Project Cost Estimators
Home Extension Cost Calculator
Estimates build cost for extensions per m² or ft²
Updated 13 May 2026 · Live
What this tool does
Estimates build cost for extensions per m² or ft².
Formula Used
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How the home extension cost calculator works
Estimates build cost for extensions per m² or ft². The calculator takes your dimensions and supplier rates, applies a standard UK trade formula, and returns a quantity with an indicative cost. Every figure is an estimate — site conditions always move the final number.
Typical UK project cost estimators wastage
Cost estimates carry an inherent range — a ±20% spread is normal at planning stage, tightening to ±5% once drawings are tendered. Our defaults reflect common UK trade allowances, and can be adjusted upwards for non-standard geometry or downwards where experience supports a lower figure.
What this tool does not do
It does not replace a professional quote, factor regional pricing, assess structural adequacy, or confirm Building Regulations compliance. Those remain the responsibility of a suitably qualified designer, surveyor, or your building control officer.
On-site considerations for home extension cost
Rates in this tool are April 2026 UK medians. Regional variation of ±30% is normal — London and the South East run higher, Scotland and the North typically lower.
Building Regulations and compliance
VAT treatment differs by project type: new-builds are zero-rated for a qualifying end user, listed-building work may qualify for 5%, normal renovation is 20%. When in doubt, a pre-application enquiry to the local authority can give early clarity, which tends to be less costly than retrospective correction.
Before you order
Break your budget into build cost, professional fees (8–15%), and contingency (10–20%) from the outset. Blending them hides overruns until it is too late to respond. Cross-checking the calculator’s output against a supplier quote helps catch differences in pricing assumptions — ask for exact product specifications (grade, finish, batch number) and confirm delivery timescales against your programme.
Adjusting the defaults
Every input in this calculator is editable. Enter your own dimensions, supplier prices, and wastage allowance — the output recalculates instantly. If the defaults feel off for your region or project type, your own numbers always override them.
Using this home extension cost calculator alongside other BuildMetricLab tools
This calculator works best as part of a planning workflow. Pair the quantity with our project contingency, labour-hours, and material-cost calculators to build a complete estimate before you pick up the phone to a supplier. All BuildMetricLab tools run entirely in your browser — no sign-up, no data sent anywhere, and every formula is shown on-page so you can audit the maths.
Sources & methodology
Estimates build cost for extensions per m² or ft². Every result is calculated from the values you enter, and all inputs are editable.
Frequently asked questions
Are home extension cost calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
What wastage percentage should I use?
The calculator defaults to the typical UK trade allowance for project cost estimators. Increase it for complex cuts, awkward shapes, or first-time DIY. The default wastage allowance reflects common trade practice; values lower than the default may underestimate offcuts.
Does this replace professional advice?
No. This tool is a planning estimator. For works that affect structure, Building Regulations, Party Wall, gas, electrics, drainage to a sewer, or similar, consult a suitably qualified professional.
Can I change the unit prices?
Yes — every price field is editable. Plug in your supplier's quote to get a total that matches your project.
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