Dormer vs Mansard vs L-Shaped Loft Conversion — Which Fits Your London Property? (2026)

Five main types of loft conversion are common on London properties: Velux, dormer, L-shaped dormer, mansard, and hip-to-gable. They cost differently, add different amounts of space, and aren't all available on every property. Here's how to pick the right one for your home.

The five types — quick reference

1. Velux loft conversion

What it is

The simplest conversion — no roof structure change. New floor, insulation, plasterboard, staircase, and Velux roof windows. The roof line stays exactly as it was.

Best for

• Detached and semi-detached properties with high existing roof apex (3m+)
• Properties in conservation areas where any visible roof change is restricted
• Budget-conscious projects
• Hobby room, study, or guest bedroom rather than master suite

Limitations

• Usable floor area is limited — only the section with adequate headroom (1.5m+) is usable; spaces under 1.5m headroom are storage or low cupboards
• May not qualify as a 'bedroom' on EPCs or valuations if standing-height area is too small
• Doesn't add a dormer's open feel — sloped ceilings throughout

Planning

Almost always permitted development. The only requirement: rooflights cannot project more than 150mm beyond the roof slope. Allow 6-8 weeks for design and a Lawful Development Certificate (recommended).

2. Standard rear dormer

What it is

A box dormer (vertical-walled extension) built into the rear roof slope. Creates a section of full standing height across the back of the loft. The front roof slope stays unchanged.

Best for

• Victorian and Edwardian terraces with hipped or pitched roofs
• Most semi-detached and end-of-terrace properties
• Adding a bedroom + en-suite at master-suite scale
• Properties not in heavily-restricted conservation areas

Limitations

• Rear elevation appearance changes — some neighbours and planning officers don't like it
• Doesn't make use of the back-addition roof (if your property has an L-shape)
• Conservation areas often restrict dormer size or materials

Planning

Permitted development covers dormers up to 40m³ (terraces) or 50m³ (semis/detached) — most standard rear dormers fit within these limits. Conservation areas usually remove this PD right; full planning required. Article 4 directions in some boroughs also remove PD for dormers.

3. L-shaped dormer (best value for Victorian terraces)

What it is

Two dormers — one on the main rear roof and one over the back addition (the kitchen extension that most Victorian terraces have). They join in the corner to form an L-shape from above. Maximum floor area for the spend.

Best for

• Victorian and Edwardian terraces with the classic L-shaped footprint (kitchen extension at rear)
• Properties wanting 2 bedrooms or 1 bedroom + bathroom + dressing room in the loft
• Anyone wanting maximum floor area without going to mansard cost

Limitations

• Only applicable to properties with a back addition — won't work on a simple rectangular footprint
• The corner detail is structurally tricky — get a builder experienced with L-shapes
• Some conservation areas restrict — check before committing

Planning

Usually within permitted development on terraces (combined volume of the L often comes in under the 40m³ limit). Worth getting a Lawful Development Certificate to confirm. Conservation areas typically need full planning.

4. Mansard loft conversion

What it is

Full reconstruction of the roof. The existing pitched roof is removed and replaced with a nearly-vertical front wall (typically 70-72° from horizontal) and a shallow-pitch roof. Creates almost full standing height across the entire loft footprint.

Best for

• Maximum possible loft floor area
• Properties where the existing roof apex is too low for other conversion types
• London Victorian terraces in non-conservation areas (Brixton, parts of Tooting, outer SW London)
• Premium projects where budget allows the higher cost

Limitations

• Most expensive type
• Significantly changes the front elevation appearance
• Rarely permitted in conservation areas
• Build time longer (12-16 weeks) than dormer types

Planning

Almost always needs full planning permission, not permitted development. Mansards change the roof line and front elevation, which is what planning policy controls most strictly. Conservation areas: usually refused. Article 4 areas: typically refused. Non-conservation areas with established mansard precedent on the street: usually approved.

5. Hip-to-gable conversion

What it is

For properties with a hipped roof (the side roof slopes down to a corner rather than going up to a gable). The hipped side is removed and replaced with a vertical gable wall. Adds usable floor area at the side of the loft. Often combined with a rear dormer for maximum space.

Best for

• Semi-detached and end-of-terrace properties with hipped roofs (very common in 1930s suburban London)
• Properties where you want extra floor area but can't justify the cost of mansard
• Combining with a rear dormer for transformative space

Limitations

• Only applies to properties with hipped roofs (most Victorian terraces have pitched roofs, not hipped)
• Side gable changes neighbour-facing elevation — may need to consider their light/overlooking
• Rarely permitted on the hipped side of a semi-detached pair facing a highway

Planning

Often within permitted development on semi-detached properties as long as you don't extend above the existing ridge height. Conservation areas usually restrict. Hip-to-gable on terraces is rare because most terraces don't have hipped roofs.

Decision framework

Use this rough guide to narrow down:

1. Do you have a back addition (L-shape footprint)? If yes — L-shaped dormer is usually best value.
2. Is your existing apex 3m+ with no back addition? Velux conversion is the cheapest route.
3. Detached or semi with hipped roof? Hip-to-gable + rear dormer is the standard combo.
4. Pitched-roof terrace, not conservation area, want maximum space? Mansard is possible.
5. Pitched-roof terrace, conservation area, want a bedroom in the loft? Standard rear dormer (within PD limits) or full planning for larger.

Common mistakes when choosing

• Going for a mansard when a smaller L-shaped would deliver 80% of the space at 60% of the cost
• Skipping the en-suite (saves £8-12k but loses £25-50k in value uplift)
• Choosing Velux only when there isn't enough headroom for it to function as a bedroom
• Not checking conservation area + Article 4 status before designing — leads to refused planning
• Picking a builder who only does dormers but isn't experienced with L-shapes (corner detailing is hard to get right)

Frequently asked questions

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