See this definition from an estate agent -
ianmaskell
Having been an estate agent for over 20 years I can assure you that the definition of a maisonette is a dwelling within a larger structure that has a private access. A flat is a dwelling within a larger block that has a communal entrance. Flat is perfectly interchangeable with Flat though not with Maisonette. Studio (whether flat or apartment) simply means that it is one room (plus bathroom) sometimes confused with bedsits which usually have one room and a communal kitchen/bathroom.
As for number of storeys, that is irrelevant as maisonettes and flats/apartments can have as many or as few as they want. It's a tad tricky having more than one floor in a studio though!
That is from a previous Q on AB -
http://www.theanswerb...n/Question271654.html
Note that 'having private access' is the definition.