Extension cost calculator
Enter your extension's size and type below to get an estimated build cost range, worked out from the cost per square metre published by Less.co.uk. It gives you a sensible starting figure before you speak to builders, and the notes underneath explain what the estimate covers and what you will need to add.
Work out your extension cost
Enter the length and width of the extension, or pick a typical size, then choose the type of extension to see an estimated build cost range. For a two storey extension, enter the floor area of both floors added together.
Or choose a typical floor area
Floor area: 0 m²
Enter your measurements or pick a size to see an estimate.
Based on the cost per square metre published by Less.co.uk (updated March 2026, checked October 2026). Less gives the South East figure for a mid range single storey extension and Midlands averages for the other types, all excluding VAT, professional fees and interior fit out such as a kitchen. Your design, ground conditions and access will affect the real price.
How to use the extension cost calculator
Start by entering the length and width of the extension in metres, and the calculator multiplies them to give the floor area. If you do not have measurements yet, pick one of the typical sizes instead, which run from 9m² to 40m². Then choose the type of extension that is closest to your plans, and the calculator shows a low and high figure based on Less.co.uk's rate per square metre for that type.
The default option is Less's South East figure for a mid range single storey extension, because it is the only regional figure Less publishes and the closest match for homes in Sussex. The other options use Less's Midlands averages for each type, and Less's own regional table puts the South East 17% above the Midlands for single storey work, so expect South East prices for those types to sit higher too. For a two storey extension, add the floor area of both storeys together before you enter it, since the rate applies to every square metre you build.
How to measure your extension
You do not need precise drawings to get a useful estimate, but a few minutes with a tape measure will make the result far more realistic than a guess. Measure the space the extension will add, not the existing room it opens into.
- Measure the length from the existing back or side wall to where the new outside wall will be.
- Measure the width across the extension, from one new wall to the other, or to the boundary for a side return.
- For an L shaped or wrap around design, split the plan into rectangles, work out each area and add them together.
- For two storeys, measure each floor and add the two areas.
- Note whether you are measuring inside or outside the walls, and ask builders which basis their prices use.
Council building control charges are banded by internal floor area, for example by Adur and Worthing Councils and Arun District Council, so it is worth knowing that figure when you come to apply. Keep in mind that permitted development limits are set by depth and height from the original house, which our guide to extension planning permission explains.
What the Less.co.uk figures include and exclude
Less.co.uk's cost per m² guide, updated in March 2026, states that its figures by type are Midlands averages excluding VAT and professional fees. Its regional figures, including the South East row used as the default here, also exclude interior fit out such as a fitted kitchen. Less does not say explicitly whether labour is included, so treat the estimate as a guide to the building work rather than a full project budget.
That means three things are not in the figure the calculator shows. VAT is the first, and GOV.UK's VAT Notice 708 says that 'work to an existing building is normally standard-rated'. Professional fees for design, engineering and approvals are the second, and fit out such as the kitchen, flooring and decoration is the third. Your own design, ground conditions and access will also move the real price away from any average.
Extras Less lists on top of the build cost
Less lists several common extras separately from its rates per square metre. Add any that apply to your plans to the calculator's range to get a fuller picture of the likely cost.
| Item | Cost | Source |
|---|---|---|
| Bi-fold doors | £3,000 to £6,000 | Less.co.uk |
| Roof lantern | £1,500 to £3,500 | Less.co.uk |
| Underfloor heating | £50 to £80 per m² | Less.co.uk |
| Fitted kitchen | £8,000 to £20,000 or more | Less.co.uk |
| Difficult ground | Adds £2,000 to £8,000 | Less.co.uk |
| Restricted access | Adds 5 to 10% | Less.co.uk |
| Professional fees | £3,000 to £10,000 | Less.co.uk |
If you need full planning permission, the householder application fee in England is £548 from 1 April 2026, according to GOV.UK. Building control charges are extra again and vary by council, so check your own council's building control page or ask a private registered building control approver for its fee.
How other sources compare
The calculator uses one source throughout so that every option compares like for like, but other UK guides publish different rates. The table gives a few of them for comparison. They are not directly comparable with Less, because each source includes different things and most do not say whether VAT is included.
| Extension | Cost per m² | Source |
|---|---|---|
| Single storey, average | Around £1,700 | MyJobQuote |
| Single storey, average | £1,800 to £3,000 | FMB |
| Single storey, overall (excludes VAT) | £2,200 to £3,300 | Homebuilding and Renovating |
| Two storey | £1,000 to £1,900 | MyJobQuote |
| Two storey | £1,000 to £2,500 | FMB |
| Double storey, standard quality | £2,000 to £2,400 | Homebuilding and Renovating |
For the full picture, including MyBuilder's figures and regional uplifts, see our guide to extension cost per m².
When to get quotes
A calculator is a good way to test whether a project fits your budget, but only builders who have seen the site can price it properly. It is worth getting quotes once you know roughly what you want to build, ideally with sketch plans or drawings so that each builder prices the same thing. Ask for itemised quotes that state whether VAT is included and list anything left out, such as the kitchen or decoration.
To compare prices from builders in your area, fill in our short form or call 01273 115520 for up to 3 free quotes from checked local builders, free and with no obligation. Our guides to single storey extension cost and two storey extension cost cover fees, planning and timings in more depth.
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Frequently asked questions
How accurate is the extension cost calculator?+
It gives a range based on Less.co.uk's published rates per square metre, so it is a starting estimate rather than a quote. Your design, specification, ground conditions and access will all affect the real price, and only a builder who has seen the site can price it properly.
Does the calculator include VAT?+
No. Less.co.uk states that its figures exclude VAT and professional fees, and its regional figures also exclude interior fit out. Add these to the estimate when you plan your budget.
How do I use the calculator for a two storey extension?+
Add the floor area of both storeys together and enter the total, then choose the double storey option. Less.co.uk gives £1,200 to £2,000 per m² for a mid range double storey rear extension, as a Midlands average.
Why does the calculator use only one source?+
Using a single source means every option is measured on the same basis, so you can compare types like for like. Other sources publish different figures, which are shown in the comparison table on this page.
Sources for the prices on this page
Prices are published UK averages and include labour. Every job is different, so the quotes you receive will reflect your own property, access and materials.
- Less.co.uk: House Extension Cost Per m² in 2026, checked October 2026
- MyJobQuote: How Much Does a Single Storey Extension Cost in the UK?, checked October 2026
- MyJobQuote: Double Storey Extension Cost Guide (UK Prices 2026), checked October 2026
- Federation of Master Builders: Single-storey extension costs, a practical guide for homeowners, checked October 2026
- Federation of Master Builders: Two-storey extension costs, checked October 2026
- Homebuilding and Renovating: How much does an extension cost in 2026?, checked October 2026
- GOV.UK: Buildings and construction (VAT Notice 708), checked October 2026
- GOV.UK: Fees for planning applications in England from 1 April 2026, checked October 2026
- Adur and Worthing Councils: Guidance on Building Regulation Charges (from 1 April 2026), checked October 2026
- Arun District Council: Building control fees and charges (from 1 April 2026), checked October 2026
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Tell us about the job and up to 3 checked local builders will be in touch with quotes. There is no charge and no obligation. Or call 01273 115520.