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Extension cost calculator · free
Extension cost calculator. London prices.
Answer one question at a time and watch your home take shape. Choose one project or several. Your indicative London price arrives at the end, on screen and by email. Free, and about two minutes.
Want your own project priced trade by trade, from your description, photos and plans? That is the AI Estimator, £9.99.
01Your home
What kind of home is it?
How the calculator works
Five questions, one clear range.
- 01
Your home
Terraced, semi detached, detached or a flat, and roughly when it was built. The model draws it in its period.
- 02
The shape
A rear extension out into the garden, a side return into the strip beside the back addition, or an L-shaped wrap around that does both.
- 03
One storey or two
A bigger ground floor, or a room on each floor built in one go, and the new floor area in square metres.
- 04
The brief
The finish level, whether you have drawings, where planning stands, and whether the house is in a conservation area or listed.
- 05
Your estimate
An indicative London range on screen, by email and as a PDF. Add a loft, a kitchen or bathrooms and they are priced together, line by line.
What London projects cost
What an extension costs in London
| The work | Typical cost, ex VAT | Read more |
|---|---|---|
| New extension space, standard finish | £2,650 to £2,900 per m² of new floor | Extension costs |
| A single storey extension, about 20 m² of new floor, flat roofed, plus making good the room it joins | Around £57,000 | Extension costs |
| A two storey side return, about 48 m² of new floor | Around £128,000 | Two storey extensions |
| A complete extension project, once finishes and fittings are included | From around £85,000 | Extension costs |
From typical London jobs priced on our estimator in October 2026, calibrated to inner west London: Fulham, Chelsea and Wandsworth. Prime central London projects often price 8 to 25 percent above these figures, and outer London commonly 10 to 20 percent below. All figures exclude VAT and are indicative, not a quotation.
What the price includes
- The building work of the new space, from the foundations to the roof
- Opening up the back of the house and making good the room the extension joins
- Fitting the windows, doors, any rooflights, sanitaryware and tiles
- Your choice of finish level: essential, standard or high
What it leaves out
- VAT
- Professional fees: design, structural engineering, building control and party wall surveyors
- Planning and statutory costs
- The products themselves: windows, doors, rooflights, sanitaryware and tiles are priced for fitting, with the products supplied by you
- Kitchen units, worktops and appliances
Good to know
Frequently asked questions
- How accurate is an extension cost calculator?
- It gives an indicative range, not a quotation. Ours uses the per square metre rates MVV publishes from typical London jobs priced on its estimator, calibrated to inner west London, so it is a realistic starting budget. The final price follows a survey of the house, the drawings and the planning position.
- How much does a 20 m² extension cost in London?
- A single storey extension of about 20 m² of new floor, flat roofed, plus making good the room it joins, came to around £57,000 ex VAT on typical London jobs priced on our estimator in October 2026. Our minimum for a complete extension project, once finishes and fittings are included, is around £85,000.
- How much does a two storey extension cost in London?
- A two storey side return adding a kitchen extension with a bedroom and shower room above, about 48 m² of new floor, came to around £128,000 ex VAT. New space at a standard finish runs about £2,650 to £2,900 per square metre of new floor.
- Does the calculator include VAT and fees?
- No. Every figure is ex VAT and leaves out professional fees, planning and statutory costs. Our design cost calculator prices the design and approvals separately.
- Is the extension cost calculator free?
- Yes. It is free, with no card and no account. You leave an email address so the range and the PDF can reach you.
- Does it work outside London?
- It is calibrated to London. Prime central London projects often price 8 to 25 percent above its figures and outer London commonly 10 to 20 percent below. Outside London the figures are likely to run high.
Read more: House extension cost in London · Rear extension or side return · House extensions in London · Loft conversion cost calculator
Estimates are indicative ranges, exclusive of VAT, professional fees, planning and statutory costs. Every MVV project is priced individually after a site visit. The estimate is where the conversation starts, not where it ends.