Rear dormer · London houses
The most popular loft conversion in London. We check Permitted Development, handle party wall notices and Building Regulations, and build under a written JCT contract.
A rear dormer loft conversion in London costs about £45,000–£70,000 in 2026 and takes 6–12 weeks on site. It is usually Permitted Development if it stays within 40 m³ (terraces) or 50 m³ (semis and detached houses) and the house is not in a conservation area. Flats and listed buildings need planning permission.
Last updated 3 October 2026
A rear dormer is a box-shaped extension built out of the rear roof slope. It creates a full-height wall with windows, which turns the sloping, unusable edge of the loft into proper floor space. It is the most common loft conversion in London because it adds the most usable area for the least cost, and on most houses it can be built without a planning application.
On a typical Victorian or Edwardian terrace, a rear dormer provides a double bedroom and a bathroom or en-suite, with a new staircase rising from the floor below.
A rear dormer works well when:
If your semi or end-of-terrace has a hipped side roof, a dormer is often combined with a hip-to-gable conversion. If you have a rear outrigger, an L-shaped dormer may give you a second bedroom.
A rear dormer is usually Permitted Development under Class B, provided it meets all of these conditions:
If you are close to any limit, or want written proof for a future sale, we can apply for a Certificate of Lawfulness. In conservation areas and Article 4 areas a full planning application is required. Read our guides to Permitted Development and conservation areas.
| Specification | Typical build cost | What you get |
|---|---|---|
| Basic rear dormer (terrace) | £45,000–£60,000 | One bedroom, new stair, insulation, Building Regs sign-off |
| Rear dormer with en-suite | £55,000–£70,000 | Bedroom plus shower room, plumbing and extraction |
| High-specification dormer | £70,000+ | Zinc or slate cladding, larger area, bespoke joinery and glazing |
Figures exclude VAT, architect and engineer fees, planning fees and furnishings. Use the Loft Conversion Cost Calculator for a personalised estimate.
Measured survey, layout options and a decision on the planning route.
Structural calculations, Building Regulations, any Certificate of Lawfulness or planning application, and party wall notices.
Scaffold, steelwork, dormer shell, roof covering, windows, first fix, stair and second fix.
Snagging walkthrough, Building Control completion certificate and handover.
Most problems come from rushed design. The usual ones are a stair that does not fit without losing a bedroom, a dormer that breaches the volume limit once earlier roof extensions are counted, missing party wall notices, and insulation that overheats in summer. A proper survey and volume calculation at the start prevents all four.
Our party wall guide and fire safety guide cover two of the most common delays.
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In 2026 a basic rear dormer on a terraced house typically costs £45,000–£60,000, and a dormer with an en-suite £55,000–£70,000. Size, structure, specification and access move the figure. Figures are build-only, before VAT and professional fees.
Usually not. A rear dormer is normally Permitted Development if it stays within 40 m³ on a terraced house or 50 m³ on a semi-detached or detached house, sits back 20 cm from the eaves, stays below the ridge and uses similar materials. In a conservation area, on a flat or listed building, or where an Article 4 Direction applies, you need a planning application.
Typically 6–12 weeks on site depending on size and whether it includes a bathroom. Before that, allow 3–4 months for drawings, structural calculations, Building Regulations and party wall notices.
A dormer is a box added to the rear roof slope that creates a vertical wall and extra headroom. A mansard rebuilds the whole upper roof with near-vertical sides and a shallow top. A mansard gives more space and costs more, and almost always needs planning permission, while a rear dormer is usually Permitted Development.
Yes, but the stair position is the usual design challenge. Building Regulations Part B requires a protected escape route or escape windows, fire doors and linked smoke alarms. We design the stair and escape route at the start so the dormer works with the floor below.
Cost figures are indicative 2026 London ranges for planning purposes, build cost only and excluding VAT, professional fees and fittings. They are not a quotation. Planning and Permitted Development rules depend on your specific address, so always confirm with your local council or a Certificate of Lawfulness.
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