Dormer window

A dormer is a roofed structure, often containing a window, that projects vertically beyond the plane of a pitched roof.

Dormers are commonly used to increase the usable space in a loft and to create window openings in a roof plane. The term "dormer" is commonly used to refer to a "dormer window" although a dormer doesn't necessarily have to contain a window. 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.

Some of the different types of dormer are:

Gable fronted dormer

Hip roof dormer

Flat roof dormer

Shed dormer

Wall dormer

Eyebrow dormer

Link dormer

Lucarne on a church spire

The word "dormer" is derived from the Middle French dormeor, meaning "sleeping room", as dormer windows often provided light and space to attic-level bedrooms.

One of the earliest uses of dormers was in the form of lucarnes, slender dormers which provided ventilation to the spires of gothic churches and cathedrals. An early example is the spire of Christ Church Cathedral, Oxford.

Dormer windows have been used in domestic architecture in Britain since the 16th century. Dormer windows were popularised by architect Francois Mansart, who used dormers extensively in the Mansard roofs he designed for 17th-century Paris.

Today dormers are a widespread feature of pitched roof buildings.

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 of 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.

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Wikipedia