What building work costs
How Much Does an Extension Cost? UK Prices by Size and Type
The cost of an extension starts with its size, then moves with the quality of the build, the finishes and how easy your site is to work on. Here are the UK prices for a small, medium and large extension, the cost per m², what a kitchen or bathroom adds, the hidden costs and contingency on top of the build, and how long the work takes.
House extension cost by size
A single storey extension costs £30,000 to £140,000, on Checkatrade’s house extension cost figures. The guide’s prices by size and quality exclude VAT.
| Extension | Size | Range | Average |
|---|---|---|---|
| Small, standard quality | 20m² | £40,000 to £56,000 | £48,000 |
| Medium, standard quality | 30m² | £60,000 to £84,000 | £72,000 |
| Large, standard quality | 50m² | £100,000 to £140,000 | £120,000 |
| Premium quality | 30m² | £75,000 to £110,000 | £92,500 |
The standard rows assume average quality, and at 30m² a premium build lifts the average from £72,000 to £92,500, on the same figures.
Rear extension cost on a terraced house
A rear extension on a terraced house costs £30,000 to £42,000 for a small 15m² build, £60,000 to £84,000 for a medium 30m² build and £100,000 to £140,000 for a large 50m² build, on Checkatrade’s figures. Side extensions and extensions to semi-detached houses are priced at £1,800 to £3,000 per m² in the same guide.
A rear extension builds out into the back garden, and a wraparound extension combines a rear and a side extension into one L-shaped space, Checkatrade’s single storey extension planning guide says.
Extension cost per m²
Checkatrade’s guide sets a shell-only build against a full extension per m², and prices a two-storey and a first-floor extension, all excluding VAT.
| Type | Based on | Range | Average |
|---|---|---|---|
| Shell only | Per m² | £1,200 to £1,700 | £1,450 |
| Full extension | Per m² | £1,800 to £3,000 | £2,400 |
| Two-storey extension | 60m² | £108,000 to £180,000 | £144,000 |
| First-floor extension, such as over a garage | 25m² | £45,000 to £75,000 | £60,000 |
Do not multiply a per m² rate by your floor area to price your own extension. The prices are not pro-rata, and a smaller extension can cost more per m² than a larger one, the guide says.
A double storey extension typically costs 50% to 70% more than a single storey one, not double, Checkatrade’s double storey guide says.
What a kitchen or bathroom adds
A kitchen adds £15,000 to £35,000 to the cost of an extension and a bathroom adds £5,500 to £9,000, excluding VAT, on Checkatrade’s figures.
On the average three-bed UK house, Nationwide research found that an additional bathroom can add 6% to the value, a double bedroom 12% and a double bedroom with an en-suite 23%, as reported by Checkatrade.
What affects the cost of an extension
Size comes first among the cost factors in Checkatrade’s house extension guide, because the build cost of materials and labour is usually the biggest cost. Ease of access, the quality of the build, the finishes and fittings, and the windows and doors move the price too.
Checkatrade’s prices are not location-based, and builders in London and the South East typically charge more, the guide says. The prices also assume:
- an extension to a normal two-storey house, at average market costs, specifications and construction methods
- average quality unless stated, with average electrical and plumbing costs
- good access, no specialist equipment and normal foundation depths with no complications
- no contaminated materials to remove and no refurbishment of the existing space
Poor access or complicated foundations take a site outside those assumptions, and outside the prices in the tables.
Hidden costs on top of the build price
House extension drawings by an architect or architectural technician cost roughly £350 to £950, with more to pay to submit a planning application, on Checkatrade’s figures. In many circumstances, permitted development rights let you extend a house without applying for planning permission, the Planning Portal says.
A kitchen is the biggest addition to a medium extension’s price.
A medium 30m² extension with a kitchen
| Medium 30m² extension, standard quality | £60,000 to £84,000 |
|---|---|
| Kitchen added to the extension | £15,000 to £35,000 |
Total £75,000 to £119,000
The total excludes VAT, on Checkatrade's figures. It leaves out the drawings, groundworks such as moving drains or removing tree roots, clearing the site, decorating and a contingency.
Work through the rest of the hidden costs on Checkatrade’s list before you settle the budget.
Hidden costs to price in
- Groundworks for your soil type, moving drains or removing tree roots
- Clearing trees, shrubs, awnings and patios from the space
- Removing any contaminated materials
- Telling your home insurer before work starts
- Decorating once the build is finished
Contingency for an extension
Checkatrade’s single storey extension planning guide recommends a contingency of 20% for unforeseen costs. Settle the budget with that money held back before you ask builders to quote.
Do not commit the whole budget to the build price
On a medium 30m² extension priced at £60,000 to £84,000, excluding VAT, a 20% contingency is £12,000 to £16,800. Without it, any unforeseen cost has to be paid from money outside your budget.
Comparing extension quotes
Write a brief before you contact builders, starting with why you want the extension, since it helps when you compare quotes, Checkatrade’s planning guide says. Make design decisions such as flooring and finishes up front too, so every builder prices the same job.
Writing the brief builders quote from
Do this
A 30m² single storey rear extension for a family kitchen and dining room, built to the attached drawings. Mid-range kitchen units, a tiled floor and glazed doors to the garden. Access is down the side of the house. Please price the kitchen and the decorating as separate lines.
Not this
Kitchen extension on the back of the house, how much roughly?
A builder typically adds around 15% to 25% to its labour and materials cost to project manage an extension, on Checkatrade’s figures, so ask whether each quote includes project management. The cost can often be spread across milestones in the build, the same guide says.
Among the Nottingham builders compared for extension work, South Notts Builders in Keyworth offers free home visits and advice for house extensions and can provide building work with an insurance-backed guarantee, and Edge Building & Construction in Burton Joyce builds rear, side and wraparound extensions.
How long an extension takes to build
Most single storey extensions take between 3 and 6 months from groundworks to completion, depending on the size, the weather and how quickly materials and tradespeople can be scheduled, Checkatrade’s planning guide says.
Delays are very common, especially with several contractors, Checkatrade’s house extension guide says. When the work is finished, get a completion certificate from building control so that selling the house later raises no issues, the same guide advises.
Common questions
How much does a single storey extension cost?
A single storey extension costs £30,000 to £140,000, on Checkatrade's figures. At standard quality, a 20m² extension costs £40,000 to £56,000, a 30m² extension £60,000 to £84,000 and a 50m² extension £100,000 to £140,000, excluding VAT.
How much does an extension cost per m²?
An extension costs £1,800 to £3,000 per m², with an average of £2,400, excluding VAT, on Checkatrade's figures. A shell-only build costs £1,200 to £1,700 per m². The prices are not pro-rata, and a smaller extension can cost more per m² than a larger one.
How much does a kitchen extension cost?
A medium 30m² kitchen extension costs £75,000 to £119,000, excluding VAT, on Checkatrade's figures, which is £60,000 to £84,000 for the extension plus £15,000 to £35,000 for the kitchen. Drawings, at roughly £350 to £950, and the other hidden costs come on top.
Does an extension add value to a house?
Yes, an extension may add 10% to 20% to a home's value, Checkatrade's house extension cost guide says. Checkatrade's single storey extension planning guide says a single storey extension typically adds 5% to 8%, and that a local estate agent can give a more specific estimate.
Do you need an architect for an extension?
No, an architect is not required, Checkatrade's single storey extension planning guide says, and for smaller, simpler extensions some homeowners work directly with a builder. An architect or a design-and-build company can turn your brief into plans and help coordinate the trades. Drawings from an architect or architectural technician cost roughly £350 to £950, on Checkatrade's figures.
How long does an extension take to build?
Most single storey extensions take 3 to 6 months from groundworks to completion, Checkatrade's single storey extension planning guide says. The time depends on the size, the weather and how quickly materials and tradespeople can be scheduled. Delays are very common, especially with several contractors, Checkatrade's house extension cost guide says.
Where the figures come from
- Checkatrade, house extension cost read 2026-09-17
- Checkatrade, single storey extension planning guide read 2026-09-17
- Checkatrade, double storey extension cost read 2026-09-17
- Planning Portal, extensions and permitted development read 2026-09-17
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