Flats and storage

My client has built some luxury flats in Manchester and these are now for sale. The apartments are being sold on a 999-year lease and are therefore zero rated because they are new build. In some cases the owners have paid an extra £20 000 for an allocated parking space in the basement. This is also zero rated if it is bought at the same time as the apartment. However I have two queries.

First some parking spaces are unsold so flat owners who moved in at the beginning of the project have asked whether they can buy an extra space eg if they are a two-car family. Would these sales still be zero rated or is there a different VAT treatment because the owners have already bought their flats and moved in?

Second in the basement there are also some allocated areas for...