Mansard Loft Conversions in London: What They Are, What They Cost
A mansard loft conversion replaces the existing rear roof slope with a near-vertical rear wall and a shallow-pitched flat roof on top. The result is maximum usable floor area and maximum headroom — but at higher cost and with planning permission required.
What Is a Mansard Conversion?
Unlike a dormer, which punches a box out of an existing sloping roof, a mansard changes the form of the entire rear roof section. The rear slope is rebuilt as a near-vertical wall (typically 70° from horizontal) with a flat or very slightly pitched roof on top. The resulting space has full-height walls across the full width and depth of the rear roof.
Mansard conversions are most common in London on Victorian terraces in conservation areas and borough-wide in areas like Chelsea, Kensington, Islington, and Hackney where there is an established character of mansard roofs.
Planning Permission for Mansard Conversions
A mansard conversion almost always requires planning permission — it is not permitted development because it significantly changes the roof form and is usually visible from the rear. In conservation areas, the design must reference the established mansard form on the street.
Some London boroughs have supplementary planning guidance on mansard design — specifying the roof pitch, setback from the front eaves, and materials — to ensure new mansards are consistent with the existing character.
What Does a Mansard Conversion Include?
- Full structural alteration of the rear roof — rafter removal, new wall framing, new structural plate
- Near-vertical rear wall — typically timber frame clad in slate, tile, or zinc
- New flat roof on top (warm roof construction with falls to outlets)
- Full-width rear windows — typically floor-to-ceiling sliding or pivot windows
- Structural floor upgrade
- Loft access (staircase)
- Full insulation to Part L standard
- Internal lining, plastering, second fix
Typical Costs in London
| Specification | Approximate cost |
|---|---|
| Basic mansard (1 bed, no en-suite) | £65,000–£90,000 |
| Standard mansard (1 bed + en-suite) | £85,000–£120,000 |
| Premium mansard (2 beds or bedroom + study + bathroom) | £110,000–£160,000+ |
Costs are higher than dormer conversions because of the structural scope of the rear wall rebuild.
RCB Design & Build
RCB Design & Build delivers mansard loft conversions across London, including design coordination, planning, structural engineering, and full build delivery.