How much does a house extension cost in the UK?
Updated 19 October 2026 · Reviewed by Steve Wolfe
A single-storey extension in the UK typically costs £2,200 to £4,000 per square metre including VAT, so about £48,000 to £80,000 for 20 m² before a kitchen and fees. A two-storey extension costs about £80,000 to £150,000 for 40 m² over two floors. Size, glazing and where you live move the price most.
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Typical costs
Typical UK prices including VAT, for the build and a basic finish. Not including a kitchen, bathroom fittings or professional fees.
| Type | Typical cost |
|---|---|
| Small single-storey, about 15 m² £35,000–£60,000 For example 3 m deep and 5 m wide across the back of a terrace or semi. | £35,000–£60,000 |
| Single-storey, about 20 m² £48,000–£80,000 A typical kitchen-diner extension, 4 m by 5 m. | £48,000–£80,000 |
| Large single-storey, about 30 m² £70,000–£115,000 A wider or deeper rear extension, or a wraparound (side and rear). | £70,000–£115,000 |
| Two-storey, about 40 m² over two floors £80,000–£150,000 For example a side or rear extension with a room downstairs and a bedroom above. On site: 8 to 14 weeks | £80,000–£150,000 |
Our reading of the published guides listed under Sources, rounded and including VAT. Most guides quote per square metre before VAT (about £1,800 to £3,300), so check which you're comparing. Add professional fees and a contingency.
What changes the price
- Size. Most of the cost scales with floor area, so every extra square metre adds roughly £2,200 to £4,000. Small extensions cost more per square metre, because the fixed costs (foundations, the opening into the house, drainage) are spread over less space.
- Single or two storey. A second storey adds a lot of space for less than double the cost, because it shares the foundations and roof. HomeBuilding & Renovating puts it at up to twice the space for 50 to 70% more money.
- Glazing and the roof. Bifold doors and a roof lantern are popular and expensive. In our example below, they move the price from about £42,000–£58,000 to £55,000–£78,000.
- Opening up the back of the house. Knocking through for an open-plan room needs steel beams, and wider openings need more steel.
- The ground and drains. Poor ground, nearby trees or a drain under the footprint can mean deeper foundations or moving the drain.
- Access. If there's no side access, everything comes through the house or over the top, which takes longer.
What's usually not in the price
- A kitchen. Checkatrade puts a new kitchen in an extension at about £15,000 to £35,000 on top.
- A bathroom or shower room. About £5,500 to £9,000 (Checkatrade).
- Professional fees. Drawings, structural calculations, planning and building control often add 10 to 20% (HomeBuilding & Renovating says about 10%, the FMB 15 to 20%).
- A party wall surveyor, if a neighbour doesn't consent.
- A contingency. Keep at least 10% for things found once digging starts.
London and the regions
Materials cost much the same across the country; labour, access and parking are what change. Building cost data from BCIS puts inner London about 21% above the UK average and outer London about 14% above, with the South East about 7% above and the North East and Yorkshire about 11% below. Some guides suggest London costs 25 to 50% more, which is more than the BCIS figures show.
Before you get quotes
- Get drawings first, so every builder prices the same thing.
- Ask what's included: foundations, steel, drainage, the roof and glazing, electrics and heating, plastering, flooring and decorating.
- Check whether prices include VAT. Builders often quote before VAT.
- Get two or three quotes, and ask each builder how long it will take and how they'll handle access.
An example Ballpark
What Ballpark gave for this job on 16 October 2026. It’s an example we worked out, not a customer’s price.
Single-storey rear extension on a semi in Birmingham, about 4 m by 5 m, for a bigger kitchen-diner. Flat roof with a roof lantern and bifold doors. Kitchen not included.
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Basic flat-roof extension £42,000–£58,000
Shell, flat roof with fixed rooflights, standard sliding doors, plastering and basic electrics for 20 square metres.
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Extension with lantern, bifoldsClosest to the job £55,000–£78,000
Foundations, cavity walls, flat roof with roof lantern, bifold doors, plastering, electrics, underfloor-ready floor and basic finishing.
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Higher-spec extension £75,000–£100,000
As described with premium glazing, underfloor heating, structural steelwork for a wide opening, and high-end finishes.
Based on
- About 20 square metres, no major structural surprises
- Excludes kitchen units and appliances
- Excludes architect, planning and building control fees
- Mid-range bifold doors and lantern
What a trade will ask you
Have answers ready and your quotes (and your ballpark) will be closer to the real price.
- Single or two storey, and roughly how deep and wide?
- Rear, side or wraparound?
- What will go in it? A kitchen, a bathroom or WC, or just living space?
- What kind of roof and doors? Flat with a lantern, pitched, bifolds or sliding doors?
- Do you have drawings or planning permission yet?
- Is there side access to the back garden, or will everything come through the house?
Common questions
Do I need planning permission for an extension?
In England, often not for a modest one. A single-storey rear extension can usually go 3 m beyond the back of the original house (4 m if it's detached) and up to 4 m high under permitted development. You can go up to 6 m (8 m detached) if you tell the council first and the neighbours don't successfully object, which takes up to 42 days. Two-storey extensions must stay within 3 m of the back wall and at least 7 m from the rear boundary. Side extensions must be single-storey and no wider than half the house. Conservation areas and flats have tighter rules, and Wales and Scotland have their own, so check with your council.
Do I need building regulations approval?
Yes. An extension needs building regulations sign-off for the foundations, structure, insulation, drainage, fire safety and ventilation, whether or not it needs planning permission.
Do I need a party wall agreement?
If you're building on or at the boundary with a neighbour, or digging foundations near theirs, you must give them notice between two months and a year before work starts (England and Wales).
Can you build an extension for £50,000?
Often, yes, for a small single-storey extension of about 15 to 20 m² with a basic finish, outside London. Add a new kitchen, a lot of glazing or a two-storey design and it's unlikely.
Do I need an architect?
You'll need drawings for building regulations and usually for planning, plus structural calculations for any steel. An architect or an architectural technologist can do them. Budget roughly 10 to 20% of the build cost for fees.
How long does an extension take?
About 8 to 14 weeks on site for a two-storey extension, and less for a simple single-storey one, once drawings and any approvals are in place.
Are your prices including VAT?
Yes. Every price on this page includes VAT. Many builders and guides quote per square metre before VAT, so check which you're comparing.
What would yours cost?
Describe your extension and get a ballpark for your house in a couple of minutes. Free, and you don’t have to get quotes.
Get my ballparkSources
- HomeBuilding & Renovating, how much does an extension cost (Feb 2026). Per m² by finish level for single and two-storey extensions, before VAT, with a quantity surveyor
- Checkatrade, house extension cost guide (Jun 2026). Per m² and by size before VAT, plus what a kitchen or bathroom adds
- Checkatrade, double-storey extension cost (Aug 2026). Per m² before VAT and time on site
- Federation of Master Builders, single-storey extension costs (Mar 2026). Per m² range and professional fees
- BCIS location factors (December 2025), published in the Department for Education's school capital allocations 2026 to 2027 (Dec 2025). Regional building cost differences
- GOV.UK, permitted development rights for householders, technical guidance (Class A) (Sep 2019). Size limits for extensions in England
- GOV.UK, party walls and building work. Notice periods for work at the boundary