govuk-frontend/dist/govuk/components/checkboxes/template-small-with-conditional-reveal.html

GOV.UK Frontend contains the code you need to start building a user interface for government platforms and services.

<div class="govuk-form-group">
  <fieldset class="govuk-fieldset">
  <legend class="govuk-fieldset__legend">
    How do you want to be contacted?
  </legend>
  <div class="govuk-checkboxes govuk-checkboxes--small" data-module="govuk-checkboxes">
    <div class="govuk-checkboxes__item">
      <input class="govuk-checkboxes__input" id="how-contacted" name="how-contacted" type="checkbox" value="a" data-aria-controls="conditional-how-contacted">
      <label class="govuk-label govuk-checkboxes__label" for="how-contacted">
        a thing
      </label>
    </div>
    <div class="govuk-checkboxes__conditional govuk-checkboxes__conditional--hidden" id="conditional-how-contacted">
      <label class="govuk-label" for="context-email">Foo</label>
<input class="govuk-input govuk-!-width-one-third" name="context-email" type="text" id="context-email">
    </div>
    <div class="govuk-checkboxes__item">
      <input class="govuk-checkboxes__input" id="how-contacted-2" name="how-contacted" type="checkbox" value="b">
      <label class="govuk-label govuk-checkboxes__label" for="how-contacted-2">
        another thing
      </label>
    </div>
  </div>
</fieldset>
</div>