Dormers are commonly used to increase the usable space in a loft and to create window openings in a roof plane. A dormer is often one of the primary elements of a loft conversion. As a prominent element of many buildings, different types of dormer have evolved to complement different styles of architecture. When the structure appears on the spires of churches and cathedrals, it is usually referred to as a lucarne.
One of the earliest uses of dormers was in the form of lucarnes, slender dormers which provided ventilation to the spires of English Gothic churches and cathedrals. An early example are the lucarnes of the spire of Christ Church Cathedral, Oxford. Dormer windows have been used in domestic architecture in Britain since the 16th century.
Today dormers are a widespread feature of pitched roof buildings.
Some of the different types of dormer are:
Gable fronted dormer (shallow instance wholly glazed)
Hip roof dormer
Flat roof dormer
Lucarne on a church spire
In some localities, permission must be sought for construction of dormers and other features. In England and Wales, the General Permitted Development Order states classes of development for which such planning permission is not required. Such rights are only applicable outside conservation areas, national parks, Areas of Outstanding Natural Beauty or The Broads. Dormers may introduce imbalance in the street scene and be seen as inappropriate within the local setting of streets and buildings.
In Vancouver, there are regulations for laneway houses stating the minimum setback of the face of the dormer from the wall below, with exceptions. This is to prevent overshadowing neighbouring yards. 
dormer shed flat gable.
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