Quoteio’s free calculator prices a standard 20m² single storey rear extension at £54,000 to £62,000 including VAT and professional fees at West Midlands rates, and £71,000 to £88,500 in inner London. It builds each price part by part from rates on 41 completed Living Design & Build jobs, then adjusts it for your postcode. This guide explains how the number is worked out, what it covers and what it leaves out, so you know how far to trust it.
- Single storey rear, 5m × 4m: £54,000 to £62,000
- Single storey side return, 6m × 2.5m: £50,000 to £57,500
- Double storey rear, 5m × 4m on each floor: £86,500 to £99,500
- Kitchen extension, 6m × 4m with bifolds and a lantern, mid spec: £71,500 to £82,500
Price your own extension in a few minutes, or see prices by region.
How does the calculator work out a price?
Most online calculators multiply your floor area by one price per square metre. That is quick, but it hides the fact that a small extension still needs footings, a roof, drainage and electrics, so its cost per square metre is much higher. We price the parts of a build instead:
- Structure from your measurements. Your length and width give the floor area, the perimeter and the wall area. Footings are priced per linear metre, the slab and screed per square metre, walls by area after openings, and the roof by type.
- Services from real jobs. Electrics and plumbing are priced from a regression across those jobs: a fixed start-up cost plus a cost per square metre, because a small room still needs a new circuit and pipework.
- Labour is added at the ratio we see between labour and materials on real final accounts.
- Extension type. A double storey or over-garage extension carries a multiplier for the extra structure.
- Your rooms, windows and doors. A kitchen, bathroom, bifolds or a roof lantern are added at their own prices.
- Your postcode. The total is adjusted with our cost index, which covers all 121 UK postcode areas.
- Fees and VAT. Drawings, a structural engineer, building regulations, the planning fee and 20% VAT are added.
The full method, including the index for every region, is on our methodology page.
What does a 20m² extension break down into?
Here is the calculator’s own breakdown for a standard spec 5m × 4m single storey rear extension at West Midlands rates. Trade lines are ex VAT, with VAT added at the end.
| Item | Cost |
|---|---|
| Foundations & slab | £4,300 |
| Screed | £1,700 |
| External walls | £4,406 |
| Roof | £3,100 |
| Fascias & guttering | £1,980 |
| Plastering | £1,800 |
| Skirting & architrave | £630 |
| Scaffold | £819 |
| Drainage | £575 |
| Electrics | £4,488 |
| Plumbing | £4,713 |
| Labour | £16,251 |
| Architectural drawings | £1,200 |
| Structural engineer | £800 |
| Building regulations | £500 |
| Planning application | £528 |
| Subtotal ex VAT | £47,790 |
| VAT at 20% | £9,558 |
| Calculated total | £57,348 |
| Range shown | £54,000 to £62,000 |
Structure only: no kitchen, no glazed doors, no lantern. Add those in the calculator and they appear as their own lines.
Why do you get a range rather than one number?
No honest estimate is a single figure. Ground conditions, access and the builder you choose all move the final price, so we show the calculated total as a range. How wide it is depends on how much evidence sits behind it:
| Where the extension is | Range for a 20m² rear extension |
|---|---|
| West Midlands, where our job data comes from 6% below to 8% above | £54,000 to £62,000 |
| Elsewhere in the UK, priced from the index (South West shown) 8% below to 10% above | £55,500 to £66,000 |
| Inner London 10% below to 12% above | £71,000 to £88,500 |
The range runs a little further above the calculated price than below it, because overruns are more common than savings on real jobs.
How do extension prices compare across the UK?
The same two projects priced for every region. Use the region links for the full picture where you live, for example the East Midlands or the North East.
| Region | 20m² single storey rear | 24m² kitchen extension, mid spec |
|---|---|---|
| Inner London | £71,000 to £88,500 | £93,000 to £115,500 |
| Outer London | £64,000 to £79,500 | £84,000 to £104,500 |
| South East | £59,500 to £71,000 | £78,500 to £94,000 |
| East of England | £57,000 to £68,000 | £75,500 to £90,000 |
| South West | £55,500 to £66,000 | £73,500 to £88,000 |
| East Midlands | £52,000 to £62,500 | £69,500 to £83,000 |
| West Midlands | £54,000 to £62,000 | £71,500 to £82,500 |
| North West | £52,000 to £62,500 | £69,500 to £83,000 |
| Yorkshire and the Humber | £50,500 to £60,500 | £67,500 to £81,000 |
| North East | £49,500 to £59,500 | £66,500 to £79,500 |
| Wales | £50,500 to £60,500 | £67,500 to £81,000 |
| Scotland | £52,500 to £63,000 | £70,000 to £83,500 |
| Northern Ireland | £47,500 to £56,500 | £63,500 to £76,000 |
How do you use the calculator?
It takes a few minutes and seven short steps:
- Choose the project: extension, loft conversion, garage conversion, new build, renovation or landscaping.
- Pick the extension type. You can tick more than one, for example a rear and a side extension for a wraparound.
- Enter the size. Measure the outside footprint of each part in metres. If you are unsure, pace it out in the garden; a rough figure still gives a useful range.
- Add the rooms going inside, such as a kitchen, utility, WC or bathroom.
- Add windows and doors: bifolds, French doors, a roof lantern, Velux windows.
- Pick a spec level. Standard, mid or high changes the walls, roof, glazing and fittings.
- Enter your postcode and email. The postcode sets your local prices; the email is where we send a copy. There is no account to create.
You see the range straight away, with every line of the breakdown. Start your estimate.
Which answers change the price most?
If you want to see what moves the number, run the calculator a few times and change one answer each time. The biggest swings usually come from:
- Going up a storey. The double storey rear extension above costs £86,500 to £99,500 against £54,000 to £62,000 for the single storey version of the same footprint: more, but far less than double, because the foundations, drainage and much of the roof are shared.
- Putting a kitchen inside it. The kitchen, bifolds and lantern are what take the 24m² kitchen extension to £71,500 to £82,500.
- Spec level. Facing brick and a pitched slate roof at high spec cost noticeably more than block walls and a warm flat roof at standard.
- Your postcode, as the regional table shows.
What does the estimate leave out?
The calculator prices the extension itself. It does not include, unless you add them:
- Ground conditions nobody can see yet: poor soil, tree roots, a sewer under the footprint.
- Knocking through to the existing house, with a steel beam and making good.
- Moving a boiler, gas meter or soil pipe.
- Kitchen appliances, flooring, decorating and furniture.
- Landscaping after the build.
A builder will confirm these after a site visit. Ask them to price the same items the estimate lists, so you can see exactly where any difference comes from.
How should you use the number?
Treat it as a budget before you spend money on drawings, and as a sense check when quotes arrive. If three builders all come in well above the range, ask what they have included that the estimate hasn’t. If one comes in far below it, ask what they have left out. For a deeper look at what each type of extension costs, read our UK extension cost guide and the single storey extension guide.
Common questions
Is the Quoteio extension cost calculator free?
Yes. There is no charge and no account to create. We ask for your name, email and postcode at the end so we can price your area and send you a copy of the estimate.
How accurate is an online extension cost calculator?
It gives you a realistic budget range, not a quote. Ours is built from trade-level rates on completed jobs and adjusted for your postcode, and the range is tighter where we hold our own job data (6% below to 8% above in those postcodes) and wider elsewhere. A builder can only firm it up after seeing the site.
Does the estimate include VAT and fees?
Yes. Every extension estimate includes 20% VAT, architectural drawings, a structural engineer, building regulations and the householder planning fee for your nation. A party wall surveyor is added for side extensions, which are usually on or near a boundary.
Why does the same extension cost more in London?
Labour, materials, site access and parking all cost more in London, and party wall work is more common. A 20m² single storey rear extension that costs £54,000 to £62,000 in the West Midlands comes out at £71,000 to £88,500 in inner London.