Cost FAQ
How much does a mansard roof cost?
Build: £78,000-£110,000 single mansard, £120,000-£180,000 full London-wide mansard.
Build: £78,000-£110,000 single mansard, £120,000-£180,000 full London-wide mansard.
A London mansard typically costs £70,000 to £110,000 to build, because the existing roof is removed and rebuilt and the party walls are raised.
A rear dormer is added to an existing roof. A mansard replaces the roof: the existing structure comes off, the party walls are raised, new purlins and a ridge beam are installed, and the whole new storey is weathered in natural slate or lead with traditional dormer windows in the steep front slope.
The floor below usually needs strengthening, scaffolding and temporary weather protection run for longer, and conservation-grade materials — natural slate, lead cheeks, timber sash windows — cost more than the alternatives.
A householder application at £548, determined in eight weeks. Consent usually turns on whether neighbouring roofs in the terrace already carry mansards, and on whether the design follows the borough's mansard guidance — lower slope at 70 to 72 degrees, set behind the parapet, dormers aligned with the windows below.
Raising a party wall is section 2 work under the Party Wall etc. Act 1996 with one month's notice to each adjoining owner. On a mid-terrace that is two awards, commonly £900 to £1,400 each with an agreed surveyor.
Where the terrace already carries mansards and the house is otherwise at its ceiling, a mansard is often the only way to add a full storey. Where it would be the first on the terrace, the planning risk is real and a rear dormer at half the cost is usually the better proposition.
How much does a mansard roof cost? in a City of London context: City of London Corporation (City of London Corporation) runs 27 conservation areas including the Barbican Conservation Area and the Smithfield Conservation Area, and an Article 4 direction is in force there, so permitted development cannot be assumed. That decides how how much does a mansard roof cost? is handled: £548 and 8 weeks for a City of London householder application, or £274 for the Lawful Development Certificate that records how much does a mansard roof cost? as lawful. The law behind how much does a mansard roof cost? sits in the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| London build cost | £70,000–£110,000 |
|---|---|
| Our mansard drawing package | |
| Planning route | Full householder application, £548 |
| Lower slope pitch | 70–72 degrees |
| Party structure notice | One month per neighbour |
| Party wall fees | £900–£1,400 per neighbour |
Rarely. It usually exceeds the ridge and alters the street-facing slope.
Materially, yes. An established pattern on the terrace is the strongest argument.
Longer than a dormer, because the roof is removed and rebuilt.
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