House Extension Cost Calculator
Estimate the cost of a house extension, build cost per m², architect and structural engineer fees, planning and building control, plus a low-high price range.
Estimate only. This is an indicative estimate, not a quote, always get quotes from architects and tradespeople before you commit to a budget. Actual costs vary significantly by region, ground conditions and specification.
Enter your extension's floor area
Your build cost, extra fees and total estimate appear here as you type.
How extension costs are estimated
The bulk of an extension's cost is the build itself, priced per square metre of floor area, this varies by extension type (a wrap-around costs more per m² than a single-storey box) and by the finish you choose, from basic fittings to a premium specification. On top of the build, most extensions need an architect (typically around 10% of the build cost), a structural engineer for calculations, a householder planning application, building control sign-off, and, if you're building near a boundary, a party wall agreement with your neighbour. Because real costs vary by region, ground conditions, glazing and the builder you choose, this calculator shows a likely range (±15%) alongside the headline estimate. Treat every figure here as indicative, get itemised quotes from architects and builders before you commit.
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