A dormer (or dormer window) is a structural projection built out of a sloping roof, containing one or more windows and covered by its own small roof. It creates additional headroom and natural light in an otherwise cramped loft or attic space. Dormers are classified by their roof type: a shed dormer has a shallow-pitch or flat roof; a gable dormer has a pitched roof with a triangular gable end at the front; a hipped dormer has a roof sloping on three sides with no vertical gable. Box dormers are large shed dormers spanning most of the roof width and are common in loft conversions.

Do dormers need planning permission?

Rear dormers are often Permitted Development (PD) and do not need planning permission, subject to conditions. PD rules for dormers require: the dormer must not be higher than the existing roof; it must not overhang the outside wall of the house; on a principal or side elevation visible from a highway it is not PD; the roof material must match the existing house as far as practicable; and windows in a side elevation must be obscure-glazed. Front dormers (facing the street) almost always require planning permission. Properties in conservation areas, listed buildings, or subject to Article 4 Directions face additional restrictions.

What is a loft conversion dormer?

A loft conversion dormer is a dormer added as part of converting an attic into habitable living space. The most common type for loft conversions is the box dormer (or rear dormer) - a large shed-roofed projection across most of the rear roof width, maximising floor area and headroom. A Velux conversion uses roof lights (flush with the slope) rather than a dormer and is cheaper but provides less headroom. An L-shaped dormer combines a rear and side dormer, common on Victorian terraces to maximise the conversion footprint.

Do dormers need Building Regulations approval?

Yes - dormer construction always requires Building Regulations approval. Key areas covered include: structural adequacy of the dormer framing and its connection to the existing roof structure (Part A); fire safety - a loft conversion creates a new storey requiring protected escape routes and fire detection (Part B); thermal performance - new dormer walls, roof, and floor must meet U-value targets (Part L); and ventilation of the new habitable space (Part F). A loft conversion involving a dormer is a significant structural project requiring careful design and Building Regulations sign-off before, during, and after construction.

Roofing noun

Dormer

/ ˈdɔːmə /

Also known as: dormer window, box dormer, shed dormer, gable dormer, hipped dormer

Definition

A dormer is a structural projection built vertically out of a sloping roof, containing one or more windows and covered by its own small roof. Dormers create headroom and natural light in loft spaces. They are described by roof type: shed (flat or shallow-pitch), gable (pitched with a triangular front), or hipped (sloping on three sides). A box dormer is a large shed dormer spanning most of the roof width - the standard approach for loft conversions.

In practice

Rear dormers are commonly built under Permitted Development rights without planning permission, provided they stay within PD conditions (not higher than the ridge, no front elevation, matching materials, obscure-glazed side windows). Front dormers almost always need planning permission. In conservation areas, any dormer visible from a public highway is likely to need permission and must respect the character of the area - box dormers are typically refused on front or side elevations in conservation areas in favour of smaller gable or hipped dormers.

Structurally, a dormer requires cutting through existing rafters - the openings are trimmed with headers and trimmers, and the loads redistributed to adjacent rafters or new structural members. Ceiling joists are typically upgraded to serve as floor joists for the new loft room. The dormer cheek walls (sides) are usually timber-framed and clad in tile hanging, lead, zinc, or render. The flat or shallow roof of a shed dormer requires a proper flat-roof build-up with falls to drain - typically EPDM rubber or GRP fibreglass over insulated deck.

Building Regulations

Structure (Part A)Dormer framing and roof alteration must be structurally designed

Fire (Part B)New storey requires protected staircase and fire detection

Thermal (Part L)Dormer walls, roof, and floor must meet U-value targets

Planning (PD)Rear dormers often PD - front dormers nearly always need PP

Approved Document B requires that a loft conversion creating a new habitable storey above two existing floors must have a protected escape staircase (30-minute fire-resisting enclosure from the loft floor to the final exit). This typically means fire doors to all rooms off the staircase and mains-linked smoke alarms on each landing. Part L requires the dormer envelope to meet U-values: 0.18 W/m2K for walls and 0.15 W/m2K for the roof as new elements. The existing roof void surrounding the dormer may also need upgrading if accessible.

Full Building Regulations guidance

Dormer

Just so you know...