Glossary
Mansard roof extensions and where London allows them
A near-vertical roof extension with a small upper slope. Common rear-of-terrace addition in London.
A near-vertical roof extension with a small upper slope. Common rear-of-terrace addition in London.
A mansard roof has two slopes on each side — a steep lower slope, typically around 70 degrees, and a shallow upper slope of about 30 degrees — with dormer windows set into the steep face. It creates the most usable loft volume of any roof form and is the type most often accepted in London conservation areas.
Because on Georgian and early Victorian terraces they read as original. Mansards were a standard nineteenth-century way of adding a storey, and where a terrace already carries several, adding another maintains rather than disrupts the rhythm. Several London boroughs publish mansard design guidance precisely because they expect applications.
The design usually has to follow that guidance closely: the lower slope pitched between 70 and 72 degrees, set behind the existing parapet, dormers vertically aligned with the windows below, and traditional materials — natural slate, lead cheeks, timber sash windows.
Rarely. A mansard almost always alters the roof slope fronting a highway and usually exceeds the existing ridge, both of which take it outside Class B. In a conservation area Class B is withdrawn entirely. Assume a full householder application at £548 over eight weeks.
The existing roof is removed and rebuilt, so the party walls are raised — a party structure notice with one month's notice to each neighbour. New purlins and a ridge beam are designed by the engineer, the floor below is usually strengthened, and Part B requires a 30-minute protected stair from the new storey to the final exit, which on a four-storey terrace often means a full escape strategy.
A worked Barnet example of mansard roof: with 20 conservation areas — the Hampstead Garden Suburb Conservation Area and the Mill Hill Conservation Area among the best known — London Borough of Barnet (Barnet Council) sees mansard roof constantly, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Where mansard roof pushes a Barnet scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.
| Lower slope pitch | Typically 70–72 degrees |
|---|---|
| Upper slope pitch | Around 30 degrees |
| Permitted development | Rarely; usually full application |
| Application fee | £548 householder |
| Party structure notice | One month |
| Our mansard package from |
Almost always yes. Class B rarely covers it and never in a conservation area.
Materially, yes. An established pattern on the terrace is the strongest argument.
More than any other loft form — typically a full additional storey.
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