Can every London house have a mansard?

No. Mansards always need full planning permission, and approval varies by borough and by the property's character. Conservation areas often favour mansards because they are historically appropriate to many Georgian and Victorian London streets. Modern post-war terraces and semi-detached suburban properties typically do not get mansard approval — the design would be out of character. Builderr's planning team checks mansard feasibility at the initial consultation.

Is a mansard worth the extra cost over a dormer?

If you are in a property where a mansard will be approved (typically inner London Georgian or Victorian terrace) and you can afford the larger budget, the mansard usually wins on absolute value uplift — particularly if it creates a fourth bedroom that brings the property into a higher buyer bracket. If your property is outer London or planning approval is uncertain, a dormer is the lower-risk, faster, more predictable choice.

How long does each take to build?

Dormer: 8-12 weeks on site, plus 6-10 weeks pre-construction (design, building regs, LDC, party wall). Total 14-22 weeks. Mansard: 12-16 weeks on site, plus 14-20 weeks pre-construction (design, planning permission, building regs, party wall). Total 26-36 weeks.

Can I combine a dormer and mansard?

Some London conversions combine a mansard rear with a hip-to-gable side extension (gives the third side a vertical wall and an extra dormer). This 'mansard hip-to-gable' is common on semi-detached corner properties. It requires full planning. Cost is towards the top of mansard range (£120,000–£160,000).

Dormer vs Mansard Loft Conversion — London 2025 Comparison

Dormer Loft vs Mansard Loft Conversion — Which Is Better?

A dormer loft is cheaper (£60,000–£95,000), faster (8–12 weeks), permitted development on most London houses, and creates a single bedroom plus ensuite. A mansard loft costs more (£95,000–£140,000), takes longer (12–16 weeks), always needs full planning, but creates a full floor with two bedrooms and an ensuite — adding more value. Choose dormer for speed and budget, mansard for maximum space and ROI.

What is a dormer loft conversion?

A dormer loft conversion adds a box-shaped structure projecting vertically from the rear (or side) roof slope, creating headroom and floor area in the loft without altering the front or other roof slopes. The existing ridge stays in place; the rear roof slope is cut back and the dormer structure is built off the new flat-roof deck. Standard dormer types: full-width rear dormer (the most common London type — extends across the full rear of the roof), half-width dormer, L-shape dormer (one over main roof, one over rear outrigger), hip-to-gable plus dormer (on semi-detached, converts the hip end to a gable wall then adds a rear dormer). Dormers are typically permitted development on London houses (subject to volume limits) and don't change the front elevation, which makes them planning-friendly.

What is a mansard loft conversion?

A mansard loft conversion rebuilds the entire roof into a steep, near-vertical rear and side roof with a small flat top. The roof shape is essentially demolished and reconstructed — old tiles off, rafters removed, new structure built with internal walls at 70-72° pitch and a flat roof deck. The result is a near-full storey of habitable floor area, typically two bedrooms plus ensuite (or three small bedrooms). Mansards are characteristic of London — they originated in Paris in the 17th century and were widely built on Georgian and early Victorian London terraces. Modern mansards always require full planning permission because they materially alter the roof shape; many London conservation areas favour mansards as historically appropriate.

Cost comparison: dormer vs mansard

On the same London terrace, a standard dormer with one bedroom + ensuite costs £65,000–£85,000 in 2025. A mansard creating two bedrooms + ensuite on the same property costs £95,000–£140,000. The mansard is 45-65% more expensive because: (1) the entire existing roof structure is demolished and rebuilt; (2) the front elevation may require lead-clad mansard return (visible from street, expensive material); (3) more square metres of habitable space is built (typically 40-55m² vs 25-35m² for a dormer); (4) full planning permission adds 8-12 weeks and £400+ in fees; (5) the build is longer (12-16 weeks vs 8-12 weeks) so the contractor's overhead is higher.

Which adds more value?

A mansard typically adds more absolute value than a dormer because it creates more floor area and an extra bedroom. Typical London zone 2-3 value uplifts: dormer £85,000–£130,000 (15-22% on a £600k house); mansard £130,000–£200,000 (20-28% on the same house). However, the ROI ratio is often similar because the mansard costs significantly more to build. Per pound spent, a dormer often returns slightly better ROI in outer London (zones 4-6) where four-bed premiums are smaller; a mansard often wins in inner London (zones 1-3) where four-bed and five-bed premiums are very high.