Mansard · Maximum space · Planning required
The loft conversion that creates the most usable space. We prepare the design and planning application, handle party wall and Building Regulations, and build under a written JCT contract.
A mansard loft conversion in London costs about £75,000–£110,000+ in 2026 and takes 10–16 weeks on site. It almost always needs full planning permission because it changes the roof shape. In conservation areas, design, slate materials and street context decide the outcome.
Last updated 3 October 2026
A mansard replaces the existing sloping roof with a new structure that has near-vertical sides, typically at about 70 degrees, and a shallow flat top. Because the sides are almost vertical, nearly all of the new storey is usable floor space with full headroom. Mansards are a traditional London roof form, common on Georgian and Victorian terraces in central areas, and they are often the form that planners prefer in sensitive streets.
For a side-by-side comparison read our Mansard vs Dormer guide. If you need to keep cost down, look at a rear dormer.
A mansard needs a full householder planning application. Council planners usually look at:
A pre-application conversation with the council can save months on a sensitive street. See our guides to conservation areas and how long planning takes in London.
| Specification | Typical build cost | What you get |
|---|---|---|
| Standard mansard (terrace) | £75,000–£110,000 | Rebuilt roof, one or two rooms, bathroom, Building Regs sign-off |
| Large or high-specification mansard | £110,000–£130,000+ | Slate cladding, bespoke dormers, larger area, premium finishes |
Figures exclude VAT, professional fees and planning fees. A mansard carries more design and planning cost than a dormer. Try the Loft Conversion Cost Calculator for your own estimate.
Measured survey, design options and, where useful, a pre-application discussion.
Planning application, structural design, Building Regulations and party wall notices.
Scaffold and temporary roof, structural frame, new roof, windows, services, stair and finishes.
Snagging walkthrough, Building Control completion certificate and handover.
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In 2026 a mansard typically costs £75,000–£110,000, and £130,000 or more for large or prime-postcode projects. It costs more than a dormer because the whole upper roof is rebuilt, but it creates the most usable space.
Almost always. A mansard changes the shape of the roof significantly, so it falls outside Permitted Development limits. A householder planning application is required, and in conservation areas the design, materials and street context are examined closely.
Often yes, particularly where the street already has mansards or the terrace has an established roof pattern, but it is never Permitted Development there. Design quality, materials such as slate and traditional dormer joinery, and the view from the street all matter. Determination can take longer.
About 10–16 weeks on site. Before that, allow time for design, the planning application (the statutory period for householder applications is eight weeks, though many take longer), Building Regulations and party wall notices.
A mansard gives more headroom and floor area and a roof profile that suits many period terraces, but costs more and needs planning permission. A rear dormer is cheaper and usually Permitted Development. The right choice depends on your roof, your street and your budget.
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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