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Built circa 1870 two semi-detached cottages at Mentmore masquerade as one Mock Tudor style house.

Semi-detached housing (often abbreviated to semi in the UK, Canada, and Australia, as in "three-bedroom semi, " or outright renamed duplex in Boston, 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.

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

During the 19th century a father and son architectural partnership, the Shaws, drew up some of the very first designs for semi-detached housing in London. Examples of their work can be seen in Chalk Farm, North London. In the British housing boom of the 1920s and 1930s semi-detached houses sprang up in suburbs throughout the country, and were popular with middle class home owners who preferred them to terrace houses. The design of many of these house, highly characteristic of the era, was heavily influenced by the Art Deco movement, taking influence from mock Tudor, chalet styles and even ship design.

In the immediate post-war years many council houses also followed the 'semi' format, giving many Britons a first experience of private garden space.

In Australia, a semi-detached house is a different form of real property title from a townhouse. A semi-detached house sits on a single property, owned in its entirety by the owner of the semi-detached house; a townhouse has a strata title or more recently know as a community title in South Australia. (See further at townhouse). Semi-detached houses only come in pairs, whereas townhouses may number more than two, attached together. In Sydney, semi-detached houses, still referred to as 'semis' were briefly popular at the beginning of the 20th century and many examples may be found in inner suburbs such as Drummoyne. However this style quickly gave way to the 'modern' style of detached housing which allowed better motor vehicle access amongst other benefits.

Edwardian 'semis' in Dubbo, New South Wales

During the house price boom in the years to 2004 many UK property developers found they could create value by demolishing semi-detached houses and building two detached houses on the same site, often with a very narrow gap between the new units.

In Canada, some semi-detached homes have linked basements, such that the houses do not have individual basements. These are called linked semi-detached homes.

Technically suburban semi-detached housing in Markham, Ontario

Semi-detached homes are very popular and representative of Toronto, and resemble Toronto's version of a New York City brownstone. They were first built after the First World War and continued to be built well into the 1950s, when suburban bungalows were beginning to be built. Many of them were built in a questionable manner, leading to a massive wave of re-modelling in cities engaged in gentrifying neighbourhoods. They are most common in the section of 'Old Toronto,' although they may be found in older sections of the city's suburbs (and other Ontario cities.)

• Duplex

• Shaw Hardwick architectural history site

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