UK cost guide · September 2026
How much does a house extension cost in 2026?
Our example is for a 25 m² extension; yours may be smaller, so pace it out for your own figure. A mid-range single-storey rear extension of 25 m² costs about £63,500 to £78,000 for the building work, including VAT, in an average UK area outside London (£2,500 to £3,100 per m²). Fees and approvals add £4,300 to £8,800 and a 15% contingency adds £9,500 to £11,700, so plan for £77,500 to £98,500 in all for 25 m², a room about 5 m by 5 m. Double-storey extensions cost less per m² (£2,300 to £2,900), wraparounds more (£2,800 to £3,500). London is about 35% higher.
An extension usually needs three people: an architect or designer to draw it, a structural engineer to work out the steel beams and foundations, and a builder. Get drawings first, then ask at least three builders to quote on the same drawings, so every price covers the same work.
Work out your house extension cost
Loading the calculator…
Already have a builder’s price? Check where your quote sits against this range. Still deciding what to do first? Read where to start a home renovation.
House extension prices in 2026
Ranges are for a typical home outside London, including VAT, before fees and contingency. Use the calculator for your size, region and finish.
| Type | Basic | Mid-range | Premium |
|---|---|---|---|
| Single-storey rear (25 m²) | £51,500 to £63,000 | £63,500 to £78,000 | £81,500 to £99,000 |
| Double-storey (40 m²) | £72,000 to £91,000 | £91,000 to £115,000 | £120,000 to £149,000 |
| Wraparound or side return (30 m²) | £68,500 to £83,000 | £83,000 to £104,000 | £108,000 to £133,000 |
Single-storey rear house extension cost by size
| Floor area (example size) | Basic | Mid-range | Premium | Mid-range per m² |
|---|---|---|---|---|
| 6 m² (room 3 m × 2 m) | £19,700 to £24,000 | £22,500 to £28,000 | £27,000 to £33,000 | £3,800 to £4,600 |
| 9 m² (room 3 m × 3 m) | £24,500 to £30,500 | £29,000 to £36,000 | £35,500 to £43,500 | £3,200 to £4,000 |
| 12 m² (room 4 m × 3 m) | £30,000 to £36,500 | £35,500 to £43,500 | £44,000 to £54,000 | £3,000 to £3,600 |
| 15 m² (room 5 m × 3 m) | £35,000 to £42,500 | £42,000 to £51,500 | £53,000 to £64,000 | £2,800 to £3,400 |
| 18 m² (room 6 m × 3 m) | £40,000 to £48,500 | £48,500 to £59,500 | £61,500 to £74,500 | £2,700 to £3,300 |
| 20 m² (room 5 m × 4 m) | £43,000 to £53,000 | £53,000 to £65,000 | £67,000 to £81,500 | £2,600 to £3,200 |
| 24 m² (room 6 m × 4 m) | £50,000 to £61,000 | £61,500 to £75,500 | £78,500 to £95,500 | £2,600 to £3,100 |
| 30 m² (room 6 m × 5 m) | £60,000 to £73,000 | £74,500 to £91,000 | £96,000 to £116,000 | £2,500 to £3,000 |
| 40 m² (room 8 m × 5 m) | £77,000 to £93,500 | £96,000 to £118,000 | £125,000 to £151,000 | £2,400 to £2,900 |
| 50 m² (room 10 m × 5 m) | £93,500 to £114,000 | £118,000 to £144,000 | £154,000 to £186,000 | £2,400 to £2,900 |
Sizes are internal floor area. Small extensions cost more per m² because some costs, like opening up the back wall and connecting the drains, don't shrink with size. Our ranges allow for that. Building work only: add fees and a contingency (see what to budget below), or enter your own size in the calculator.
Building 3 m or 6 m deep? Our 3 metre and 6 metre extension guides price each width by its outside size, which is how planning limits are measured in England, Wales and Scotland, and set out what to budget and how deep you can build without planning permission.
House extension costs by region
Labour costs vary across the UK far more than materials. House extension prices in London run about 35% above the UK average outside London, and in Northern Ireland about 18% below it.
| Region | Mid-range cost | Per m² |
|---|---|---|
| UK average (outside London) | £63,500 to £78,000 | £2,500 to £3,100 |
| London | £86,000 to £105,000 | £3,400 to £4,200 |
| South East | £73,000 to £89,500 | £2,900 to £3,600 |
| East of England | £68,000 to £83,500 | £2,700 to £3,300 |
| South West | £66,000 to £81,000 | £2,600 to £3,200 |
| West Midlands | £61,500 to £75,500 | £2,500 to £3,000 |
| East Midlands | £60,500 to £74,000 | £2,400 to £3,000 |
| Scotland | £60,500 to £74,000 | £2,400 to £3,000 |
| North West | £59,000 to £72,500 | £2,400 to £2,900 |
| Yorkshire and the Humber | £58,500 to £72,000 | £2,300 to £2,900 |
| Wales | £57,000 to £70,000 | £2,300 to £2,800 |
| North East | £56,000 to £68,500 | £2,200 to £2,700 |
| Northern Ireland | £52,000 to £64,000 | £2,100 to £2,600 |
Which region is my town in? Brighton, Oxford and Southampton are in the South East; Cambridge and Norwich in the East of England; Bristol, Bath and Exeter in the South West; Birmingham and Coventry in the West Midlands; Nottingham and Leicester in the East Midlands; Glasgow and Edinburgh in Scotland; Manchester and Liverpool in the North West; Leeds, Sheffield and Hull in Yorkshire and the Humber; Cardiff and Swansea in Wales; Newcastle and Sunderland in the North East; and Belfast in Northern Ireland. Somewhere else? Type your postcode into the calculator above and it says which region’s prices it uses.
Want it for your area? See local prices and council building control fees for 92 council areas in England and Wales (not every council yet), or extension costs in Scotland, with building warrant fees.
Fees and approvals
Design, engineering and approvals come on top of the building work. Building control: 47 of the 91 councils we checked in England and Wales charge between £740 and £1,000 for a 10 to 40 m² extension, so the table shows that typical range; your council’s own charge is on its local page, and every council is on building control fees by council. We have local pages for 92 council areas, but Winchester publishes only building notice charges, so the full plans figures cover 91.
| Fee | Typical cost | Notes |
|---|---|---|
| Measured survey | £600 to £1,800 | |
| Building regulations drawings | £2,100 to £3,700 for 25 m² | Grows with floor area: £1,400 to £2,500 for 6 m² and £3,400 to £5,800 for 60 m² or more. |
| Planning drawings | £1,300 to £2,400 for 25 m² | Grows with floor area: £1,000 to £1,900 for 6 m² and £1,700 to £3,200 for 60 m² or more. Only with a planning application. |
| Structural engineer | £870 to £2,300 for 25 m² | Grows with floor area: £530 to £1,200 for 6 m² and £1,100 to £3,200 for 40 m² or more. |
| Building control (council, full plans) | £740 to £1,000 | About half the councils we checked in England and Wales (47 of 91) charge within this range for a 10 to 40 m² extension; current schedules run from £587 to £1,876. |
| Householder planning application (England) | £548 | Only if your project needs planning permission. Optional. |
| Prior approval for a larger home extension (England) | £249 | Only for a single-storey rear extension in England going more than 3 m beyond the back of the original house (4 m if detached), up to 6 m (8 m if detached), and not on protected land such as a conservation area. You use it instead of a planning application, not as well. Optional. |
| Party wall surveyor (per neighbour, one surveyor agreed by both sides) | £1,100 to £1,800 | If your neighbour wants their own surveyor, the cost roughly doubles, and the person having the work done normally pays both. Nothing to pay if your neighbour agrees in writing. Optional. |
The calculator adjusts design and engineer fees for your region, so its figures can differ slightly from this table. Council pages use that council’s own building control charge.
Extension budgets for 20, 30 and 40 m²
The building work is only part of what an extension costs. These budgets add the fees and approvals most projects need (a measured survey, building regulations drawings, a structural engineer and council building control) and a 15% contingency, for the same single-storey rear extension at three sizes. Small extensions cost more per m² because some costs, like opening up the back wall and connecting the drains, don't shrink with size. Our ranges allow for that. Drawings and the structural engineer grow with the size of the extension: building regulations drawings cost about £1,400 to £2,500 for 6 m² and £2,600 to £4,600 for 40 m², and the engineer £530 to £1,200 and £1,100 to £3,200, including VAT, UK average. Planning drawings, about £1,000 to £1,900 for 6 m² and £1,400 to £2,800 for 40 m², are needed only if the extension needs a planning application; most single-storey rear extensions within permitted development don’t.
| Floor area | Building work | Fees and approvals | 15% contingency | Budget to plan for |
|---|---|---|---|---|
| 20 m² | £53,000 to £65,000 | £4,000 to £8,200 | £7,900 to £9,700 | £65,000 to £82,500 |
| 30 m² | £74,500 to £91,000 | £4,600 to £9,400 | £11,200 to £13,700 | £90,000 to £114,000 |
| 40 m² | £96,000 to £118,000 | £5,100 to £10,600 | £14,400 to £17,600 | £116,000 to £146,000 |
Each figure is rounded on its own, so a row can differ from its parts by a few hundred pounds. A planning application (£548 in England) with its planning drawings, only if you need one, and a party wall surveyor (only if you build on or near a shared wall) are not in these budgets. Open a size in the calculator: 20 m², 30 m² or 40 m².
If the extension includes a new kitchen, add £9,000 to £16,000 for a mid-range kitchen of typical size, supplied and fitted. A 30 m² kitchen extension then comes to £100,000 to £133,000 in all. Appliances beyond a basic set, furniture and anything else you buy yourself are not in these figures, so list them separately in your budget.
Two-storey extensions: what the floor area means
Price per m² for a two-storey extension is quoted on the total new floor area over both storeys, not the area of one floor. A two-storey extension with a room 5 m by 4 m inside on each floor adds 40 m² of floor.
| Extension | Building work | Per m² of floor | Budget to plan for |
|---|---|---|---|
| Single-storey, 20 m² | £53,000 to £65,000 | £2,600 to £3,200 | £65,000 to £82,500 |
| Two-storey, 40 m² (20 m² on each floor) | £91,000 to £115,000 | £2,300 to £2,900 | £110,000 to £143,000 |
| Single-storey, 40 m² | £96,000 to £118,000 | £2,400 to £2,900 | £116,000 to £146,000 |
A second storey costs less per m² because the foundations, roof and site setup serve twice the floor area. It still costs more in total, and it often needs planning permission and a larger fee for drawings.
A sample extension quote, annotated
This is a hypothetical quote for a 30 m² single-storey rear extension, written the way many real quotes are. The price looks reasonable, but it leaves out several things you will still pay for.
Hypothetical quote: rear extension, 30 m², to the approved drawings
Building work, including steel beam and opening into the house£79,500
Provisional sum: alterations to drainage£2,500
Total including VAT£82,000
Excludes: kitchen supply and fitting, structural engineer, building control fees. Client to hold a contingency.
| On the quote | What it means | Add to your budget |
|---|---|---|
| £82,000 for the building work | Inside the typical range for 30 m² of building work (£74,500 to £91,000), so the price itself is not a warning sign. | Nothing extra |
| Provisional sum of £2,500 for drainage | A provisional sum is an allowance, not a fixed price. The final cost depends on what the builder finds. Ask what it covers and how it will be adjusted. | Could go up or down |
| Excludes the kitchen | The new room needs a kitchen the quote does not price. | £9,000 to £16,000 |
| Excludes the structural engineer | Steel beam and foundation calculations are needed for building control. | £960 to £2,600 |
| Excludes building control fees | Paid to the council (or a private building control firm), not the builder. | £740 to £1,000 |
| Building regulations drawings and survey | Already paid or due to your designer, so not on a builder's quote. | £2,900 to £5,800 |
| No contingency | Hold 15% of the building work and kitchen for things nobody can see yet. | £13,700 to £14,700 |
Budget to plan for this job: £109,000 to £122,000, against a quote of £82,000. Each figure is rounded on its own, so a row can differ from its parts by a few hundred pounds. Compare quotes on the same basis with the quote comparison spreadsheet, and see the costs that most often sit outside a quote in hidden costs of an extension.
Have a real quote? Check where it sits for your size and area, or read is my extension quote fair? Kitchen prices by size are in the kitchen cost guide, and your council's building control charge is on its local page.
What's included in these prices
Included
- Foundations, walls, roof, insulation and weatherproofing
- Windows and external doors to suit the finish (standard at Basic, better at Mid-range, high quality at Premium)
- First fix and second fix electrics (the wiring before plastering, then the sockets and lights after), heating and plumbing to the new space. An old fuse board may need replacing to take the new circuits, and the electrical work needs an Electrical Installation Certificate and building regulations notification.
- Plastering, flooring and decoration to a finished shell
- Opening into the existing house (steel beam included in the rate)
- Site setup (scaffolding, protection and site welfare, such as a toilet for the workers), skips and removal of the soil dug out
Not included
- A fitted kitchen (add it in the calculator)
- Design, engineering, building control and planning fees (shown separately)
- Party wall surveyors, if your neighbour needs one
- The extra cost of bifold or sliding doors and roof lanterns (glazed boxes set on a flat roof). The price allows for ordinary windows and doors to suit the finish, not bifolds, sliding doors or a lantern, so an upgrade costs the difference
- Energy calculations (SAP) if the windows, glazed doors and rooflights come to more than a quarter of the new floor area: in England building control asks for them then (council building control guide)
- Principal designer duties if more than one firm works on the job: under the CDM 2015 regulations the designer who coordinates the project usually takes them on, so ask whether their fee covers it (HSE)
- Landscaping, driveways and work elsewhere in the house
- Your contingency
What drives the cost
- Size and shape. Small extensions cost more per m² because some costs, like opening up the back wall and connecting the drains, don't shrink with size. Our ranges allow for that. Simple rectangles cost less than L-shapes and wraparounds.
- Glazing. Wide sliding doors, bifolds and roof lanterns are the biggest upgrade. An aluminium bifold door set about 4 m wide costs about £5,200 to £6,800 supplied and fitted, including VAT. The building work already allows for standard doors in that opening, so the upgrade costs the difference.
- Ground conditions and drains. Clay soil, nearby trees or a drain under the footprint can mean deeper foundations, or a build-over agreement (your water company’s permission to build over or near its sewer).
- Access. Terraced houses with no side access mean materials come through the house, which adds labour and time.
- Specification. The finish you choose moves the price a long way: flooring, kitchen, lighting and heating all count.
- Where you live. London prices run about 35% above the UK average outside London and the South East about 15%. The North East, Wales and Northern Ireland are below that average.
Planning, regulations and VAT
Many single-storey rear extensions in England are permitted development, meaning they can be built without a planning application, within size limits set out by the Planning Portal. Those rights don’t cover every house: a flat, or a house where the council has removed them, needs an application. Larger rear extensions can use the prior approval route (fee £249), which you can tick in the calculator for a single-storey rear extension, in place of a planning application. If you do need planning permission, a householder application in England costs £548.
Almost every extension needs building regulations approval (a small conservatory can be exempt if it meets the Planning Portal’s conditions), usually on full plans, meaning your drawings are checked and approved before work starts. Electrical work is notifiable under Part P of the building regulations in England and Wales: it must be reported to building control, which an electrician registered with a competent person scheme (a government-approved scheme whose members can sign off their own work) does for you.
Party wall. In England and Wales, if you build on or at the boundary, cut into a shared wall (for example to rest a steel beam on it) or dig foundations close to a neighbour’s, you must give notice to every neighbour affected: one, or two if the work touches walls on both sides, which on a terrace usually means both. If a neighbour agrees in writing within 14 days, there is nothing to pay. If they don’t reply, or don’t agree, a party wall surveyor sets out how the work will be done: one surveyor agreed by both sides costs about £1,100 to £1,800 for each neighbour, including VAT. If a neighbour wants their own surveyor, the cost roughly doubles, and the person having the work done normally pays both (GOV.UK explanatory booklet). The Party Wall etc. Act 1996 does not apply in Scotland or Northern Ireland (the Act says where it applies).
Checklist: what to ask builders to confirm
Print this or save it as a PDF, one copy for each quote. Mark each line included, separate or not confirmed as quotes come in. An empty amount means unknown, not free.
| Item | What to confirm | Included? Amount? |
|---|---|---|
| Design and investigations | Drawings, measured survey, structural calculations and any trial holes (small holes dug to check the ground before the foundations are designed). | |
| Permissions | Planning (if needed), building control and party wall agreements. | |
| Site setup | Access, protection of the existing house, demolition, skips and waste. | |
| Groundworks and drains | Groundworks (digging out and the foundations), ground conditions and any drain diversions (moving a drain that runs where the extension will go). | |
| Structure and envelope | The envelope is everything that keeps the weather out: walls, steel beam, roof, insulation, windows and doors. | |
| Services | Heating, electrics, plumbing, ventilation and changes to existing systems. | |
| Fixtures and finishes | Kitchen, flooring, joinery (doors, skirting and other woodwork), lighting and decoration. | |
| External work | Steps, paving, drainage outside doors and garden reinstatement. |
Questions people ask
How much does a 4 x 5 metre extension cost?
Measured outside, the way planning limits are measured in England, Wales and Scotland, a single-storey extension 4 m deep and 5 m wide gives a room of about 15.5 m² and costs about £43,000 to £53,000 for a mid-range finish, including VAT, in an average UK area outside London, before fees (September 2026 prices). If 4 m × 5 m is the room inside (20 m²), it costs about £53,000 to £65,000. Small extensions cost more per m² because some costs, like opening up the back wall and connecting the drains, don't shrink with size. Our ranges allow for that.
Is it cheaper to build a double-storey extension?
Per square metre, yes. A mid-range double-storey extension costs about £2,300 to £2,900 per m² including VAT against £2,500 to £3,100 for single-storey, because the foundations and roof serve twice the floor area.
Do extension prices include VAT?
Most building work carries VAT at 20%. Ask each builder to confirm whether their price includes it. Our figures include it by default. To see them without VAT, choose Before VAT under VAT in the calculator. Small builders below the VAT threshold may not charge it on their work, though they still pay it on materials: choose The builder isn't VAT registered to see that price.
How much does an extension cost in London?
About £86,000 to £105,000 for a 25 m² mid-range single-storey extension, including VAT and before fees. That is roughly 35% above the UK average outside London.
Does an extension add value?
Often, but not always by more than it costs. Check sold prices for extended houses on your street before you commit, and speak to a local estate agent.
How much does a 30 m² extension cost?
A 30 m² single-storey rear extension costs about £74,500 to £91,000 for the building work, mid-range and including VAT, in an average UK area outside London (September 2026 prices). With fees, approvals and a 15% contingency, plan for £90,000 to £114,000. Add £9,000 to £16,000 if it includes a new kitchen.
How much does a 6 metre extension cost?
It depends on the width. Measured outside, as the planning limits are in England, Wales and Scotland, a single-storey rear extension 6 m deep and 4 m wide has about 18.5 m² of floor inside and costs about £49,500 to £61,000 for the building work, mid-range and including VAT, in an average UK area outside London (September 2026 prices). See 6 metre extension costs for other widths, what to budget and prior approval.
What does an extension price per m² include?
Our per m² prices cover the finished shell: foundations, walls, roof, insulation, windows and doors to suit the finish, electrics, heating, plastering, flooring and decoration, plus the opening into the house. They do not include a kitchen, professional fees, building control or your contingency.
Sources and method
Our ranges combine published 2026 UK cost guides, industry data and our own survey of council fees. We check them every quarter. They are budget ranges, not quotes: always get at least three detailed written quotes. Read how Estiquote estimates work, or download the full cost data.
- Checkatrade: House extension cost guide (June 2026)
- Federation of Master Builders: Single-storey extension costs (March 2026)
- Homebuilding & Renovating: How much does an extension cost (February 2026)
- Build It: Extension cost guide 2026 (August 2026)
- MyJobQuote: House extension cost guide (August 2026)
- Federation of Master Builders: Two-storey extension costs (January 2026)
- Checkatrade: Side return extension cost (September 2026)
- Harper Latter Architects: Wraparound extension cost guide (April 2026)
- Harper Latter Architects: Measured building survey cost (August 2026)
- Checkatrade: Architect fees (an older source: page updated September 2024, prices January 2025)
- HomeOwners Alliance: Architect fees (January 2026)
- MyJobQuote: Architect prices (September 2026)
- MyJobQuote: Structural engineer cost (September 2026)
- Estiquote: Building control fees by council (September 2026)
- Planning Portal: Application fees in England from 1 April 2026
- Checkatrade: Party wall surveyor cost (June 2026)