L-shaped dormer · Victorian terraces with outriggers
The loft conversion for Victorian terraces with a rear outrigger. More floor area than a single dormer, with space for two bedrooms or a bedroom and bathroom.
An L-shaped dormer loft conversion in London costs about £60,000–£80,000 in 2026 and takes 10–14 weeks on site. It can be Permitted Development, but the 40 m³ terrace limit is easy to exceed, so a volume check or planning application is often needed.
Last updated 3 October 2026
An L-shaped dormer combines a rear dormer on the main roof with a second dormer over the rear outrigger, the narrower back wing found on many Victorian and Edwardian terraces. Seen from above the two dormers form an L. The result is a much larger floor area than a single dormer, with space for two bedrooms or a bedroom, bathroom and landing.
If your budget is tighter, a rear dormer is the lower-cost route. If you want to maximise space on a central London terrace and planning allows, compare a mansard.
A terraced house has a 40 m³ Permitted Development limit for roof extensions. A large L-shaped dormer can reach that limit quickly, particularly if there is an earlier roof extension. Before any design is finalised we calculate the volume, then choose between three routes:
See our guides to Permitted Development and Article 4 Directions.
| Specification | Typical build cost | What you get |
|---|---|---|
| Standard L-shaped dormer | £60,000–£80,000 | Two rooms or bedroom plus bathroom, new stair, Building Regs sign-off |
| High-specification L-shaped dormer | £80,000+ | Zinc or slate cladding, larger bathroom, bespoke joinery and glazing |
Figures exclude VAT, professional fees, planning fees and furnishings. Check your own figure with the Loft Conversion Cost Calculator.
Measured survey, volume calculation and a decision on the planning route.
Structural design, Building Regulations, Certificate of Lawfulness or planning application, and party wall notices.
Scaffold, steelwork across both roofs, dormer shells, roofing, windows, first fix, stair and second fix.
Snagging walkthrough, Building Control completion certificate and handover.
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In 2026 an L-shaped dormer in London typically costs £60,000–£80,000 at standard specification, and more for large or high-specification projects. It costs more than a single rear dormer because there is more roof to build, more steel and a larger bathroom and landing area.
Not always, but it is the loft type most likely to need a planning check. The Permitted Development volume limit is 40 m³ on terraced houses, and an L-shaped dormer can use all of it or more. If the combined volume exceeds the limit, or the house is in a conservation area or Article 4 area, you need a planning application.
Victorian and Edwardian terraces with a rear outrigger (the lower back wing). The dormer runs across the main roof and along the outrigger roof, giving room for two bedrooms or a bedroom, bathroom and landing area.
About 10–14 weeks on site, after 3–4 months of design, Building Regulations, planning route and party wall notices.
Yes, in almost every case. Steel beams bear on the party wall, and the outrigger often shares a wall with the neighbour's outrigger, so notices are normally needed for both adjoining owners at least two months before structural work starts.
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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