Mansard Roofs Explained: The Loft Conversion for Period Properties

A mansard loft conversion effectively adds a new storey within the roof. The existing roof slope, usually at the rear and sometimes at the front too, is rebuilt at a very steep angle with a flat roof on top. Windows sit in neat dormers within the sloping face. The result is almost full head height across the whole floor and rooms that feel like part of the house rather than a converted attic.

Why mansards suit London period houses

The mansard is a traditional roof form in London, and you will see original examples across Georgian and Victorian streets. Because of that, many London planning departments see a well designed mansard as an appropriate way to extend a terrace, particularly where neighbours already have one. Councils often publish design guidance setting out the angle, materials, window style and how the mansard should relate to the parapet and party walls.

Mansard versus dormer

• Space: a mansard gives more usable floor area and head height than a dormer, because the whole roof slope is rebuilt.
• Appearance: a mansard usually looks more traditional and integrated, especially in a street of period houses.
• Planning: mansards almost always need full planning permission, whereas many rear dormers fall under permitted development.
• Cost and programme: a mansard involves more structural work and usually costs more and takes longer.

Planning permission for a mansard

Because a mansard changes the shape of the roof significantly, it is generally outside permitted development and needs full planning permission. In conservation areas the design will be judged carefully against local character. Precedent on your street helps, and a well prepared application with drawings that follow the council’s guidance gives the best chance of approval.

What the build involves

1. Planning permission, building regulations approval and party wall notices to both neighbours where the walls are shared.
2. Scaffolding with a temporary roof, so the house stays dry while the old roof is removed.
3. Structural steel and new floor joists, often building up the party walls.
4. The new mansard frame, flat roof, slate or tile cladding to the steep slope, and dormer windows.
5. Stairs, fire protected escape route, insulation, services and interior finishes.

How we handle mansard conversions on London’s Prestige projects

We coordinate the full process: architectural drawings, planning, structural design, building regulations, party wall procedures, the temporary roof and the build, with one project manager and regular updates. On site, our team pays close attention to the details planners care about, from slate coursing to lead work and window proportions. See how a new top floor transformed our Corbyn Street project, and learn about our loft conversion service. Book a site visit and we will tell you whether a mansard is realistic on your street.