How accurate is the estimate?

It is an indicative London market range for budgeting, not a quotation. Only a contractor pricing a defined drawing package can give a firm number.

Build cost per square metre calculator

Q: What does the build cost per square metre include?

It estimates an indicative London build rate by project type. Typical London figures are £2,200–£3,200 per m² for a single-storey extension, £1,900–£2,800 for a loft conversion, and £4,000–£6,500 for a basement. Statutory fees and professional fees are separate.

Build-cost-per-sqm for London extensions and conversions.

Interactive calculator UI scheduled for Phase 4, until then.

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What this estimates

This tool estimates an indicative London build rate by project type. As a London market range, expect £2,200–£3,200 per m² for a single-storey extension, £1,900–£2,800 for a loft conversion, and £4,000–£6,500 for a basement. That is a budgeting range, not a quotation.

What moves the number?

Project type, which is the largest single variable — below-ground work costs roughly twice above-ground work per square metre.
Specification, particularly glazing, and whether fit-out is inside or outside the rate.
Access. A terraced house with no side access adds labour to every material movement.
Structural complexity: number of openings, spans, and whether foundations are standard or designed.
Contract type, since a design and build price bundles design risk that a traditional contract prices separately.

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.