A duplex house plan has two living units attached to each other, either next to each other as townhouses, condominiums 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 three-family (or triplex) and fourplex (or quadplex/quadruplex) or a more general multiplex. 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 single-family house.
