Dormer loft conversions in London: rear, L-shaped and full-width

Published London guides for 2026 put a rear dormer at £55,000–£80,000, a dormer with an en-suite at £60,000–£92,000 and an L-shaped dormer at £70,000–£105,000. Those are market ranges compiled from several companies' cost guides, not our quotes, and some include VAT while others do not. Planning or certificate fees, building control, party wall surveyors and sanitaryware come on top. Our loft conversion cost guide lists the sources and the fees.

Per square metre, usually yes, because the walls, roof structure and floor already exist and nothing has to be dug. Published London figures put a single-storey extension at £2,650–£4,500 per square metre to build before the kitchen and glazing, with side-return totals starting around £40,000, so a rear dormer at £55,000–£80,000 typically buys more floor for the money. The better question is which room the house needs: a dormer adds a bedroom, an extension adds living space.

Usually not on a house. A rear dormer within 40 cubic metres on a terrace or 50 on a semi or detached house is permitted development, as long as it sits back from the eaves, stays below the ridge and matches the roof. A dormer on the principal elevation facing the road, a dormer on a flat, and any dormer in a conservation area or on an Article 4 street needs a planning application (Planning Portal, loft conversions, checked September 2026). Confirm with your borough; we apply for a lawful development certificate either way.

Yes, on every dormer, whether or not planning permission is needed. The new floor, the steels, the dormer frame, insulation, the stair, ventilation and the fire strategy for the whole house are all controlled, and building control inspects the work as it goes rather than at the end. The completion certificate is what proves the room is a bedroom rather than a boarded loft when you come to sell or remortgage.

A flat roof laid without enough fall, so water ponds and finds the join at the cheeks. Cladding that was never detailed at the corners. Insulation fitted tight to the deck with no ventilation, which shows up as condensation the first winter. A stair with 1.8 metres of headroom because the dormer was sized to the room rather than the flight. And the non-technical one: notices served late, so the neighbour's two-month period pushes the start into the weather.

The carpentry is within reach of a good joiner, but a dormer is not a carpentry job. It needs a structural engineer's design for the floor and the frame, party wall notices under an Act with its own procedure, a building control application with inspections at each stage, and a fire strategy for the floors below. Done without those, the room cannot be called a bedroom and a buyer's solicitor will find out.

Since 25 April 2024, building work done without planning permission in England becomes immune from enforcement only after ten years; work substantially completed before that date keeps the old four-year limit (gov.uk planning enforcement guidance, checked September 2026), so check the position with your council. For a dormer it is a backstop, not a plan: a buyer's solicitor asks for the certificate and the building control completion, indemnity insurance does not fix a missing certificate, and our loft conversion cost guide covers the rule in full.

Not a dormer. The published London ranges start at £55,000 for a rear dormer, and the lowest advertised starting figures describe a bare room without the fire-door work on the floors below. If £20,000 is the budget, a rooflight conversion in a roof that already has the height is the only type that comes close, and waiting until the budget matches the job beats building half of one; our loft conversion cost guide sets out what each type costs and why.

A terrace normally takes a flat-roofed rear dormer across most of the width, because the back slope is the only face that can grow within permitted development, and an L-shaped dormer where there is a back addition to build over. A semi or end-of-terrace with a hipped roof often gets more from a hip-to-gable first, with the dormer built out to the new gable. Pitched or gable-fronted dormers are the answer where the planning officer wants the roof to keep its shape.

A Velux conversion in a roof that already has the height, because it adds no dormer structure or cladding and rarely needs a planning application, so the published London range starts around £30,000 rather than £55,000. On a terrace with a low ridge the slope takes most of the floor, and a rear dormer is usually the better value per usable square metre; the loft conversion cost guide compares the two type by type.