Dormer Loft Conversions

A rear box dormer turns much of the loft floor into full-height space. It replaces the sloping rear roof with a vertical wall, so the usable area typically extends from the ridge line to the new back wall. How much you gain depends on the width of the house, the height of the ridge and the size of dormer allowed. Under permitted development, a dormer's volume counts towards the 40 cubic metre (terraced) or 50 cubic metre (detached and semi-detached) allowance for roof additions.

In practice, a rear dormer on a typical London terrace or semi can often fit a double bedroom with an en-suite, or a bedroom and a separate study. At the survey we draw a simple layout, so you can see what fits before you commit.

Often not, for a rear dormer on a house in England. It is usually permitted development if:

• Volume: The roof additions stay within 40 cubic metres (terraced) or 50 cubic metres (detached and semi-detached), including any earlier roof extensions.
• Position: The dormer is not on the roof slope facing the road, and it is set back from the eaves, normally by at least 20 centimetres where practical.
• Height: It is no higher than the highest part of the existing roof.
• Materials: The outside looks similar to the existing house.
• Side windows: Any side-facing windows are obscure-glazed and non-opening below 1.7 metres.
• No balconies or raised platforms.

A dormer on the front roof slope, or on a flat, maisonette or listed building, needs permission. So does a dormer in many conservation areas and where an Article 4 direction applies. We check your property and can apply for a Lawful Development Certificate for written confirmation.

• Cladding: Tile-hanging to match the existing roof is the most common finish. Brick, render, slate or timber cladding are other options, subject to planning.
• Roof: A durable flat roof, such as GRP (fibreglass) or a single-ply membrane, laid to falls with proper drainage.
• Windows and doors: The new rear windows, often with a Juliet-style glazed door where planning allows, set out to suit the room layout.
• Roof windows: Roof windows can also go in the front slope, to bring light into the rest of the loft.

A dormer conversion needs building regulations approval. The main structural work is new steel beams to carry the dormer and the remaining roof, and new floor joists, all designed by a structural engineer. Fire safety needs a protected escape route down the stairs, fire-resisting doors and interlinked smoke alarms. The walls, roof and floor must be insulated to current standards, and the room needs ventilation and a permanent staircase with enough headroom. We arrange the building control inspections and hand you the completion certificate.

On terraced and semi-detached houses, the steel beams for a dormer usually bear into the party wall, and the dormer's side wall is often built up alongside it. Both need a notice under the Party Wall etc. Act 1996 before work starts. We tell you at the survey which notices are needed and allow time for them in the programme.