Worked example: 20m² rear extension · UK average · no optional extras
Quick budgeting answer: Our UK-average model places this example at £70,500, with lower and higher construction-rate scenarios of £52,500–£88,000. These are planning allowances, not a narrow predicted contractor quote.
Illustrative model results for the precise example shown, not current quotes from builders. Budgets include an assumed 20% VAT on construction, professional/approval allowances and 10% contingency. A survey or different specifications could change them.
Construction-only benchmark versus an all-in budget
The Federation of Master Builders cites approximately £1,800–£3,000 per m² for single-storey extension construction as a broad starting range. Multiplying that range by a floor area does not automatically include architectural or structural design, approvals, VAT, a fitted kitchen or contingency. Confirm the scope of any published rate before relying on it.
| Single-storey floor area | Construction-only multiplication at £1,800–£3,000/m² | Our model expected planning budget |
|---|---|---|
| 15m² | £27,000–£45,000 | £52,500 |
| 20m² | £36,000–£60,000 | £70,500 |
| 30m² | £54,000–£90,000 | £105,500 |
The right-hand column is not a competing construction-only rate: it contains the calculator’s additional allowances. For the illustrated 20m² example the expected full-budget amount works out at about £3,525 per m². That is the result of this exact set of assumptions, not a standard price to apply to every extension.
Six items that can sit outside a quoted £/m² figure
Measured surveys, designer or architect services, structural engineering and certain approvals.
Whether the quoted building rate is inclusive or exclusive of tax, and which treatment applies.
Unexpected foundation requirements, drains and restricted access conditions.
Steelwork and temporary propping needed to join old and new rooms.
Bifolds, wide sliders, large rooflights and non-standard window sizes.
Cabinetry, appliances, floor coverings, decoration and changes to the original room.
Use a rate to shortlist designs, not approve spending
- Measure the proposed area on a dimensioned sketch or drawing.
- Check whether a published rate is for construction or the entire finished project.
- Add realistic allowances for approval costs, professional input, VAT and unexpected conditions.
- Compare contractors using the same drawings and written specifications before making financial commitments.
Enter your own dimensions, region, design type and known extras. The result shows a low, expected and high planning scenario with stated allowances. More specific details improve the usefulness of the estimate, but do not replace a site survey or written quotes.
Calculate your own extension budget
The examples above describe specific assumptions. Change the dimensions, region, finish and extras to explore your project.
Calculate my extension price →Common questions
What is a typical UK single-storey extension cost per square metre?
The Federation of Master Builders cites roughly £1,800–£3,000 per m² as a broad single-storey construction starting point. Site conditions and specification can push prices outside it.
Why is your model more expensive than the building-only £/m² rate?
Its full planning budget adds assumed VAT on construction, professional and approval fees, contingency and some project-size effects. Those are not necessarily included in a quoted building rate.
Can I multiply a single rate by any extension size?
For an initial construction-only illustration, yes, but it is unlikely to give a reliable all-in price. Fixed costs, design complexity, access and the existing structure matter.
Pricing context: Federation of Master Builders construction benchmark; the full-budget numbers are generated by our published illustrative model, which makes additional assumptions. For permissions in England see Planning Portal. Other UK nations have different rules. Always obtain site-specific professional advice and comparable written builder quotes.