maisonette (a corruption of 'maisonnette', from the french word for "little house") has no strict definition, but the oed suggests "a part of a residential building which is occupied separately, usually on more than one floor and having its own outside entrance." it differs from a flat in having, usually, more than one floor, with a staircase internal to the dwelling leading from the entrance floor to the upper (or, in some cases, lower) other floor. this is a very common arrangement in much post-war british public housing, serving both to reduce costs by reducing the amount of space given to access corridors and to emulate the 'traditional' two-storey terrace house to which many of the residents would have been accustomed.