Its definitely not fair, in fact its insulting, the ground floor flat will be wheelchair accessible, but the one thats up fifteen odd granite steps won't be. In the same way that it won't have access to the garden.
Spoke to engineer, he feels that there is no issue with regards to planning and no point in applying for retention as long as the bank and insurance people are happy. He did point out that there "may" be a moral duty (in a legal way) to install fire alarms, extinguishers and escapes, but as pointed out above that would be part of the plan anyway.
He did also mention soundproofing which on a timber floor is not very financially viable to get totally right, but the double slabbing of the ceiling plasterboard of the ground floor flat would help both this and the fireproofing.