Introduction
A dormer loft conversion is one of the most popular ways to add a bedroom and bathroom to a London home. By extending the existing roof to create a vertical-sided addition, a dormer conversion dramatically increases the usable floor area and ceiling height in the loft space. But not all dormer conversions are the same. The type, size, and positioning of the dormer affects the planning status, the structural approach, the amount of floor space gained, and the cost.
What Is a Dormer?
A dormer is a structural addition to a sloped roof, projecting upward to create a flat or pitched roof section with vertical walls. Unlike a Velux (roof light) conversion — which adds windows into the existing slope without changing the roof's profile — a dormer changes the shape of the roof itself. The effect is to replace part of the sloped ceiling with vertical walls and a near-horizontal ceiling, creating rooms that feel like conventional habitable spaces rather than rooms under a slope.
Types of Dormer Loft Conversion
Rear Dormer — the most common type in London. A rear dormer extends across all or part of the rear slope of the roof, creating a flat-roofed rectangular addition. It typically runs from just above the eaves to just below the ridge. A full-width rear dormer maximises the floor area gained and commonly produces a generous double bedroom with an en-suite bathroom.
Hip-to-Gable Conversion — available on semi-detached or end-of-terrace properties with a hipped roof. A hip-to-gable conversion replaces the sloped end hip with a vertical gable wall, dramatically increasing the floor area and often combined with a rear dormer to create an L-shaped arrangement.
L-Shaped Dormer — combines a full rear dormer with a hip-to-gable or side return dormer, typically available on semi-detached or end-of-terrace properties with a rear outrigger. The result is a very large loft floor area, often enough for two bedrooms and a bathroom.
Mansard Conversion — involves restructuring the entire roof to create steeply sloped sides (typically 72 degrees) with a nearly flat top. A full-width dormer runs across the rear elevation. This creates the maximum possible floor area but is more structurally complex and expensive. Planning permission is required in almost all cases.
Front Dormer — less common because most planning authorities are cautious about changes to the front elevation. Front dormers are more likely to require a planning application and may not be permitted in conservation areas.
Does a Dormer Require Planning Permission?
In most cases, rear dormers on houses can proceed under permitted development rights — without a full planning application — provided they stay within the size limits set by the Town and Country Planning (General Permitted Development) Order. Key conditions include: the dormer must not exceed the highest part of the existing roof, must not overhang the outer face of the wall, must be set back from the eaves by at least 20cm, materials should be similar in appearance to the existing house, and side-facing dormer windows must be obscure-glazed.
Permitted development rights do not apply to flats, in conservation areas, in Article 4 direction zones, or where PD rights have been removed. A Certificate of Lawful Development is recommended even for permitted development schemes — it's formal written confirmation from the local authority that your conversion is lawful.
Building Regulations for Dormer Conversions
All dormer loft conversions require Building Regulations approval, regardless of planning status. Building Control will assess structural integrity of the new dormer and floor, fire escape routes and means of escape, thermal insulation and energy performance, ventilation and natural light, and staircase design and headroom. A structural engineer is required to produce calculations for the structural floor and dormer framing. Building Control will inspect key stages of the work.
What Does a Dormer Conversion Cost?
Costs vary significantly by type, specification, and size. We don't publish fixed prices because the variables — property size, roof type, specification level, structural complexity, drainage connections — are too significant to generalise accurately. What we provide is a detailed, itemised estimate based on the specific project after reviewing the property and proposed design. Contact RCB Design & Build to discuss the options for your property.