How Much Does a House Extension Cost? (UK 2026 Guide)
Adding an extension is one of the most effective ways to gain space and value without moving, but the first question everyone asks is the hardest to answer online: how much does a house extension cost? The honest answer is that it depends on the type, size, specification and where you live. This 2026 guide gives you realistic UK price ranges by extension type, by footprint and by region, the cost per m² figures builders actually use, every fee and hidden cost most online estimates leave out, three worked examples and what it all means if you are extending in Southampton. Use it to set a budget with your eyes open before you speak to anyone.
House extension costs at a glance (2026)
| Extension type | Typical size | Typical build cost (ex VAT) | Cost per m² |
|---|---|---|---|
| Side return | 8–15 m² | £30,000 – £60,000 | £2,500 – £4,000 |
| Single-storey rear | 15–25 m² | £40,000 – £90,000 | £1,800 – £3,300 |
| Kitchen extension (incl. kitchen) | 20–30 m² | £60,000 – £130,000 | £2,500 – £4,500 |
| Wrap-around | 25–40 m² | £70,000 – £140,000 | £2,200 – £3,500 |
| Double-storey | 30–50 m² (both floors) | £70,000 – £180,000+ | £1,800 – £3,200 |
| Lean-to / small rear | 6–10 m² | £20,000 – £35,000 | £2,800 – £4,000 |
The national average for a single-storey extension is commonly quoted at around £50,000 to £55,000. That figure hides an enormous spread: the same 20 m² footprint can cost £36,000 or £75,000 depending on region, the ground, the structure, the glazing and the finish. The rest of this guide explains where that spread comes from.
Cost per m²: the number builders actually use
Most builders think in cost per square metre. For 2026 the working ranges are:
| Finish level | Cost per m² (ex VAT) | What you get |
|---|---|---|
| Basic | £1,500 – £2,000 | Simple rectangular footprint, standard windows, basic finishes, no structural work to the existing house |
| Standard | £2,000 – £3,000 | Good glazing, steel opening into the house, quality plaster and flooring, underfloor heating as an option |
| Premium | £3,000 – £4,500+ | Large-span glazing, roof lantern, high-specification finishes, complex structure or awkward ground |
| Shell only | £1,300 – £1,900 | Watertight structure with no internal fit-out, for finishing yourself or in stages |
Per-m² is a useful sanity check, but it flatters larger extensions and penalises small ones. A lot of the cost is fixed: you need foundations, a roof, drainage and a connection to the existing house whether the extension is 10 m² or 30 m². That is why a compact side return often costs more per m² than something twice its size.
Cost per m² by region
Labour and material rates vary widely across the UK. As a guide for a standard-finish single-storey extension in 2026:
| Region | Cost per m² (ex VAT) | Compared with national average |
|---|---|---|
| London | £2,500 – £4,500 | +20 to 40% |
| South East (incl. Hampshire, Surrey, Sussex) | £2,000 – £3,500 | +10 to 20% |
| South West and East of England | £1,800 – £3,000 | +5 to 10% |
| Midlands | £1,700 – £2,800 | around average |
| North of England | £1,500 – £2,500 | -10 to 15% |
| Scotland and Wales | £1,600 – £2,700 | -5 to 15% |
Southampton and the wider Hampshire area sit in the South East band, which is why our local figures further down are above the headline national numbers.
Extension cost by size
If you already know roughly how big you want to go, this is the quickest way to get a ballpark. These are single-storey figures for a standard finish, excluding VAT, fees and kitchen:
| Footprint | Floor area | Typical build cost (ex VAT) |
|---|---|---|
| 3m x 3m | 9 m² | £20,000 – £30,000 |
| 3m x 4m | 12 m² | £25,000 – £38,000 |
| 4m x 4m | 16 m² | £30,000 – £45,000 |
| 4m x 5m | 20 m² | £36,000 – £55,000 |
| 4m x 6m | 24 m² | £42,000 – £65,000 |
| 5m x 6m | 30 m² | £50,000 – £80,000 |
| 6m x 7m | 42 m² | £65,000 – £105,000 |
| 50 m² double-storey (25 m² per floor) | 50 m² | £80,000 – £150,000 |
Notice how the cost per m² falls as the footprint grows. A 3m x 3m extension can cost over £3,000 per m²; a 42 m² one can come in under £2,000 per m² for the same specification.
Single-storey extension costs
Single-storey extensions are the most common project in the UK and the most varied in price.
Rear extensions
The classic project: extend the back of the house by three to six metres to create a kitchen-diner or family room. Expect £40,000 to £90,000 for 15 to 25 m² at a standard finish. Rear extensions are the most likely to fall under permitted development and the easiest to build because access is usually straightforward.
Side-return extensions
Common on Victorian and Edwardian terraces, a side return fills the narrow strip beside the original kitchen. At 8 to 15 m² they are small but structurally busy, usually needing a steel to open the side of the house, a party wall agreement with the neighbour and careful drainage, so budget £30,000 to £60,000 and a high cost per m².
Wrap-around extensions
A wrap-around combines a rear and side extension in an L-shape and typically creates 25 to 40 m² of new space for £70,000 to £140,000. They rarely qualify as permitted development and almost always need a full planning application.
Lean-to and small rear extensions
A modest 6 to 10 m² lean-to for a utility room or a bigger kitchen costs £20,000 to £35,000. Small, but the fixed costs of foundations and roof mean they are never as cheap as their size suggests.
Double-storey extension costs
A double-storey extension adds a ground-floor room and a bedroom or bathroom above for £70,000 to £180,000 or more depending on size and specification. Because the second storey shares the foundations and roof with the first, a two-storey build usually costs 50 to 70 per cent more than a single-storey extension of the same footprint rather than double, which makes it the best value per m² you can get. The trade-offs are that double-storey extensions usually need full planning permission and take longer, and a bathroom upstairs adds fit-out cost.
First-floor and over-garage extensions
Building on top of an existing single-storey part of the house or a garage avoids new foundations if the existing ones are adequate, which an engineer must confirm. Expect £40,000 to £90,000 for a typical bedroom-sized room, with the risk that the existing foundations need strengthening.
Kitchen extension costs
Most rear extensions are really kitchen extensions, and the kitchen is where the budget can double. A 20 to 30 m² kitchen extension costs £60,000 to £130,000 with the kitchen included. Breaking that down:
- Build: £40,000 to £80,000
- Kitchen units, worktops and appliances: £10,000 to £40,000 or more, depending on specification
- Kitchen services: plumbing, extraction, underfloor heating and electrics, £4,000 to £10,000
If you are weighing up the kitchen itself, our guide to the full house renovation cost covers kitchen and bathroom fit-out prices in more detail.
Where the money goes: a typical cost breakdown
Understanding the split helps you see where a quote is padded and where it is not:
| Element | Share of build cost | On a £60,000 build |
|---|---|---|
| Groundworks and foundations | 10 – 15% | £6,000 – £9,000 |
| Structure, walls and steel | 20 – 25% | £12,000 – £15,000 |
| Roof | 10 – 15% | £6,000 – £9,000 |
| Windows, doors and glazing | 8 – 12% | £5,000 – £7,000 |
| Electrics, plumbing and heating | 10 – 12% | £6,000 – £7,000 |
| Internal finishes (plaster, floors, decoration) | 20 – 25% | £12,000 – £15,000 |
| Professional fees and approvals | 5 – 8% | £3,000 – £5,000 |
Labour is typically 40 to 45 per cent of the total, which is why regional wage rates move the price so much.
What actually drives the price
Structural work. Opening up the back of the house into an open-plan kitchen-diner usually means removing a load-bearing wall and installing a steel beam (RSJ), designed by a structural engineer. A single steel typically costs £250 to £450 to design and £800 to £2,500 supplied and fitted depending on span, plus the labour to install it and make good. Steel, foundations and any underpinning are among the least visible but most expensive elements of a build.
Ground conditions and drainage. Clay soils, nearby trees and poor ground can force deeper foundations, adding thousands. If the extension crosses a public sewer or a manhole sits where you want to build, you may need a build-over agreement with your water company (Southern Water in the Southampton area) and to divert the drain or relocate the manhole, at £1,500 to £5,000.
Glazing and doors. Aluminium bifold doors, large sliding doors, roof lanterns and skylights transform how an extension feels, and they carry a real cost. Budget £3,000 to £7,500 for quality aluminium bifolds supplied and fitted, £1,800 to £4,500 for standard sliding doors, and £1,500 to £4,000 for a roof lantern. Our bifold vs sliding doors guide compares them in detail.
Kitchens and bathrooms. If your extension houses a new kitchen or a bathroom, the fit-out can add £10,000 to £40,000 or more on its own. This is the single biggest reason two extensions of the same size can have wildly different price tags.
Finish level. Flooring, tiling, underfloor heating, lighting and decoration are where a project reads as either premium or budget. They are also where it is easiest to control spend without compromising the structure.
Professional and regulatory fees
The build cost is not the whole story. Budget for these on top:
| Fee | Typical cost (2026) |
|---|---|
| Architect or architectural designer | £1,500 – £5,000, or 3 – 10% of build cost for a full service |
| Structural engineer | £500 – £1,500 |
| Planning application (householder, England) | £548 |
| Lawful development certificate | £274 |
| Prior approval (larger home extension) | £249 |
| Building control (full plans or building notice) | £600 – £1,200 |
| Party wall surveyor | £700 – £1,500 per neighbour with a shared surveyor; £2,000 – £3,000+ if each side appoints their own |
| Community Infrastructure Levy | Possible on extensions over 100 m²; the self-build extension exemption usually removes it, but you must claim it before starting |
Planning fees in England now rise each year, so check the current figure before you apply. Our guide to planning permission for extensions explains when you need an application at all.
Hidden and often-forgotten costs
A trustworthy quote lists these separately rather than folding them into one vague number. If a price looks suspiciously low, one of these is usually missing:
- Site survey and soil investigation where ground conditions are uncertain: £300 to £1,000
- Scaffolding: £550 to £1,150 per week for a double-storey build
- Skips and waste: £250 to £350 per skip, and a build needs several
- Drainage diversion or build-over agreement: £1,500 to £5,000
- Temporary kitchen or accommodation if the build takes yours out of action
- Making good the garden, patio and paths after the build: £1,000 to £5,000
- Redecorating the rooms that connect to the extension
- Site insurance and a structural warranty if you want one
- Contingency: hold back 10 to 15 per cent for what is found once the ground is opened
- VAT at 20 per cent on the whole job, as most extensions are standard-rated
Contingency and VAT
Two lines that catch people out more than any other. First, contingency: even a well-surveyed project can uncover soft ground, an unmapped drain or a wall that is not what it seemed, and a 10 to 15 per cent buffer turns those from a crisis into a line item. Second, VAT: extensions to existing homes are standard-rated at 20 per cent, so a £60,000 quote excluding VAT is a £72,000 bill. Always ask whether a quote includes VAT before comparing it with another. Ours state it clearly.
Extension vs loft conversion vs garage conversion
An extension is not always the cheapest way to add a room:
| Option | Typical cost (ex VAT) | Best for | Watch out for |
|---|---|---|---|
| Single-storey extension | £40,000 – £90,000 | Open-plan living, kitchens | Uses garden; may need planning |
| Loft conversion | £30,000 – £60,000+ | Extra bedroom and en-suite | Head height; stairs eat space below |
| Garage conversion | £10,000 – £25,000 | Office, playroom, utility | Loses parking; insulation and damp-proofing needed |
| Garden room or annexe | £15,000 – £60,000 | Office, gym, guest space | Not connected to the house; planning limits |
If the aim is a bigger kitchen or a connection to the garden, nothing beats an extension. If it is an extra bedroom, a loft conversion is often better value.
Shell-only vs turnkey
Some builders offer a shell-only price: the extension built and watertight with no internal fit-out, at £1,300 to £1,900 per m². It suits people who want to do the second fix and decoration themselves or spread the cost over time. A turnkey price covers everything to the finished room. Be careful comparing the two; a shell-only quote can look like a bargain until you add the electrics, plumbing, plastering, flooring and decoration you still have to buy.
How long does an extension take?
| Stage | Typical duration |
|---|---|
| Design and drawings | 4 – 8 weeks |
| Planning application (if needed) | 8 weeks, or 6 weeks for prior approval |
| Structural calculations and building regulations drawings | 2 – 4 weeks |
| Party wall process (if needed) | 1 month's notice for excavation near a neighbour's foundations, 2 months for work to a party wall |
| Build: single-storey | 12 – 16 weeks |
| Build: double-storey | 16 – 24 weeks |
From first conversation to finished room, allow six to nine months for a single-storey extension and nine to twelve for a double-storey one. Kitchen lead times, bespoke glazing and weather are the usual reasons a programme slips.
How to keep costs under control
You can shape the budget without cutting corners:
- Get the design right first. Changes on paper are free; changes once the steel is in are not.
- Keep the footprint simple. A rectangle is cheaper than an L-shape or a curve for the same floor area.
- Prioritise the structure, phase the finishes. You can upgrade flooring or a kitchen later; you cannot easily redo foundations.
- Choose one team for the whole job. A single contractor handling every trade avoids the gaps, delays and finger-pointing that inflate costs when work is split across subcontractors.
- Insist on a line-by-line written quote. It is your best defence against mid-build surprises.
- Order long-lead items early. Glazing and kitchens can take eight to twelve weeks; ordering late stalls the site and costs money.
Does an extension add value?
As a rough guide, a well-designed extension adds value roughly in proportion to the floor area it creates. In the South of England that is often £1,500 to £2,500 per m² added, so a 25 m² kitchen extension can add £40,000 to £60,000. Open-plan kitchen-diners and extra bedrooms add the most; a poorly planned extension that steals the garden can add little. The hard limit is your street's ceiling price: if similar houses top out at £450,000, spending £120,000 on a £380,000 house will not all come back. Extend for how you want to live first, and treat added value as a bonus.
What does a house extension cost in Southampton?
Costs in Southampton and across the South East sit above the UK average, because labour and materials cost more here. In practice, single-storey extensions typically run £60,000 to £130,000 and double-storey extensions £100,000 to £220,000, depending on size, specification and whether a kitchen fit-out is included. Local factors to plan for: Southern Water build-over agreements where drains run under the proposed footprint, conservation areas in places like Winchester, Romsey and parts of Fareham borough that remove permitted development rights, and the New Forest National Park boundary west of Totton where the planning authority changes. Our area pages for Fareham, Hedge End, Totton, Chandler's Ford, Winchester, Eastleigh and Romsey cover the local planning picture for each.
If you are weighing up a larger project, it can also be worth comparing the cost of extending against a full house renovation, which sometimes delivers more usable space per pound.
Three worked examples
1. Kitchen extension on a Victorian semi (our Locks Heath project). A single-storey rear extension opening the kitchen into a garden-facing kitchen-diner with a separate utility, steel beam, bifold doors and matched period brickwork. Build £55,000 to £75,000; kitchen £18,000 to £30,000; glazing £6,000 to £9,000; fees and approvals £4,000 to £6,000. Total £85,000 to £120,000 for the scope and standard delivered, and a compact build that finished in ten weeks on site. See the Locks Heath case study.
2. Side-return extension on a terrace, 12 m². Steel to open the side of the original kitchen, party wall agreement with one neighbour, roof lights, new drainage run. Build £32,000 to £45,000; party wall and fees £3,000 to £5,000; kitchen retained and extended £6,000 to £12,000. Total £41,000 to £62,000.
3. Double-storey rear extension, 45 m² over two floors. Kitchen-diner below, bedroom with en-suite above, full planning application. Build £95,000 to £130,000; kitchen £15,000 to £25,000; bathroom £8,000 to £14,000; fees, planning and party wall £6,000 to £10,000. Total £124,000 to £179,000.
All three exclude VAT. Add 20 per cent for the final bill.
What a good extension quote should include
Before you compare quotes, check each one covers:
- Groundworks and foundations, with an allowance stated for deeper foundations if needed
- Structure, steel and roof, with the engineer's design included or listed separately
- Windows, doors and glazing with the brand and specification named
- First and second fix electrics and plumbing, including underfloor heating if wanted
- Plastering, flooring, decoration and external making good
- Building control fees and who is applying
- Skips, scaffolding and site welfare
- A payment schedule tied to stages, not dates
- Whether VAT is included
- A contingency, or a clear statement that variations will be priced and agreed in writing
Get an accurate extension quote
We visit your property, assess the structure and ground conditions, and give you a fixed written quote broken down line by line, with VAT stated clearly. No obligation, no pressure.
The ranges in this guide will get you to a realistic budget, but every property is different. The only way to know what your extension will actually cost is a proper site visit and a detailed quote, which is exactly where we would start.
Frequently asked questions
How much does a 4m x 4m extension cost?
A 4m by 4m single-storey extension (16 m²) typically costs £30,000 to £45,000 excluding VAT for the build to a standard finish, before a kitchen fit-out and professional fees. Small extensions cost more per square metre than large ones because foundations, roof and the connection to the house are fixed costs whatever the floor area.
Is a double-storey extension cheaper per square metre?
Yes, and often by a wide margin. On a 25 m² footprint a single storey might cost £50,000 while adding the second floor takes the total to £80,000 to £85,000 rather than £100,000, because the foundations, roof and connection to the house are only paid for once. The catches are that two-storey extensions usually need full planning permission and a bedroom or bathroom upstairs adds fit-out cost.
What is a shell-only extension?
A shell-only or watertight-shell extension is the structure built and weatherproof with no internal fit-out: foundations, walls, roof, windows and doors, but no plastering, electrics, plumbing, flooring or decoration. It typically costs £1,300 to £1,900 per m² and suits people who want to finish the inside themselves or in stages.
Do I need a structural engineer for an extension?
Almost always. The engineer designs the foundations and any steel beams, which building control will want calculations for, and a load-bearing wall removal to create an open-plan space cannot be signed off without them. Expect £500 to £1,500 for a typical extension, more if the ground or structure is unusual.
Which part of an extension costs the most?
For a kitchen extension the kitchen itself is usually the single biggest line, at £10,000 to £40,000 or more. For the build, structure and walls, including any steel, is the largest share at around 20 to 25 per cent, followed by internal finishes and the groundworks and foundations.
Can I live in the house during an extension build?
Usually, yes. Most of the work happens outside the existing footprint, so the disruption is concentrated around the knock-through at the end when the new space is joined to the house. If the extension replaces your kitchen, plan a temporary kitchen for a few weeks. We sheet off the work area and agree the knock-through date with you in advance.
How long does a house extension take to build?
A single-storey extension typically takes twelve to sixteen weeks on site and a double-storey extension sixteen to twenty-four weeks. Add two to four months before that for design, planning or prior approval and structural calculations. Ground conditions, kitchen lead times and weather can shift the programme, so a written schedule with realistic contingency matters.
How much does planning permission cost for an extension?
Around £640 to apply in England in 2026: the £548 householder fee plus the Planning Portal's online charge of about £90. A lawful development certificate for a permitted development extension is £274 and prior approval £249. Add £800 to £2,500 for the drawings, whichever route you take, and remember many single-storey rear extensions need no application at all.
Is it cheaper to extend or move house?
For most people an extension is cheaper than moving once you add up stamp duty, agent and legal fees, removals and the price gap to a bigger house, and you keep the location you chose. The exception is where the value of your street cannot absorb the spend, so check your local ceiling price before committing to a large extension.
How much value does an extension add to a house?
Enough to cover the cost on the right street, and not enough on the wrong one. In the South of England a well-designed extension typically adds £1,500 to £2,500 per m² of new floor area, so a 25 m² kitchen extension can add £40,000 to £60,000, most on open-plan kitchen-diners and extra bedrooms. Check what fully extended houses on your road sell for before you commit, because that ceiling price is the real limit.