Yes you pay VAT on extras.
The view taken by HMRC is that because manufacturers ‘optional extras’ are for general use and not specifically designed for disabled people, they will attract VAT at the prevailing rate.
Adaptions specifically designed for disabled people are VAT free subject to the HMRC rules as laid out in:
Obviously, Motability have a block VAT exemption on the actual vehicle, but VAT is payable on the optional extras as they are designed for general use.