Dormer Types for London Loft Conversions: Choosing the Right Option

A rear dormer is the most common dormer addition in London. It is a box-like structure projecting from the rear slope of the roof, typically extending to near the full width of the rear elevation and to near the ridge height.

• **Planning status:**
• A rear dormer on most London terraced and semi-detached houses can be built under Permitted Development (PD) if it meets the specific PD criteria:
• •Must not exceed the highest part of the original roof (the ridge)
• •Must be set back from the eaves by at least 200mm
• •Must not project within 200mm of the existing ridge
• •Rear-facing materials must not be of a kind prohibited by PD rules (cladding is restricted in some situations)
• •Not permitted in a designated area (Conservation Area, AONB, National Park) — most of these rules are relaxed for rear dormers outside conservation areas; in conservation areas rear dormers typically require full planning permission
• •The volume added must not exceed 40m³ for terraced houses or 50m³ for detached and semi-detached (this limit is rarely reached for a single rear dormer)

**What it gives:** A full-width rear dormer typically creates a near-full floor area at standing height across the rear half to two-thirds of the loft space. Combined with Velux rooflights to the front slope, a rear dormer can give a large open-plan bedroom, or a bedroom plus bathroom.

**Cost:** A standard rear dormer loft conversion (1 bedroom + 1 bathroom): £45,000–£70,000.

An L-shaped dormer combines a rear dormer with a smaller side dormer over the back addition (the outrigger or back extension) of a Victorian terraced house. The 'L' shape covers both the main rear slope and the outrigger rear slope, providing significantly more floor area.

**Planning status:** An L-shaped dormer typically falls under Permitted Development in the same way as a standard rear dormer, subject to the same criteria. The fact that it covers both the main roof and the outrigger is generally within PD limits for a single dwelling — but should be confirmed against the specific criteria for the relevant borough and house type.

• **What it gives:**
• The L-shape typically provides 20–35m² of usable floor area at standing height, typically allowing:
• •A large bedroom over the main house
• •A second bedroom or bathroom over the outrigger
• •A separate bathroom or en-suite

The L-shaped dormer is the conversion of choice for a typical 3-bed Victorian or Edwardian terraced house with an outrigger — it is the format that most efficiently converts the available roof space.

**Cost:** An L-shaped dormer loft conversion (2 rooms): £55,000–£85,000.

**Hip-to-gable conversion:** Semi-detached and some detached houses have hipped roofs — the roof slopes on all four sides including the side elevation. A hip-to-gable conversion extends the roof at the side by replacing the hip with a new vertical gable wall, creating additional volume within the roof at the side elevation. This is often combined with a rear dormer to maximize the usable loft area.

Planning status: hip-to-gable extensions are generally Permitted Development for semi-detached houses (subject to the usual PD criteria), but this must be confirmed as the specific rules for each house type apply. For detached houses, the volume and positioning criteria are somewhat different.

What it gives: combined with a rear dormer, a hip-to-gable conversion can create 40–60m² of usable floor area — comparable to a full extension. It is often used to create a master bedroom with ensuite or a large family bathroom.

Cost: hip-to-gable plus rear dormer: £55,000–£90,000.

**Mansard conversion:** A mansard roof replaces the existing sloping roof with a near-vertical flat-topped structure, creating maximum internal volume. The near-vertical sides of a mansard (typically at 72° or steeper) dramatically increase the usable floor area compared with a standard slope. Mansard conversions are most common in central London where Victorian terraced houses are densely packed and rear dormers alone are insufficient to create genuinely usable space.

Planning status: mansard conversions almost always require full planning permission — they represent a more significant change to the roofscape than a standard dormer and rarely fall within PD rules. In many conservation areas in central London, mansard conversions are an established precedent and are relatively straightforward to obtain permission for, provided they follow the accepted design typology (zinc or lead-covered mansard face, Crittall-style roof windows). Outside conservation areas, or where the design departs from precedent, planning may be more contentious.

What it gives: a mansard typically creates 30–50m² of usable floor area (depending on the footprint and the roof structure), with full standing height across most of the space. It is the premium option for maximising loft space in central London.

Cost: mansard conversion (1–2 bedrooms): £80,000–£130,000+.