Mansard roof cost calculator

What moves the number?

• The existing roof is removed and rebuilt, and the party walls are raised on both sides of a mid-terrace.
• New purlins and a ridge beam, plus strengthening of the floor below.
• Conservation-grade materials: natural slate, lead cheeks and timber sash windows.
• Longer scaffolding and temporary weather protection than any other loft type.
• Two party wall awards on a mid-terrace, commonly £900–£1,400 each with an agreed surveyor.

What sits outside the build cost?

Statutory fees go to the council and are set nationally for England: a householder planning application is £548 determined in 8 weeks, larger home extension prior approval is £249 over 6 weeks, and a Lawful Development Certificate is £274. Building control is separate again at typically £500 to £1,200 in London.

How should you use the figure?

As a budget envelope before design, not as a target after it. Build cost should drive the design rather than follow it: a scheme drawn without a budget is a scheme that gets value-engineered after planning consent, which usually means a second application. Fix the range first, design to it, then price the defined drawing package with two or three contractors.

Ranges are indicative London market rates observed across domestic projects and are not a quotation. Statutory figures are the England rates in force in 2026 under the Town and Country Planning (Fees for Applications) (England) Regulations. Building control charges are set by each local authority or registered building control approver.