Thanks for your question - I ma Sam adn I am one of the UK tax experts here on Just Answer.
The extension itself will NOT be VAT free only work within it such as
ramps, doorways and passages bathrooms, washrooms and lavatories installation or repair and maintenance of lifts preparation or restoration work
As this relate solely for the use of the individual (in this case your daughter) to use.
SO have a chat with your builder as this should have been addresses when you contracted him to undertake the work so the appropriate paperwork was completed and if any of the above will be included within this build and he should VAT free the materials and work for these specific aspects of the work.
I have added a link here to the HMRC website sheet that advises what is and what is not VAT free when relating to disabled alterations. https://www.gov.uk/government/publications/vat-notice-7017-vat-reliefs-for-disabled-people/vat-notice-7017-vat-reliefs-for-disabled-and-older-people
Let me know if I can assist further