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 may also be referred to as a duplex.
The term "duplex" can also be extended to three-unit and four-unit buildings, or they can 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.
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 urban planning, the term duplex is used more specifically. Major Canadian cities sometimes use the term duplex to refer specifically to a building with one unit built above another. Edmonton defines Duplex Housing as "development consisting of a building containing only two Dwellings, with one Dwelling placed over the other in whole or in part with individual and separate access to each Dwelling". Calgary defines Duplex Dwelling as "a building which contains two Dwelling Units, one located above the other, with each having a separate entrance". Toronto proposes in their new Zoning Bylaw to define Duplex Building as a building that has only two dwelling units, and one dwelling unit is entirely or partially on top of the other dwelling unit. Halifax defines Duplex Dwelling as "the whole of a dwelling that is divided horizontally into two separate dwelling units, each of which has an independent entrance".