A dormer window is a structural extension projecting from a sloping roof, creating additional headroom and floor space within a loft. Dormers are one of the most popular ways to create usable loft space, and many can be built without planning permission under the permitted development rights that apply to loft conversions.

Permitted Development for Rear Dormers

Under Class B of Part 1 of the General Permitted Development Order, certain loft conversions including dormers are permitted development. The permitted development rules allow a rear dormer provided the additional roof volume does not exceed 40 cubic metres for a terraced house or 50 cubic metres for a detached or semi-detached house. The dormer must not be on the front roof slope or on a roof slope that faces a highway. It must not exceed the highest part of the existing roof. The materials used on the external face of the dormer must be similar in appearance to those of the existing house. A hip-to-gable extension is permitted under the same rules within the volume limits.

Side dormers are permitted development under these rules provided they face away from a highway and are within the volume limit. Front dormers, which face toward the street, are not permitted development and always require planning permission.

When Planning Permission Is Required

Planning permission is required for any front-facing dormer, for dormers that exceed the permitted development volume limits, for any dormer on a flat or maisonette, and for dormers at listed buildings or in conservation areas. In conservation areas, the permitted development rights for roof extensions are removed, meaning all dormers in these areas require a planning application regardless of their size or position.

Prior Approval for Some Loft Conversions

Some larger loft conversions using the larger homes extension prior approval route may require prior approval from the local planning authority, though this is primarily relevant to extensions rather than dormers specifically. The standard loft conversion permitted development rights under Class B do not require prior approval, only compliance with the conditions described above.

Building Regulations

All dormer loft conversions, regardless of planning status, require Building Regulations approval. This covers the structural design of the dormer and the floor below, the thermal performance of the new roof and walls, fire safety measures including fire-resisting doors and an escape window, and the staircase to the new space. Building Regulations compliance is a separate requirement from planning and must be obtained before or during construction, with a completion certificate issued at the end of the project.

Summary

Rear and side dormers within the permitted development volume limits and away from highways are generally permitted development and do not require planning permission. Front dormers, dormers exceeding volume limits, dormers in conservation areas, and dormers at listed buildings require planning permission. Building Regulations approval is always required. A Lawful Development Certificate provides formal confirmation of permitted development status and is a useful document when selling the property.