Extension Cost Calculator UK
Get an indicative cost range for a house extension in the UK. Adjust the size, storeys, construction type and specification for a budget estimate based on current UK build rates.
Estimated Extension Cost
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to —
Indicative range. Not a quote. Excludes architect fees (typically 5–10% of build cost) and structural engineer fees.
Cost Breakdown
| Base Build | — |
| Foundations | — |
| Kitchen Fit-Out | — |
| Wet Services | — |
| Contingency (10%) | — |
| Estimated Total | — |
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How We Estimate
- • Estimates assume a straightforward rear or side extension with standard access and conditions.
- • Costs exclude architect fees (typically 5–10% of build cost) and structural engineer fees.
- • Permitted Development applies to many single-storey rear extensions under 60 m². Full planning may be required for double-storey or larger.
- • This is an indicative estimate, not a quote.
- • Last updated: Q2 2026.