There seems to be a general notion that accessible housing, i.e. all on one level with no steps for access, must be in individual units such as bungalows or blocks of flats solely for the elderly.  There could be a case for making a 2 floor dwelling two units with an accessible unit on the ground floor and a similar but non accessible, possibly a starter unit above.  This would allow people needing accessible accommodation to integrate better.  With careful design (for soundproofing etc) this should be acceptable.