Salon apartment is a term linked to the exclusive apartments built as part of multi-family houses in Belgrade and in certain towns in Yugoslavia in the first decades of the 20th century.
A maisonette could encompass Tyneside flats, pairs of single-storey flats within a two-storey terrace. Their distinctive feature is their use of two separate front doors onto the street, each door leading to a single flat. “Maisonette” could also stretch to cottage flats, also known as ‘four-in-a-block flats’, a style of housing common in Scotland.
The vast majority of apartments are on one level, hence “flat”. Some, however, have two storeys, joined internally by stairs, just as many houses do. One term for this is “maisonette”, as above. Some housing in the United Kingdom, both public and private, was designed as scissor section flats. On a grander level, penthouses may have more than one storey, to emphasise the idea of space and luxury. Two storey units in new construction are sometimes referred to as “townhouses” in some countries (though not usually in Britain).
Duplex description can be different depending on the part of the US, but generally has two to four dwellings with a door for each and usually two front doors close together but separate—referred to as ‘duplex’, indicating the number of units, not the number of floors, as in some areas of the country they are often only one story. Groups of more than two units have corresponding names (Triplex, etc.). Those buildings that have a third storey are known as triplexes. See Three-decker (house)
In the United States, regional forms have developed, see vernacular architecture. In Milwaukee, a Polish flat or “raised cottage” is an existing small house that has been lifted up to accommodate the creation of a basement floor housing a separate apartment, then set down again, thus becoming a modest pair of dwellings. In the Sun Belt, boxy small apartment buildings called dingbats, often with carports below, sprang up from the 1950s.