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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.
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.
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 home is generally held as a Torrens Titled property, whilst a townhouse is a Strata Titled unit. 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 narrow lots early in the city's history, and in the late 19th century they were built in areas such as The Annex and Cabbagetown in assorted styles: Gothic Revival, Queen Anne, Second Empire, bay and gable.   The building type was arguably most popular during the first few decades of the 20th century.
Semi-detached homes continued to be built in the post-war period, often alongside detached types such as the bungalow. They remain popular with developers today because they are cheaper to build than detached houses. According to the 2006 census Toronto had more than 139,000 semis, more than any other Canadian city by a wide margin.  Red-brick semis are a common sight throughout downtown and older suburbs, helping define the character of the city's neighbourhoods.
In rare cases semi-detached homes have linked basements and are called linked semi-detached. These are distinct from linked homes, which appear detached but are connected 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.
- "My Pink Half of The Drainpipe", written and performed by Bonzo Dog Doo-Dah Band, satirizes neighbourly relations and ownership of property, referring to the painting of only one half of the drainpipe that runs down the exact centre of the dividing line between properties
- 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" (PDF). 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
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