Buying a conversion flat with a loft

[craigjm]

A friend of mine is buying a top floor flat in a converted victorian house in London. The flat they are buying is the only one at the top of the house and they have access to the loft from their hall. Obviously nobody else has access to it so they can store their crap in there etc but what would the position be generally if they wanted to convert the loft? My thought is that the sale means that they own the loft as it is in the plans but I think the roof would surely be a communal asset and as such they wouldnt be able to fit dormers etc. The situation appears unclear and I am hoping their conveyancing solicitor can sort it out but has anyone here been in a similar position and know the score?