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, Western Canada, and Saudi Arabia.

A townhouse, townhome, town house, or town home, is a type of terraced housing. A modern townhouse is often one with a small footprint on multiple floors. In a different British usage, the term originally referred to any type of city residence of someone whose main or largest residence was a country house.