Duplex House

A duplex house plan has two living units attached to each other, either next to each other via townhouses or above each other like apartments. By contrast, a building comprising two attached units on two distinct properties is typically considered semi-detached or twin homes but is also called a duplex in parts of the Northeastern United States.

The term "duplex" is not 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.