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. 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.
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 consists of 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.
During the 19th century, a father and son architectural partnership, John Shaw, Sr. and John Shaw, Jr., 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 houses, highly characteristic of the era, was heavily influenced by the Art Deco movement, taking influence from Tudor Revival, chalet style, 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. The semi is now the most common dwelling type in England, yet because it is typically suburban and ordinary, little research into its origins and development has been carried out, and semis are under-represented in heritage listings. 
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 known as a community title in South Australia. Semi-detached houses come only 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.
The semi-detached house was seen as a good fit for downtown Toronto's long, narrow lots early in the city's history, and their popularity was well established by the late 19th century. Victorian examples can be seen in areas such as The Annex, but the style was arguably most popular during the first few decades of the 20th century. They continued to be built well into the 1950s, often alongside detached types such as the bungalow. Red-brick semis are a common sight throughout downtown neighbourhoods and older suburbs; in fact, they are so typical of Toronto they could be seen as its answer to the New York City brownstone.
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. This should not be confused with linked homes which appear detached, but there is a linkage below ground.
In the UK semi-detached houses are the most common property type, accounting for 32% of UK housing transactions and 32% of the English housing stock as of 2008. But there has been a decline. Between 1945 and 1964, 41% of all properties built were semis, but after 1980 they fell to 15%.
- "Semi-detached suburban Mr. James", written and performed by Manfred Mann, a song about a lost love marrying a man living in a small suburban house, released in 1966 (Fontana TF 757), reached No. 2 in the UK charts.
- Lofthouse, Pamela (2012). "The Development of English Semi-detached Dwellings During the Nineteenth Century". Papers from the Institute of Archaeology (Ubiquity Press) 22: 83–98. doi:10.5334/pia.404. Retrieved 17 March 2013.
- Anon. "Special Feature 2: Semi-Detached Properties". Nationwide: House prices. Nationwide. Retrieved 12 July 2010.
- The Guardian Wednesday 20 January 2010, Patrick Collinson, "50 years on: homes are more expensive but loos are indoors" London p.17