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. the term duplex is largely unknown in the uk; there, buildings containing two dwellings with a common vertical wall are known as "semi-detached" or just "a semi" (the i pronounced "ee" not "eye"), but the dwellings are usually two storeys (excluding any use of the roof space). this form of construction is very common, and built as such rather than a later conversion.