Semi-detached

Semi-detached housing (often abbreviated to semi in the UK, Canada and Australia, as in "three-bedroom semi", and occasionally referred to as se-tenant houses) consists of pairs of houses built side by side as units 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. Confusingly, this style is sometimes colloquially called a duplex in New England, in certain other parts of the United States and in most of Canada (elsewhere 'duplex' usually 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.

This type of housing can be thought of as being a half-way state between terraced or row housing and single-family detached homes. Terraced housing is constituted by continuous row houses with open spaces at the front and back, while semi-detached houses have front, rear and any one side open spaces, and individual detached houses have open spaces on all sides.