, `asbestos-at-home`
+- `cancer-diagnosis`: `cancer-diagnosis`
+- `cancer-diagnosis-relatives`: `cancer-diagnosis-relatives`
+- `cancer-diagnosis-relatives-age`: `cancer-diagnosis-relatives-age`
+- `smoking-duration`: `age-started-smoking`, `age-stopped-smoking` (shown when any configured condition matches), `periods-stopped-smoking`
+- `smoking-type`: `smoking-type`
+- `smoking-status`: `smoking-status`
+- `smoking-status-current`: `smoking-status-current`
+- `tobacco-smoking`: `smoking-frequency`, `smoking-quantity`
+- `tobacco-smoking-change`: `smoking-frequency-change`, `smoking-quantity-change`, `smoking-years-change`
+- `smoking-change`: `smoking-change`
+
+## Runtime tobacco messages
+
+- Tobacco sub-flow pages use `getSmokingContentQuestionOverrides()` before validation. The validator sees the runtime heading, input name, selected value and tobacco-specific variant.
+- `age-started-smoking`, `age-stopped-smoking`, `periods-stopped-smoking`, `smoking-status`, `smoking-frequency`, `smoking-quantity`, `smoking-change`, `smoking-frequency-change`, `smoking-quantity-change`, and `smoking-years-change` can replace their base `required` text with contextual text generated from the runtime heading.
+- For headings beginning `How often`, the required message is `Select ...`; for `How old`, `How much`, `How many`, or `How long`, the message is `Enter ...` for text inputs and `Select ...` for single-choice inputs; yes/no headings become `Select whether ...`.
+- For numeric tobacco quantity text inputs, non-rolling-tobacco and non-shisha types require whole numbers. The integer message is generated as `[answer phrase] must be a whole number`.
+- For shisha `another_amount`, the conditional input messages are `Enter the number of hours`, `Number of hours must be a number`, `Number of hours must be 0.5 or more`, and `Number of hours must be [maxHours] or fewer`. `maxHours` is 24 for daily/non-shisha, 168 weekly, 744 monthly, and 8760 yearly.
+- `smoking-quantity-change` adds a comparison error when a `greater` answer is not greater than the original amount, or a `fewer` answer is not fewer than the original amount after normalising by frequency. The message is generated as `[quantity phrase] must be more/fewer/less than [original quantity] [original frequency]`.
+
+## Question messages
+
+### accept-terms
+
+Question: Confirm you agree to the terms of use
+
+Type: `multiple`
+
+Answer key: `acceptTerms`
+
+Page: `accept-terms`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Confirm that you have read and agree to the terms of use | `#accept-terms` |
+
+### phone-questionnaire
+
+Question: Have you previously completed a lung cancer risk questionnaire by phone in the last 12 months?
+
+Type: `single`
+
+Answer key: `phoneQuestionnaire`
+
+Page: `phone-questionnaire`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have previously completed a lung cancer risk questionnaire by phone | `#phone-questionnaire` |
+
+### smoker
+
+Question: Have you ever smoked tobacco?
+
+Type: `single`
+
+Answer key: `smoker`
+
+Page: `smoker`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever smoked tobacco | `#smoker` |
+
+### date-of-birth
+
+Question: What is your date of birth?
+
+Type: `date`
+
+Answer key: `dateOfBirth`
+
+Page: `date-of-birth`
+
+Validation: required; type: date
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter your date of birth | `#dateOfBirth-day` |
+| `invalid` | Enter a real date of birth | `#dateOfBirth-day` |
+
+### face-to-face-appointment
+
+Question: Do you need to leave the online service and ask for a face-to-face appointment?
+
+Type: `single`
+
+Answer key: `faceToFaceAppointment`
+
+Page: `face-to-face-appointment`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you need to leave the online service and ask for a face-to-face appointment | `#face-to-face-appointment` |
+
+### height-metric
+
+Question: What is your height in centimetres?
+
+Type: `text`
+
+Answer key: `height`
+
+Page: `height-metric`
+
+Validation: required; type: number; minimum 139.7; maximum 243.8
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter your height in centimetres | `#height-metric` |
+| `invalid` | Enter your height in centimetres using numbers | `#height-metric` |
+| `min` | Height in centimetres must be 139.7cm or more | `#height-metric` |
+| `max` | Height in centimetres must be 243.8cm or fewer | `#height-metric` |
+
+### height-imperial
+
+Question: What is your height in feet and inches?
+
+Type: `text_group`
+
+Answer key: `height`
+
+Page: `height-imperial`
+
+Validation: grouped input items: feet (required, number, whole number, min 0); inches (required, number, whole number, min 0, max 11); group total: min 55, max 96
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `feet.required` | Enter your height in feet | `#height-imperial-feet` |
+| `feet.invalid` | Enter your height in feet using numbers | `#height-imperial-feet` |
+| `feet.integer` | Height in feet must be a whole number | `#height-imperial-feet` |
+| `feet.min` | Height in feet must be 0ft or more | `#height-imperial-feet` |
+| `inches.required` | Enter your height in inches | `#height-imperial-inches` |
+| `inches.invalid` | Enter your height in inches using numbers | `#height-imperial-inches` |
+| `inches.integer` | Height in inches must be a whole number | `#height-imperial-inches` |
+| `inches.min` | Height in inches must be 0in or more | `#height-imperial-inches` |
+| `inches.max` | Height in inches must be 11in or fewer | `#height-imperial-inches` |
+| `total.min` | Height must be between 4 feet 7 inches and 8 feet | `#height-imperial-feet` |
+| `total.max` | Height must be between 4 feet 7 inches and 8 feet | `#height-imperial-feet` |
+
+### weight-metric
+
+Question: What is your weight in kilograms?
+
+Type: `text`
+
+Answer key: `weight`
+
+Page: `weight-metric`
+
+Validation: required; type: number; maximum 2 decimal places; minimum 25.4; maximum 317.5
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter your weight in kilograms | `#weight-metric` |
+| `invalid` | Enter your weight in kilograms using numbers | `#weight-metric` |
+| `decimalPlaces` | Weight in kilograms must have 2 decimal places or fewer | `#weight-metric` |
+| `min` | Weight in kilograms must be 25.4kg or more | `#weight-metric` |
+| `max` | Weight in kilograms must be 317.5kg or fewer | `#weight-metric` |
+
+### weight-imperial
+
+Question: What is your weight in stones and pounds?
+
+Type: `text_group`
+
+Answer key: `weight`
+
+Page: `weight-imperial`
+
+Validation: grouped input items: stones (required, number, whole number, min 0); pounds (required, number, whole number, min 0, max 13); group total: min 56, max 700
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `stones.required` | Enter your weight in stones | `#weight-imperial-stones` |
+| `stones.invalid` | Enter your weight in stones using numbers | `#weight-imperial-stones` |
+| `stones.integer` | Weight in stones must be a whole number | `#weight-imperial-stones` |
+| `stones.min` | Weight in stones must be 0st or more | `#weight-imperial-stones` |
+| `pounds.required` | Enter your weight in pounds | `#weight-imperial-pounds` |
+| `pounds.invalid` | Enter your weight in pounds using numbers | `#weight-imperial-pounds` |
+| `pounds.integer` | Weight in pounds must be a whole number | `#weight-imperial-pounds` |
+| `pounds.min` | Weight in pounds must be 0lb or more | `#weight-imperial-pounds` |
+| `pounds.max` | Weight in pounds must be 13lb or fewer | `#weight-imperial-pounds` |
+| `total.min` | Weight must be between 4 stone and 50 stone | `#weight-imperial-stones` |
+| `total.max` | Weight must be between 4 stone and 50 stone | `#weight-imperial-stones` |
+
+### gender
+
+Question: Which of these best describes your gender identity?
+
+Type: `single`
+
+Answer key: `gender`
+
+Page: `gender`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select which option best describes your gender identity | `#gender` |
+
+### sex
+
+Question: What was your sex at birth?
+
+Type: `single`
+
+Answer key: `sex`
+
+Page: `sex`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select your sex at birth | `#sex` |
+
+### ethnicity
+
+Question: What is your ethnic background?
+
+Type: `single`
+
+Answer key: `ethnicity`
+
+Page: `ethnicity`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select your ethnic background | `#ethnicity` |
+
+### education
+
+Question: What level of education have you completed?
+
+Type: `multiple`
+
+Answer key: `education`
+
+Page: `education`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select what level of education you have completed | `#education` |
+
+### respiratory-conditions
+
+Question: Have you ever been diagnosed with any of the following respiratory conditions?
+
+Type: `multiple`
+
+Answer key: `respiratoryConditions`
+
+Page: `respiratory-conditions`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select if you have ever been diagnosed with any respiratory conditions | `#respiratory-conditions` |
+
+### asbestos-at-work
+
+Question: Have you ever worked in a job where you might have been exposed to asbestos?
+
+Type: `single`
+
+Answer key: `asbestosAtWork`
+
+Page: `asbestos`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever worked in a job where you might have been exposed to asbestos | `#asbestos-at-work` |
+
+### asbestos-at-home
+
+Question: Have you ever lived with anyone who worked with asbestos?
+
+Type: `single`
+
+Answer key: `asbestosAtHome`
+
+Page: `asbestos`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever lived with anyone who worked with asbestos | `#asbestos-at-home` |
+
+### cancer-diagnosis
+
+Question: Have you ever been diagnosed with cancer?
+
+Type: `single`
+
+Answer key: `cancerDiagnosis`
+
+Page: `cancer-diagnosis`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever been diagnosed with cancer | `#cancer-diagnosis` |
+
+### cancer-diagnosis-relatives
+
+Question: Have any of your parents, siblings or children ever been diagnosed with lung cancer?
+
+Type: `single`
+
+Answer key: `cancerDiagnosisRelatives`
+
+Page: `cancer-diagnosis-relatives`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether any of your parents, siblings or children have ever been diagnosed with lung cancer | `#cancer-diagnosis-relatives` |
+
+### cancer-diagnosis-relatives-age
+
+Question: Were any of your relatives younger than 60 years old when they were diagnosed with lung cancer?
+
+Type: `single`
+
+Answer key: `cancerDiagnosisRelativesAge`
+
+Page: `cancer-diagnosis-relatives-age`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether any of your relatives were younger than 60 when they were diagnosed with lung cancer | `#cancer-diagnosis-relatives-age` |
+
+### age-started-smoking
+
+Question: How old were you when you started smoking?
+
+Type: `text`
+
+Answer key: `ageStartedSmoking`
+
+Page: `smoking-duration`
+
+Validation: required; type: number; minimum 1; maximum 120
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter the age you started smoking | `#age-started-smoking` |
+| `invalid` | Enter the age you started smoking using numbers | `#age-started-smoking` |
+| `min` | Age you started smoking must be 1 or older | `#age-started-smoking` |
+| `max` | Age you started smoking must be 120 or younger | `#age-started-smoking` |
+
+### age-stopped-smoking
+
+Question: How old were you when you quit smoking?
+
+Type: `text`
+
+Answer key: `ageStoppedSmoking`
+
+Page: `smoking-duration`
+
+Validation: required; type: number; minimum 1; maximum 120
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter the age you quit smoking | `#age-stopped-smoking` |
+| `invalid` | Enter the age you quit smoking using numbers | `#age-stopped-smoking` |
+| `min` | Age you quit smoking must be 1 or older | `#age-stopped-smoking` |
+| `max` | Age you quit smoking must be 120 or younger | `#age-stopped-smoking` |
+
+### periods-stopped-smoking
+
+Question: Have you ever stopped smoking for periods of 1 year or longer?
+
+Type: `single`
+
+Answer key: `periodsStoppedSmoking`
+
+Page: `smoking-duration`
+
+Validation: required; conditional: yes (required, number, min 1, max 80)
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you have ever stopped smoking for periods of 1 year or longer | `#periods-stopped-smoking` |
+| `invalid` | Total number of years you stopped smoking must be a number | `#years-stopped-smoking` |
+| `min` | Total number of years you stopped smoking must be 1 or more | `#years-stopped-smoking` |
+| `max` | Total number of years you stopped smoking must be 80 or fewer | `#years-stopped-smoking` |
+| `conditional.yes.required` | Enter the total number of years you stopped smoking | `#years-stopped-smoking` |
+
+### smoking-type
+
+Question: Have you ever smoked any of the following types of tobacco for a period of 1 year or longer?
+
+Type: `multiple`
+
+Answer key: `smokingType`
+
+Page: `smoking-type`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select the types of tobacco you have smoked | `#smoking-type` |
+
+Variants: `previous`. Variants can change type, options, hints or validation before the same validator runs.
+
+### smoking-status-current
+
+Question: Do you currently smoke any of the following types of tobacco?
+
+Type: `multiple`
+
+Answer key: `smokingStatusCurrent`
+
+Page: `smoking-status-current`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select the types of tobacco you currently smoke | `#smoking-status-current` |
+
+### smoking-status
+
+Question: Do you currently smoke?
+
+Type: `single`
+
+Answer key: `smokingStatus`
+
+Page: `smoking-status`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether you currently smoke this type of tobacco | `#smoking-status` |
+
+### smoking-frequency
+
+Question: How often do you smoke?
+
+Type: `single`
+
+Answer key: `smokingFrequency`
+
+Page: `tobacco-smoking`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select how often you smoke this type of tobacco | `#smoking-frequency` |
+
+### smoking-quantity
+
+Question: How much do you smoke?
+
+Type: `text`
+
+Answer key: `smokingQuantity`
+
+Page: `tobacco-smoking`
+
+Validation: required; type: number; minimum 1; maximum 200
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter how much you smoke | `#smoking-quantity` |
+| `invalid` | Enter how much you smoke using numbers | `#smoking-quantity` |
+| `min` | Amount smoked must be 1 or more | `#smoking-quantity` |
+| `max` | Amount smoked must be 200 or fewer | `#smoking-quantity` |
+
+Variants: `rolling_tobacco`, `shisha`. Variants can change type, options, hints or validation before the same validator runs.
+
+### smoking-change
+
+Question: Has the amount you normally smoke changed over time?
+
+Type: `multiple`
+
+Answer key: `smokingChange`
+
+Page: `smoking-change`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select whether the amount you normally smoke has changed over time | `#smoking-change` |
+
+### smoking-frequency-change
+
+Question: How often did you smoke?
+
+Type: `single`
+
+Answer key: `smokingFrequencyChange`
+
+Page: `tobacco-smoking-change`
+
+Validation: required
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Select how often you smoked this type of tobacco | `#smoking-frequency-change` |
+
+### smoking-quantity-change
+
+Question: How much did you smoke?
+
+Type: `text`
+
+Answer key: `smokingQuantityChange`
+
+Page: `tobacco-smoking-change`
+
+Validation: required; type: number; minimum 1; maximum 200
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter how much you smoked | `#smoking-quantity-change` |
+| `invalid` | Enter how much you smoked using numbers | `#smoking-quantity-change` |
+| `min` | Amount smoked must be 1 or more | `#smoking-quantity-change` |
+| `max` | Amount smoked must be 200 or fewer | `#smoking-quantity-change` |
+
+Variants: `rolling_tobacco`. Variants can change type, options, hints or validation before the same validator runs.
+
+### smoking-years-change
+
+Question: How many years did you smoke this amount?
+
+Type: `text`
+
+Answer key: `smokingYearsChange`
+
+Page: `tobacco-smoking-change`
+
+Validation: required; type: number; minimum 1; maximum 80
+
+| Rule | Message | Link |
+| --- | --- | --- |
+| `required` | Enter how many years you smoked this amount | `#smoking-years-change` |
+| `invalid` | Enter how many years using numbers | `#smoking-years-change` |
+| `min` | Number of years must be 1 or more | `#smoking-years-change` |
+| `max` | Number of years must be 80 or fewer | `#smoking-years-change` |
+
diff --git a/app/prototype_v4_3/docs/question-schema.md b/app/prototype_v4_3/docs/question-schema.md
index f4aaf43e..5f972112 100644
--- a/app/prototype_v4_3/docs/question-schema.md
+++ b/app/prototype_v4_3/docs/question-schema.md
@@ -393,7 +393,10 @@ Supported validation fields are:
 | `type: date` | Requires day, month and year to form a real date. |
 | `min` | Minimum numeric value. |
 | `max` | Maximum numeric value. |
+| `integer` | Requires a numeric answer to be a whole number. |
+| `decimalPlaces` | Maximum decimal places for a numeric answer. |
 | `items` | Per-field validation for `text_group`. |
+| `total` | Combined numeric validation for `text_group` fields. |
 | `conditional` | Validation for conditional reveal inputs. |
 
 For numeric questions, provide all relevant messages:
diff --git a/app/prototype_v4_3/lib/question-validator.js b/app/prototype_v4_3/lib/question-validator.js
index db2953f0..9b970ee2 100644
--- a/app/prototype_v4_3/lib/question-validator.js
+++ b/app/prototype_v4_3/lib/question-validator.js
@@ -12,8 +12,11 @@ const { getQuestion } = require('./questions')
  * @property {string} [type] - Type-specific validation, for example number or date.
  * @property {number} [min] - Minimum numeric value.
  * @property {number} [max] - Maximum numeric value.
+ * @property {boolean} [integer] - Whether a numeric value must be a whole number.
+ * @property {number} [decimalPlaces] - Maximum number of decimal places allowed.
  * @property {Object} [conditional] - Conditional reveal validation rules.
  * @property {Object[]} [items] - Validation rules for grouped inputs.
+ * @property {Object} [total] - Total validation rules for grouped numeric inputs.
  */
 
 /**
@@ -191,6 +194,30 @@ const mergeQuestion = (question, overrides = {}) => {
   }
 }
 
+/**
+ * Check whether a submitted numeric value is written as a whole number.
+ *
+ * @param {*} value - Submitted value.
+ * @returns {boolean} True when the submitted value is an integer string.
+ */
+const isIntegerValue = (value) => {
+  return /^-?\d+$/.test(String(value).trim())
+}
+
+/**
+ * Check whether a submitted numeric value has no more than the allowed number
+ * of decimal places.
+ *
+ * @param {*} value - Submitted value.
+ * @param {number} decimalPlaces - Maximum decimal places allowed.
+ * @returns {boolean} True when the value has an allowed decimal precision.
+ */
+const hasMaximumDecimalPlaces = (value, decimalPlaces) => {
+  const match = String(value).trim().match(/^-?\d+(?:\.(\d+))?$/)
+
+  return Boolean(match) && (!match[1] || match[1].length <= decimalPlaces)
+}
+
 /**
  * Validate a numeric value against invalid, min and max rules.
  *
@@ -203,11 +230,24 @@ const mergeQuestion = (question, overrides = {}) => {
 const validateNumber = (value, validation, errorsConfig, errors, defaultHref) => {
   const number = Number(value)
 
-  if (Number.isNaN(number)) {
+  if (Number.isNaN(number) || !Number.isFinite(number)) {
     errors.push(makeError(errorsConfig.invalid, defaultHref))
     return
   }
 
+  if (validation.integer && !isIntegerValue(value)) {
+    errors.push(makeError(errorsConfig.integer || errorsConfig.invalid, defaultHref))
+    return
+  }
+
+  if (
+    validation.decimalPlaces !== undefined &&
+    !hasMaximumDecimalPlaces(value, validation.decimalPlaces)
+  ) {
+    errors.push(makeError(errorsConfig.decimalPlaces || errorsConfig.invalid, defaultHref))
+    return
+  }
+
   if (validation.min !== undefined && number < validation.min) {
     errors.push(makeError(errorsConfig.min, defaultHref))
   }
@@ -217,6 +257,61 @@ const validateNumber = (value, validation, errorsConfig, errors, defaultHref) =>
   }
 }
 
+/**
+ * Validate the total value of grouped numeric inputs.
+ *
+ * @param {Object} groupValue - Submitted group values keyed by answer key.
+ * @param {Object} question - Normalised text_group question config.
+ * @param {ValidationError[]} errors - Mutable error collection.
+ */
+const validateGroupTotal = (groupValue, question, errors) => {
+  const totalValidation = question.validation?.total
+
+  if (!totalValidation) {
+    return
+  }
+
+  const total = (totalValidation.items || []).reduce((sum, item) => {
+    const multiplier = item.multiplier || 1
+
+    return sum + (Number(groupValue[item.answerKey]) * multiplier)
+  }, 0)
+  const totalErrors = question.errors?.total || {}
+  const defaultHref = `#${question.input?.items?.[0]?.id || question.id}`
+
+  if (totalValidation.min !== undefined && total < totalValidation.min) {
+    errors.push(makeError(totalErrors.min, defaultHref))
+  }
+
+  if (totalValidation.max !== undefined && total > totalValidation.max) {
+    errors.push(makeError(totalErrors.max, defaultHref))
+  }
+}
+
+/**
+ * Order grouped input errors by the visual input order.
+ *
+ * @param {ValidationError[]} errors - Validation errors for a text_group.
+ * @param {Object} question - Normalised text_group question config.
+ * @returns {ValidationError[]} Errors ordered by grouped input position.
+ */
+const orderGroupErrors = (errors, question) => {
+  const inputOrder = (question.input?.items || []).reduce((map, item, index) => {
+    map[`#${item.id}`] = index
+    return map
+  }, {})
+
+  return errors
+    .map((error, index) => ({ error, index }))
+    .sort((a, b) => {
+      const aOrder = inputOrder[a.error.href] ?? Number.MAX_SAFE_INTEGER
+      const bOrder = inputOrder[b.error.href] ?? Number.MAX_SAFE_INTEGER
+
+      return aOrder - bOrder || a.index - b.index
+    })
+    .map(({ error }) => error)
+}
+
 /**
  * Validate grouped inputs, such as imperial height or weight fields.
  *
@@ -227,6 +322,7 @@ const validateNumber = (value, validation, errorsConfig, errors, defaultHref) =>
 const validateInputGroup = (answers, question) => {
   const errors = []
   const groupValue = answers[question.answerKey]?.[question.input.valueKey] || {}
+  let canValidateTotal = Boolean(question.validation?.total)
 
   ;(question.validation?.items || []).forEach((itemValidation) => {
     const value = groupValue[itemValidation.answerKey]
@@ -235,15 +331,30 @@ const validateInputGroup = (answers, question) => {
 
     if (itemValidation.required && isBlank(value)) {
       errors.push(makeError(itemErrors.required, defaultHref))
+      canValidateTotal = false
       return
     }
 
     if (itemValidation.type === 'number' && !isBlank(value)) {
+      const number = Number(value)
+
+      if (
+        Number.isNaN(number) ||
+        !Number.isFinite(number) ||
+        (itemValidation.integer && !isIntegerValue(value))
+      ) {
+        canValidateTotal = false
+      }
+
       validateNumber(value, itemValidation, itemErrors, errors, defaultHref)
     }
   })
 
-  return errors
+  if (canValidateTotal) {
+    validateGroupTotal(groupValue, question, errors)
+  }
+
+  return orderGroupErrors(errors, question)
 }
 
 /**
diff --git a/app/prototype_v4_3/lib/tobacco-flow.js b/app/prototype_v4_3/lib/tobacco-flow.js
index 9c317640..71da00f7 100644
--- a/app/prototype_v4_3/lib/tobacco-flow.js
+++ b/app/prototype_v4_3/lib/tobacco-flow.js
@@ -312,6 +312,119 @@ const getSmokingComparisonQuantity = (type, answer) => {
   return getSmokingQuantity(type, answer)
 }
 
+const rollingTobaccoQuantityValues = {
+  less_than_10: 10,
+  '10_to_30': 20,
+  '31_to_50': 40.5,
+  '51_to_75': 63,
+  '76_to_100': 88,
+  more_than_100: 100
+}
+
+const smokingFrequencyRateMultipliers = {
+  daily: 7,
+  weekly: 1,
+  monthly: 0.25,
+  yearly: 1 / 52
+}
+
+/**
+ * Check whether a value can be compared as a submitted numeric answer.
+ *
+ * @param {*} value - Submitted value.
+ * @returns {boolean} True when value is numeric.
+ */
+const isComparableNumber = (value) => {
+  if (value === undefined || value === null || String(value).trim() === '') {
+    return false
+  }
+
+  const number = Number(value)
+
+  return Number.isFinite(number)
+}
+
+/**
+ * Get a normalised quantity value for cross-frequency comparisons.
+ *
+ * @param {string} type - Tobacco type key.
+ * @param {*} quantity - Submitted quantity answer.
+ * @param {string} frequency - Submitted frequency answer.
+ * @returns {number|undefined} Normalised quantity, when comparable.
+ */
+const getSmokingQuantityComparisonRate = (type, quantity, frequency) => {
+  const multiplier = smokingFrequencyRateMultipliers[frequency]
+
+  if (!multiplier) {
+    return undefined
+  }
+
+  if (type === 'rolling_tobacco') {
+    const value = rollingTobaccoQuantityValues[quantity]
+
+    return value ? value * multiplier : undefined
+  }
+
+  return isComparableNumber(quantity) ? Number(quantity) * multiplier : undefined
+}
+
+/**
+ * Build the left side of a changed-quantity comparison error.
+ *
+ * @param {string} type - Tobacco type key.
+ * @returns {string} Error phrase.
+ */
+const getSmokingQuantityChangeErrorPhrase = (type) => {
+  const heading = removeQuestionMark(getSmokingTypeHeadings(type, true).quantityHeading || '')
+    .replace(/ in a normal (day|week|month|year)$/, '')
+
+  return upperFirst(lowerFirst(heading)
+    .replace(/^(how (?:much|many) .+?) did you normally smoke$/, '$1 you normally smoked')
+    .replace(/^(how (?:much|many) .+?) did you smoke$/, '$1 you normally smoked'))
+}
+
+/**
+ * Check whether a changed-smoking quantity contradicts the selected change direction.
+ *
+ * @param {Object} params - Comparison inputs.
+ * @returns {Object|undefined} Validation error when the quantities contradict.
+ */
+const getSmokingQuantityChangeComparisonError = ({
+  page,
+  step,
+  answer,
+  changeAnswer,
+  href
+}) => {
+  if (page !== 'smoking-quantity-change' || !step.change || !answer.smokingQuantity || !answer.smokingFrequency || !changeAnswer.quantity || !changeAnswer.frequency) {
+    return undefined
+  }
+
+  const currentRate = getSmokingQuantityComparisonRate(step.type, answer.smokingQuantity, answer.smokingFrequency)
+  const changedRate = getSmokingQuantityComparisonRate(step.type, changeAnswer.quantity, changeAnswer.frequency)
+
+  if (currentRate === undefined || changedRate === undefined) {
+    return undefined
+  }
+
+  const contradictsChange = step.change === 'greater'
+    ? changedRate <= currentRate
+    : changedRate >= currentRate
+
+  if (!contradictsChange) {
+    return undefined
+  }
+
+  const comparisonText = step.type === 'rolling_tobacco' && step.change === 'fewer'
+    ? 'less'
+    : smokingChangeTypes[step.change]?.label
+
+  return {
+    text: `${getSmokingQuantityChangeErrorPhrase(step.type)} must be ${comparisonText} than ${[getSmokingComparisonQuantity(step.type, answer.smokingQuantity), getSmokingFrequencyComparisonPeriod(answer.smokingFrequency)].filter(Boolean).join(' ')}`,
+    href
+  }
+}
+
 const smokingFrequencyPeriods = {
   daily: 'a day',
   weekly: 'a week',
@@ -319,6 +432,20 @@ const smokingFrequencyPeriods = {
   yearly: 'a year'
 }
 
+const smokingFrequencyComparisonPeriods = {
+  daily: 'per day',
+  weekly: 'per week',
+  monthly: 'per month',
+  yearly: 'per year'
+}
+
+const smokingFrequencyMaxHours = {
+  daily: 24,
+  weekly: 168,
+  monthly: 744,
+  yearly: 8760
+}
+
 /**
  * Convert a smoking frequency value into a period phrase.
  *
@@ -329,6 +456,31 @@ const getSmokingFrequencyPeriod = (frequency) => {
   return smokingFrequencyPeriods[frequency] || ''
 }
 
+/**
+ * Convert a smoking frequency value into a comparison phrase.
+ *
+ * @param {string} frequency - Frequency value.
+ * @returns {string} Comparison phrase, for example `per day`.
+ */
+const getSmokingFrequencyComparisonPeriod = (frequency) => {
+  return smokingFrequencyComparisonPeriods[frequency] || ''
+}
+
+/**
+ * Get the maximum number of hours allowed for an "another amount" shisha answer.
+ *
+ * @param {string} type - Tobacco type key.
+ * @param {string} frequency - Selected smoking frequency.
+ * @returns {number} Maximum number of hours.
+ */
+const getSmokingQuantityOtherMaxHours = (type, frequency) => {
+  if (type !== 'shisha') {
+    return 24
+  }
+
+  return smokingFrequencyMaxHours[frequency] || 24
+}
+
 /**
  * Replace the default "normal day/week/month/year" phrase in a heading.
  *
@@ -585,13 +737,7 @@ const getSmokingChangeHeading = (page, type, change, changeAnswer = {}, answer =
   }
 
   if (page === 'smoking-years-change') {
-    const quantity = getSmokingQuantity(type, changeAnswer.quantity)
-
-    if (!quantity) {
-      return getQuestion('smoking-years-change').input.label
-    }
-
-    return `How many years did you smoke ${[quantity, getSmokingFrequencyPeriod(answer.smokingFrequency)].filter(Boolean).join(' ')}?`
+    return getQuestion('smoking-years-change').input.label
   }
 
   return ''
@@ -719,6 +865,16 @@ const getQuestionItemsWithLabels = (id, labels = {}, hintOverrides = {}) => {
   })
 }
 
+/**
+ * Check whether a smoking quantity must be a whole number.
+ *
+ * @param {string} type - Tobacco type key.
+ * @returns {boolean} True when decimal quantities should be rejected.
+ */
+const requiresWholeNumberSmokingQuantity = (type) => {
+  return !['rolling_tobacco', 'shisha'].includes(type)
+}
+
 /**
  * Build runtime overrides for a quantity question.
  *
@@ -733,6 +889,7 @@ const getSmokingQuantityQuestionOverrides = ({
   name,
   value,
   conditionalValue,
+  frequency,
   smokingType
 }) => {
   const question = getQuestion(page)
@@ -742,6 +899,15 @@ const getSmokingQuantityQuestionOverrides = ({
   const hint = hasHintOverride ? variantInput.hint : question.input.hint
   const questionType = variant.type || question.type
   const conditionalHref = '#smoking-quantity-other'
+  const maxHours = getSmokingQuantityOtherMaxHours(step.type, frequency)
+  const validation = {
+    ...variant.validation
+  }
+
+  if (requiresWholeNumberSmokingQuantity(step.type)) {
+    validation.integer = true
+  }
+
   const items = variant.options
     ? variant.options.map((option) => {
       const item = toComponentItem(option)
@@ -777,13 +943,13 @@ const getSmokingQuantityQuestionOverrides = ({
       suffix: questionType === 'text' ? smokingType.suffix : undefined
     },
     validation: {
-      ...variant.validation,
+      ...validation,
       conditional: {
         another_amount: {
           required: true,
           type: 'number',
           min: 0.5,
-          max: 24,
+          max: maxHours,
           answerKey: 'smokingQuantityOther',
           value: conditionalValue,
           href: conditionalHref
@@ -803,12 +969,16 @@ const getSmokingQuantityQuestionOverrides = ({
         text: 'Number of hours must be a number',
         href: conditionalHref
       },
+      integer: {
+        text: `${upperFirst(getAnswerPhraseFromHeading(heading))} must be a whole number`,
+        href: `#${question.input.id}`
+      },
       min: {
         text: 'Number of hours must be 0.5 or more',
         href: conditionalHref
       },
       max: {
-        text: 'Number of hours must be 24 or fewer',
+        text: `Number of hours must be ${maxHours} or fewer`,
         href: conditionalHref
       }
     },
@@ -841,9 +1011,10 @@ const getAnswerPhraseFromHeading = (heading = '') => {
     .replace(/^how long did you smoke /, 'how long you smoked ')
     .replace(/^how long do you currently smoke /, 'how long you currently smoke ')
     .replace(/^how long do you smoke /, 'how long you smoke ')
-    .replace(/^(how (?:much|many|long) .+?) did you smoke /, '$1 you smoked ')
-    .replace(/^(how (?:much|many|long) .+?) do you currently smoke /, '$1 you currently smoke ')
-    .replace(/^(how (?:much|many|long) .+?) do you smoke /, '$1 you smoke ')
+    .replace(/^(how (?:much|many|long) .+?) did you normally smoke(?= |$)/, '$1 you normally smoked')
+    .replace(/^(how (?:much|many|long) .+?) did you smoke(?= |$)/, '$1 you smoked')
+    .replace(/^(how (?:much|many|long) .+?) do you currently smoke(?= |$)/, '$1 you currently smoke')
+    .replace(/^(how (?:much|many|long) .+?) do you smoke(?= |$)/, '$1 you smoke')
 }
 
 /**
@@ -1048,6 +1219,7 @@ const getSmokingContentQuestionOverrides = ({
       name: `answers[${step.type}][smokingQuantity]`,
       value: answer.smokingQuantity,
       conditionalValue: answer.smokingQuantityOther,
+      frequency: answer.smokingFrequency,
       smokingType
     })
   }
@@ -1089,6 +1261,7 @@ const getSmokingContentQuestionOverrides = ({
       name: `answers[${step.type}][${smokingChange.answerKey}][quantity]`,
       value: changeAnswer.quantity,
       conditionalValue: changeAnswer.smokingQuantityOther,
+      frequency: answer.smokingFrequency,
       smokingType
     })
   }
@@ -1323,10 +1496,23 @@ const validateSmokingTypeQuestion = (req, page, step) => {
     }
   }
 
-  return validateQuestion(req.session.data.answers, page, {
+  const validationErrors = validateQuestion(req.session.data.answers, page, {
     ...overrides,
     errors
   })
+  const comparisonError = getSmokingQuantityChangeComparisonError({
+    page,
+    step,
+    answer: context.answer,
+    changeAnswer: context.changeAnswer,
+    href: `#${errorHref}`
+  })
+
+  if (comparisonError && !validationErrors.some((error) => error.href === comparisonError.href)) {
+    validationErrors.push(comparisonError)
+  }
+
+  return validationErrors
 }
 
 /**