Extension cost calculator for UK builders.
Break the extension into costs before quoting the lump sum.
Plan the main cost buckets for a UK extension: groundworks, structure, roof, first fix, second fix, preliminaries, overhead, margin, and variations.
Estimate the extension price
Use square metres and sensible allowances to rough out the job before doing a full quote.
Inputs
Example: a 32m2 extension with preliminaries, contingency, margin, and VAT.
Instant result
Estimated job cost
£85,536
£7,200 prelims, £6,336 contingency
Quote before VAT
£100,932
£15,396 profit
Client total inc VAT
£121,119
Approx price per m2 ex VAT
£3,154
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Split structure from finish
Groundworks, brickwork, steel, roof, windows, first fix, second fix, and finishes all behave differently on margin.
Add preliminaries and risk
Skips, scaffolding, welfare, access, weather, parking, delays, and design changes should not come out of profit by accident.
Plan staged payments early
A clear payment schedule protects cash flow and makes the client approval process easier once the quote is accepted.
Build the quote from the real cost buckets.
Step 1
Break the extension into structural, services, finishing, and preliminary sections.
Step 2
Add labour, materials, subcontractors, hire, waste, and access costs.
Step 3
Set margin, contingency, exclusions, and staged payment assumptions before sending.
Free calculator
Send me the extension cost calculator.
Use it to plan the first price. QuoteBuild helps turn the same estimate into a quote, payment stages, variations, and live margin tracking.
Quick answers.
Can a calculator replace a site visit?
No. It is a pricing structure, not a survey. Use it to avoid missing cost buckets, then confirm assumptions on site.
Why quote extensions in sections?
Sections make it easier to spot missing costs, explain the price to the client, and track overruns once work starts.