diff --git a/app/prototype_v4_1/controllers/question.js b/app/prototype_v4_1/controllers/question.js
index 6b3dc3e..06c3efa 100644
--- a/app/prototype_v4_1/controllers/question.js
+++ b/app/prototype_v4_1/controllers/question.js
@@ -3,6 +3,7 @@ const { renderQuestion, version, view } = require('../lib/question-renderer')
 const { getCheckYourAnswers } = require('../lib/summary')
 const {
   deleteUnselectedShishaSettingAnswers,
+  deleteUnselectedSmokingQuantityOtherAnswer,
   deleteUnselectedSmokingChangeAnswers,
   deleteUnselectedSmokingTypeAnswers,
   getFormerSmokerFallbackStep,
@@ -773,6 +774,7 @@ exports.smokingQuantity_get = (req, res) => {
 
 exports.smokingQuantity_post = (req, res) => {
   const { step, steps } = getSmokingTypeStep(req, 'smoking-quantity')
+  deleteUnselectedSmokingQuantityOtherAnswer(req.session.data.answers, step)
   const errors = step ? validateSmokingTypeQuestion(req, 'smoking-quantity', step) : []
 
   if (!step) {
@@ -859,6 +861,7 @@ exports.smokingQuantityChange_get = (req, res) => {
 
 exports.smokingQuantityChange_post = (req, res) => {
   const { step, steps } = getSmokingTypeStep(req, 'smoking-quantity-change')
+  deleteUnselectedSmokingQuantityOtherAnswer(req.session.data.answers, step, 'quantity')
   const errors = step ? validateSmokingTypeQuestion(req, 'smoking-quantity-change', step) : []
 
   if (!step) {
diff --git a/app/prototype_v4_1/data/questions.yaml b/app/prototype_v4_1/data/questions.yaml
index 97e33ee..423d47e 100644
--- a/app/prototype_v4_1/data/questions.yaml
+++ b/app/prototype_v4_1/data/questions.yaml
@@ -960,6 +960,18 @@ questions:
             value: 1_to_2_hours
           - label: More than 2 hours
             value: more_than_2_hours
+          - divider: or
+          - label: Another amount
+            value: another_amount
+            conditionalInput:
+              id: smoking-quantity-other
+              name: answers[smokingQuantityOther]
+              answerKey: smokingQuantityOther
+              label: Enter the number of hours
+              hint: Give an estimate if you are not sure
+              suffix: hours
+              inputmode: numeric
+              classes: nhsuk-input--width-4
         validation:
           required: true
     validation:
diff --git a/app/prototype_v4_1/lib/question-validator.js b/app/prototype_v4_1/lib/question-validator.js
index cf60198..35d5ecf 100644
--- a/app/prototype_v4_1/lib/question-validator.js
+++ b/app/prototype_v4_1/lib/question-validator.js
@@ -55,6 +55,21 @@ const getAnswerValue = (answers = {}, question) => {
   return value
 }
 
+/**
+ * Resolve a conditional reveal value from a runtime override or answer key.
+ *
+ * @param {Object} answers - Session answers object.
+ * @param {Object} rule - Conditional validation rule.
+ * @returns {*} Submitted conditional value.
+ */
+const getConditionalValue = (answers = {}, rule = {}) => {
+  if (rule.value !== undefined) {
+    return rule.value
+  }
+
+  return answers[rule.answerKey]
+}
+
 /**
  * Resolve a date input value from the answers object.
  *
@@ -128,7 +143,7 @@ const validateConditional = (answers, question, errors) => {
       return
     }
 
-    const conditionalValue = answers[rule.answerKey]
+    const conditionalValue = getConditionalValue(answers, rule)
 
     if (isBlank(conditionalValue)) {
       const error = question.errors?.conditional?.[triggerValue]?.required || {
diff --git a/app/prototype_v4_1/lib/summary.js b/app/prototype_v4_1/lib/summary.js
index b6702dd..3004166 100644
--- a/app/prototype_v4_1/lib/summary.js
+++ b/app/prototype_v4_1/lib/summary.js
@@ -92,6 +92,24 @@ const formatWeight = (weight = {}) => {
   return ''
 }
 
+/**
+ * Format a smoking quantity, including conditional reveal "another amount" answers.
+ *
+ * @param {string} type - Tobacco type key.
+ * @param {Object} answer - Answer object containing quantity fields.
+ * @param {string} quantityKey - Quantity answer key.
+ * @returns {string} Formatted quantity answer.
+ */
+const formatSmokingQuantityAnswer = (type, answer = {}, quantityKey = 'smokingQuantity') => {
+  if (answer[quantityKey] === 'another_amount') {
+    return answer.smokingQuantityOther
+      ? formatQuantity(answer.smokingQuantityOther, 'hour', 'hours')
+      : ''
+  }
+
+  return getSmokingQuantity(type, answer[quantityKey])
+}
+
 /**
  * Format one or more stored values using display labels.
  *
@@ -226,7 +244,7 @@ const getCheckYourAnswers = (answers = {}) => {
         }),
         makeSummaryRow({
           key: getSmokingStepHeading('smoking-quantity', type, setting, isPast, settingAnswer),
-          value: getSmokingQuantity(type, settingAnswer.smokingQuantity),
+          value: formatSmokingQuantityAnswer(type, settingAnswer),
           href: getSmokingTypeStepUrl({ page: 'smoking-quantity', type, setting })
         })
       ]
@@ -270,7 +288,7 @@ const getCheckYourAnswers = (answers = {}) => {
       }),
       type !== 'shisha' && makeSummaryRow({
         key: getSmokingStepHeading('smoking-quantity', type, undefined, isPast, answer),
-        value: getSmokingQuantity(type, answer.smokingQuantity),
+        value: formatSmokingQuantityAnswer(type, answer),
         href: getSmokingTypeStepUrl({ page: 'smoking-quantity', type })
       }),
       type !== 'shisha' && makeSummaryRow({
diff --git a/app/prototype_v4_1/lib/tobacco-flow.js b/app/prototype_v4_1/lib/tobacco-flow.js
index 790fb80..490b9b3 100644
--- a/app/prototype_v4_1/lib/tobacco-flow.js
+++ b/app/prototype_v4_1/lib/tobacco-flow.js
@@ -480,6 +480,48 @@ const getSmokingChangeAnswer = (answer = {}, change) => {
   return answerKey ? answer[answerKey] || {} : {}
 }
 
+/**
+ * Get the object that stores a smoking quantity answer for the current step.
+ *
+ * @param {Object} answers - Session answers object.
+ * @param {SmokingTypeStep} step - Current tobacco step.
+ * @returns {Object} Answer object containing the quantity answer.
+ */
+const getSmokingQuantityAnswer = (answers = {}, step = {}) => {
+  const answer = answers[step.type] || {}
+
+  if (step.change) {
+    return getSmokingChangeAnswer(answer, step.change)
+  }
+
+  if (step.setting) {
+    return getShishaSettingAnswer(answer, step.setting)
+  }
+
+  return answer
+}
+
+/**
+ * Remove a stale conditional reveal quantity when "another amount" is not selected.
+ *
+ * @param {Object} answers - Session answers object, mutated in place.
+ * @param {SmokingTypeStep} step - Current tobacco step.
+ * @param {string} quantityKey - Quantity answer key.
+ */
+const deleteUnselectedSmokingQuantityOtherAnswer = (answers = {}, step, quantityKey = 'smokingQuantity') => {
+  delete answers.smokingQuantityOther
+
+  if (!step) {
+    return
+  }
+
+  const answer = getSmokingQuantityAnswer(answers, step)
+
+  if (answer[quantityKey] !== 'another_amount') {
+    delete answer.smokingQuantityOther
+  }
+}
+
 /**
  * Build contextual comparison text for changed-smoking questions.
  *
@@ -637,6 +679,7 @@ const getSmokingQuantityQuestionOverrides = ({
   caption,
   name,
   value,
+  conditionalValue,
   smokingType
 }) => {
   const question = getQuestion(page)
@@ -645,6 +688,29 @@ const getSmokingQuantityQuestionOverrides = ({
   const hasHintOverride = Object.prototype.hasOwnProperty.call(variantInput, 'hint')
   const hint = hasHintOverride ? variantInput.hint : question.input.hint
   const questionType = variant.type || question.type
+  const conditionalHref = '#smoking-quantity-other'
+  const items = variant.options
+    ? variant.options.map((option) => {
+      const item = toComponentItem(option)
+
+      if (!item.conditionalInput) {
+        return item
+      }
+
+      const conditionalName = step.setting
+        ? `answers[${step.type}][${step.setting}][${item.conditionalInput.answerKey}]`
+        : `answers[${step.type}][${item.conditionalInput.answerKey}]`
+
+      return {
+        ...item,
+        conditionalInput: {
+          ...item.conditionalInput,
+          name: conditionalName,
+          value: conditionalValue
+        }
+      }
+    })
+    : question.items
 
   return {
     type: questionType,
@@ -659,8 +725,43 @@ const getSmokingQuantityQuestionOverrides = ({
       hintParam: hint ? { text: hint } : undefined,
       suffix: questionType === 'text' ? smokingType.suffix : undefined
     },
-    validation: variant.validation,
-    items: variant.options ? variant.options.map(toComponentItem) : question.items,
+    validation: {
+      ...variant.validation,
+      conditional: {
+        another_amount: {
+          required: true,
+          type: 'number',
+          min: 0.5,
+          max: 24,
+          answerKey: 'smokingQuantityOther',
+          value: conditionalValue,
+          href: conditionalHref
+        }
+      }
+    },
+    errors: {
+      conditional: {
+        another_amount: {
+          required: {
+            text: 'Enter the number of hours',
+            href: conditionalHref
+          }
+        }
+      },
+      invalid: {
+        text: 'Number of hours must be a number',
+        href: conditionalHref
+      },
+      min: {
+        text: 'Number of hours must be 0.5 or more',
+        href: conditionalHref
+      },
+      max: {
+        text: 'Number of hours must be 24 or fewer',
+        href: conditionalHref
+      }
+    },
+    items,
     value
   }
 }
@@ -831,6 +932,7 @@ const getSmokingContentQuestionOverrides = ({
         ? `answers[${step.type}][${step.setting}][smokingQuantity]`
         : `answers[${step.type}][smokingQuantity]`,
       value: isSettingSpecific ? settingAnswer.smokingQuantity : answer.smokingQuantity,
+      conditionalValue: isSettingSpecific ? settingAnswer.smokingQuantityOther : answer.smokingQuantityOther,
       smokingType
     })
   }
@@ -871,6 +973,7 @@ const getSmokingContentQuestionOverrides = ({
       caption: smokingType.caption,
       name: `answers[${step.type}][${smokingChange.answerKey}][quantity]`,
       value: changeAnswer.quantity,
+      conditionalValue: changeAnswer.smokingQuantityOther,
       smokingType
     })
   }
@@ -1012,6 +1115,7 @@ const validateSmokingTypeQuestion = (req, page, step) => {
 
 module.exports = {
   deleteUnselectedShishaSettingAnswers,
+  deleteUnselectedSmokingQuantityOtherAnswer,
   deleteUnselectedSmokingChangeAnswers,
   deleteUnselectedSmokingTypeAnswers,
   formatQuantity,
diff --git a/app/prototype_v4_1/views/questions/_question.html b/app/prototype_v4_1/views/questions/_question.html
index c4364e8..7ce7bbb 100644
--- a/app/prototype_v4_1/views/questions/_question.html
+++ b/app/prototype_v4_1/views/questions/_question.html
@@ -68,7 +68,7 @@
                 suffix: item.conditionalInput.suffix,
                 prefix: item.conditionalInput.prefix,
                 inputmode: item.conditionalInput.inputmode,
-                value: data.answers[item.conditionalInput.answerKey],
+                value: item.conditionalInput.value if item.conditionalInput.value is defined else data.answers[item.conditionalInput.answerKey],
                 classes: item.conditionalInput.classes,
                 errorMessage: {
                   text: errorMap[item.conditionalInput.id].text