Duplex (building)

In North America, a duplex house is a dwelling having apartments with separate entrances for two households. This includes two-story houses having a complete apartment on each floor and also side-by-side apartments on a single lot that share a common wall. By contrast, a building comprising two attached units on two distinct properties is typically considered semi-detached or twin homes but sometimes also duplex.

The term "duplex" can't be extended to three-unit and four-unit buildings, as they would be referred to with specific terms such as triplex and fourplex or quadplex/quadruplex. Because of the flexibility of the term, the line between an apartment building and a duplex is somewhat blurred, with apartment buildings tending to be bigger, while duplexes are usually the size of a normal house.

British English usage refers simply to the number of levels within an apartment unit with duplex indicating two storeys (traditionally this was referred to as a maisonette) and triplex for three. A semi-detached house would never be referred to as 'duplex'.

In dense areas like Manhattan and downtown Chicago, a duplex or duplex apartment refers to a maisonette, a single dwelling unit spread over two floors connected by an indoor staircase. Similarly, a triplex apartment refers to an apartment spread out over three floors. These properties can be quite expensive and include the most expensive property in Manhattan as of 2006 (according to Forbes Magazine), a triplex atop The Pierre hotel. In this context, an apartment locating on one floor only may be called a simplex.

In the United Kingdom, the term duplex is more often used by property professionals such as architects and estate agents and refers only to a flat or apartment on two floors connected by an inner staircase though many newer apartments have open-plan designs including mezzanines. The traditional layout is normally called a maisonette, indicating a 'house-like dwelling' raised above the ground or street, but the term "duplex" is increasingly used by estate agents because of the perceived glamour of the term.

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