What might it cost? All of it, not just the build.
This estimate adds our fees, the engineer, party wall surveyors, approvals, VAT and a contingency, so the number you plan around is the number you'll actually need.
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- C-300
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- Example rates
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- C · Oct 2026
Your estimate · example rates
Side return · 12 m² · considered
£—
Total project budget range, including VAT.
Saved estimates
The assumptions, in full.
Every house is different. These example rates are a sensible starting point for London in 2026, not a quote.
Example base rates (considered spec, inner London, before VAT): side return £3,400, rear extension £3,200, wraparound £3,500, loft £2,900, basement £5,600 per m². The range shown is ±10%. Essential spec is 15% less; exceptional is 30% more.
Up to planning: a fixed fee of £3,800–£7,500, depending on the project. Planning and technical: 7–9% of the build cost. Full service, including tender and contract administration: 11–14% of the build cost. VAT is added if you include it.
Structural engineer: £2,000–£4,500 (basements £8,000–£14,000). Party wall surveyors: £1,200–£2,000 per neighbour. Building control: £1,200–£2,200. Planning or LDC fee: £264–£528. Measured survey: £600–£1,100.
A contingency of 10–15% of the build is included. Not included: furniture, moving out, Community Infrastructure Levy (rarely due on extensions under 100 m²), garden landscaping, and any surprises found in very old houses.