Home/Cost Calculator See an indicative budget for your extension in a few quick steps. No email needed to see the results.
House Extension Cost Calculator
Indicative budget · six steps
How the estimate is worked out
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1
Construction
Floor area multiplied by a cost per m² for your extension type, roof and finish.
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2
Extras
Allowances for a new kitchen, large glazing, roof lights and structural or site works.
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3
Fees
Design, structural engineer, surveys, planning and Building Control fees.
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4
Contingency and VAT
An allowance for the unexpected, with VAT at 20% where it applies.
How much does a house extension cost in Chester?
| Extension type | Typical size | Build cost exc. VAT | All-in budget inc. fees & VAT |
|---|---|---|---|
| Small single-storey rear extensionA compact garden room or larger kitchen. About 10–14 weeks on site. | 15 m² | £27,000 – £42,000 | £42,000 – £66,000 |
| Single-storey rear extensionThe most popular type: opens the house to the garden. About 11–15 weeks. | 20 m² | £36,000 – £56,000 | £54,000 – £86,000 |
| Kitchen extensionOpen-plan kitchen-diner. Kitchen itself extra (typically £8,000–£30,000). About 12–17 weeks. | 25 m² | £45,000 – £70,000 | £66,000 – £106,000 |
| Side or side-return extensionFills the gap beside the house; often needs a party wall agreement. About 10–14 weeks. | 15 m² | £29,000 – £45,000 | £45,000 – £70,000 |
| Wraparound extensionSide and rear combined for a large open-plan space. About 14–20 weeks. | 30 m² | £57,000 – £88,000 | £83,000 – £132,000 |
| Two-storey (double-storey) extensionExtra living space below and bedrooms above. Lower cost per m². About 20–28 weeks. | 40 m² over 2 floors | £66,000 – £103,000 | £95,000 – £153,000 |
Build cost is the building work only, at a standard to mid-range finish, excluding VAT, the kitchen and fit-out. All-in budget adds design fees, structural engineer, surveys, the £548 planning fee, Cheshire West and Chester Building Control, a 10% contingency and VAT at 20%. Times are typical weeks on site. Example figures, last updated October 2026. Use the calculator above for an estimate based on your own home.
Extension cost per m²
Standard £1,800–£2,300/m², Mid-range £2,200–£2,800/m², Premium £2,800–£3,600/m².
Design fees (from 6% of build cost), structural engineer, planning (£548), Lawful Development Certificate (£274), and Cheshire West and Chester Building Control (£626-£945 inc VAT).
Fees to budget for
About the calculator
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No. It is an early budget guide based on typical costs. Your builder will quote from the final drawings.
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Build costs are based on current 2026 benchmarks for extensions in the North West. Planning and Building Control fees use the actual charges from the Planning Portal and Cheshire West and Chester Council. Fit-out, glazing and site-work allowances are typical ranges. We review the figures regularly; the date they were last updated is shown on your results.
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At this early stage, many things aren’t known yet: what the ground is like, the exact layout, which kitchen or glazing you’ll choose, and how builders will price it. The lower figure assumes things go smoothly with sensible choices; the higher figure allows for higher-spec finishes and more complex work. The range narrows as the design develops.
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Furniture, landscaping and garden work, moving out during the build, finance costs, and any unexpected problems beyond the contingency allowance. If you’re unsure whether something is covered, ask us and we’ll tell you.
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If the budget feels right, get in touch for a free initial conversation. We’ll talk through your plans, explain the likely planning route and give you a clear quote for our design and approvals service. You can then use our drawings to get accurate prices from builders.
Ready to make more of your home?
Start with an indicative budget, or tell us about your plans and we'll arrange a first call.