Extension Cost Calculator: An Indicative London Budget
Last reviewed: 6 November 2026
Enter the extension type, the total added floor area and the finish level. The calculator applies the per-square-metre rates published in our extension cost guide, adds the London allowance that same guide describes, and shows an indicative range. It is a budgeting tool: there is no address lookup, no drawings and no survey, and it is not a quote.
Extension cost estimator
Indicative only — before VAT, professional fees and contingency. Confirm a written scope and itemised quote.
How to use this estimate: enter the total added floor area across all storeys, not just the ground-floor footprint. For example, 20 m² on each of two floors means 40 m². The calculator always applies the guide's London allowance: +30% to the lower rate and +50% to the upper rate. Good finish adds 18% and high specification adds 45% to those adjusted rates, which are the multipliers published on our long-form calculator page. VAT, professional and statutory fees, contingency and property-specific work are additional.
What this range includes and excludes
- Rates: construction rates per m² from the 2026 breakdown table in our extension cost guide (published above), before VAT.
- London allowance: +30% on the lower rate and +50% on the upper rate, the allowance the same guide publishes. The calculator has no region input.
- Finish: standard ×1.00, good ×1.18, high specification ×1.45.
- Excludes: VAT at 20%; professional fees (architect, structural engineer, party wall surveyor); statutory fees (planning application, building control); contingency; kitchen, bathroom, flooring and decoration beyond the base construction rate; landscaping, drainage, and any abnormal foundation or structural work.
- Not a price: the result is rounded to the nearest £1,000 and is a budgeting range, not a quotation or a promise of a final cost.
Rates used in this calculator
| Extension type | Low rate per m² | High rate per m² | Published example |
|---|---|---|---|
| Single-storey rear | £1,800 | £2,800 | £36,000–£56,000 (20 m² total floor area) |
| Side return | £2,000 | £3,000 | £30,000–£45,000 (15 m² total floor area) |
| Double-storey rear | £1,800 | £2,500 | £72,000–£100,000 (40 m² total floor area) |
| Wrap-around | £2,000 | £3,000 | £50,000–£75,000 (25 m² total floor area) |
| Kitchen extension | £2,000 | £3,000 | £40,000–£60,000 (20 m² total floor area) |
| Loft conversion | £1,500 | £2,500 | £30,000–£50,000 (20 m² total floor area) |
| Garage conversion | £800 | £1,500 | £12,000–£22,500 (15 m² total floor area) |
| Conservatory | £1,200 | £2,000 | £18,000–£30,000 (15 m² total floor area) |
| Orangery | £2,000 | £3,000 | £30,000–£45,000 (15 m² total floor area) |
Worked examples from our published guide
The figures below are the guide's own published examples, recalculated from the rates above. They are here so you can see the arithmetic rather than take it on trust.
| Published example | Floor area | Our guide says | Calculator's rates give |
|---|---|---|---|
| Single-storey rear | 20 m² | £36,000–£56,000 | £36,000 – £56,000 |
| Side return | 15 m² | £30,000–£45,000 | £30,000 – £45,000 |
| Double-storey rear | 40 m² | £72,000–£100,000 | £72,000 – £100,000 |
| Wrap-around | 25 m² | £50,000–£75,000 | £50,000 – £75,000 |
| Kitchen extension | 20 m² | £40,000–£60,000 | £40,000 – £60,000 |
| Loft conversion | 20 m² | £30,000–£50,000 | £30,000 – £50,000 |
| Garage conversion | 15 m² | £12,000–£22,500 | £12,000 – £22,500 |
| Conservatory | 15 m² | £18,000–£30,000 | £18,000 – £30,000 |
| Orangery | 15 m² | £30,000–£45,000 | £30,000 – £45,000 |
| Single-storey rear, London, standard finish | 20 m² | about £47,000 – £84,000 | £47,000 – £84,000 |
Frequently asked questions
How much does an extension cost per square metre?
Our extension cost guide uses £1,800 to £2,800 per m² for a single-storey rear extension before VAT, fees and contingency. For extensions in London the guide adds 30% to the lower rate and 50% to the upper rate, which gives £2,340 to £4,200 per m². These are budgeting assumptions from our own published guide, not an independently verified market average.
Does this calculator include VAT?
No. The extension figures are before VAT at 20%, and before professional fees, statutory fees and contingency. That is the basis our extension cost guide uses for its construction rates. Our garage and loft calculators show figures including VAT, because that is how those guides state their rates.
Why is the London range higher than the national figures in your guide?
The guide publishes a London allowance of 30% on the lower base rate and 50% on the upper base rate. This calculator always applies it, because we build in London. The national figures are on the guide's own table for comparison.
My extension is on two floors — what do I enter?
Enter the total added floor area across every storey, not the ground-floor footprint. An extension of 20 m² on each of two floors is 40 m². For a two-storey rear extension the guide's own 40 m² example is £72,000 to £100,000 before the London allowance.
Is this a quote?
No. It is an indicative budget range from published rates, with no address, drawings or survey involved. Your property, access, condition and specification decide the real number. The next step is a free survey, where we can confirm the scope and give you a written price.
Plan your London extension
Discuss your property, plans and next steps at a free survey.
Book a free survey →Prefer to read the workings? Read the full extension cost guide.
More budgeting tools: All cost calculators · Loft conversion calculator · Garage conversion calculator