Semi-detached housing (often abbreviated to semi in the UK, Canada and Australia, as in "three-bedroom semi", sharing a party wall and usually in such a way that each house's layout is a mirror image of its twin. This style of housing, although built throughout the world, is commonly seen as particularly symbolic of the suburbanisation of the United Kingdom and Ireland, or post-war homes in Central Canada. In New England, certain other parts of the United States, and most of Canada, this style is sometimes colloquially called a duplex; elsewhere, however, "duplex" refers to a building split into two flats/apartments (one above the other). The style is usually referred to in the mid-Atlantic (particularly Philadelphia) as a twin.

Semi-detached housing consists of pairs of houses built side by side.