A semi-detached house is a single family duplex dwelling house that shares one common wall with the next house. The name distinguishes this style of house from detached houses, with no shared walls, and terraced houses, with a shared wall on both sides. Often, semi-detached houses are built in pairs in which each house's layout is a mirror image of the other's.

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.