House Extension Cost Calculator
Estimate a planning budget for a single or double-storey extension. Adjust the region, size and finish level to see the range change instantly.
Estimated project range
£50,760 – £84,600Typical estimate £67,680 · Suggested 10% contingency £6,768 · Budget incl. contingency £74,448
Postcode matching uses the postcode area to select an approximate cost region; some postcode areas cross regional boundaries, so the region can always be overridden manually. Core extension benchmark: £1,800–£3,000 per m².
How the estimate works
The calculator starts from a national cost-per-square-metre benchmark, then applies transparent adjustments for project size, storeys, specification and region. Treat the result as a planning range before VAT and before any project-specific items that are not included in your contractor quote.
1. Floor area
Enter the extension footprint so the model can establish the core build allowance.
2. Project type
Storeys and finish level change the amount of structure, labour and specification in the model.
3. Region
Postcode-aware regional factors adjust the national benchmark without presenting them as measured city-wide quote averages.
4. Contingency
Keep a separate allowance for unknown ground conditions, structural surprises and scope changes.
Read the 2026 extension cost guide →How UK Home Costs models prices →