How much does a house extension cost UK?
For a standard single storey build with a good finish, the calculator works from £1,800 to £3,000 per m², which is the most reliable way to budget because it scales with the size of the room you are adding. Turned into real totals:
| Project | Typical size | Cost range (build only, before VAT and fees) |
|---|---|---|
| Small single storey extension | 15 to 20m² | £30,000 to £60,000 |
| Medium kitchen or family room | 20 to 30m² | £45,000 to £90,000 |
| Two storey extension | 50 to 60m² | £108,000 to £180,000 |
| First floor extension over a garage | around 25m² | £45,000 to £75,000 |
These are build costs. VAT, professional fees, planning, building control and any kitchen fit out sit on top, and the calculator lists those separately so nothing is hidden. Every site is different, so treat the estimate as a working budget and confirm it with written quotes.
How the house extension cost calculator works
The calculator turns the per m² method into a single figure for your project. You choose four things:
- Extension type, such as single storey, double storey, rear, side, front, wrap around, ground floor, kitchen, conservatory or timber frame.
- Size in m². If you do not know your area, use the length and width helper and the calculator works it out for you.
- Finish level, from basic through standard to premium.
- Region, since where you live moves the figure by 20 to 40 percent.
It then shows a low to high total for the build, the cost per m² it used, and a separate line for extras like planning and building regulations. Enter your details at the end to save your estimate and get matched with tradespeople.
What affects your house extension cost
Two extensions of the same size can return very different quotes. These are the factors the calculator accounts for, and the ones a builder will price against:
| Cost driver | Effect on your estimate |
|---|---|
| Size in m² | The biggest single factor. Larger builds cost more overall, though the rate per m² often eases as the project grows. |
| Finish and specification | A basic build sits far below a premium one with bi-fold doors, underfloor heating and a fitted kitchen. |
| Number of storeys | A two storey build costs more overall but less per m², because the roof and foundations are shared. |
| Region | London and the South East run 20 to 40 percent above the national average. |
| Kitchen or bathroom fit out | A new kitchen with plumbing, gas and electrics can add £15,000 to £35,000. |
| Ground and access conditions | Poor soil, sloping plots or tight access push up groundwork and labour. |
| Structural work | Removing walls, adding steel beams or changing the roofline adds cost. |
Labour usually makes up around 35 to 50 percent of the total, with materials accounting for much of the rest.
House extension cost per m2 calculator
The per square metre method is the backbone of the calculator. To estimate any extension, measure the rough length and width of the new space, multiply them for the area in m², then multiply that by the rate for your region:
| Region | Cost per m² (single storey) |
|---|---|
| North of England and Wales | £1,600 to £2,400 |
| Midlands | £1,800 to £2,800 |
| London and the South East | £2,400 to £3,800 |
A 25m² extension in the Midlands at £2,300 per m² works out at about £57,500 for the build. The calculator does this for you and adds the extras on top.
Basic house extension cost per m2
Finish level is one of the biggest swings in the rate. For a single storey build:
| Finish level | Cost per m² | What it usually includes |
|---|---|---|
| Basic | £1,500 to £2,000 | Standard doors and windows, plain plaster, basic fittings |
| Standard | £2,000 to £2,800 | Better glazing, decent flooring, mid range kitchen or bathroom |
| Premium | £3,000 to £4,500 | Bi-fold or sliding doors, underfloor heating, high end finishes |
A basic house extension keeps the figure down by sticking to a simple footprint and standard materials. Set the calculator to basic to see the lower end for your size.
Labour and materials cost breakdown
When the calculator returns a figure, roughly 35 to 50 percent of it is labour and 40 to 55 percent is materials, with the balance going on fees and extras. This is worth knowing when you compare builder quotes, because a quote that is much cheaper on labour may be planning a faster, lower spec build, and one that is heavy on materials may be pricing a higher finish. The split shifts with the type of work, but it is a useful sense check against the estimate.
House extension cost calculator by type
The type of extension you choose is the first thing the calculator asks for, because it sets the base rate per m². A single storey build, a two storey build, a conservatory and a timber frame all price differently, even at the same size. The sections below show what each type usually costs and how the calculator handles it, so you can compare a few options before you decide. Pick the closest match to your plans, enter your size, and the calculator applies the right rate.
Single storey house extension cost
A single storey extension is the most common choice and the easiest to calculate. It usually adds a bigger kitchen, a dining space or a family room. The calculator works from £1,800 to £3,000 per m² for a standard build, higher in London. A typical 20m² single storey extension comes out at roughly £36,000 to £60,000 for the build before VAT and fees.
Single storey house extension cost per m2
For a single storey build specifically, the rate sits at £1,800 to £3,000 per m² nationally and £2,300 to £3,400 per m² in London. Smaller extensions can sit at the higher end per m², because fixed costs like the roof and foundations are spread over less floor area. Enter your size and the calculator applies the right rate for you.
Double storey house extension cost
A double storey extension adds space across two floors, usually living space downstairs and a bedroom or bathroom above. The total is higher than a single storey build, but the cost per m² is lower, often around £1,900 per m² for a standard finish, because the roof and foundations serve both floors. Select double storey in the calculator and it applies the two floor rate across your total area.
Two storey house extension cost
Two storey and double storey describe the same build, an extension over two floors. The reason it is worth pricing on its own is value for money: a two storey extension almost always works out cheaper than building two separate single storey extensions of the same total area, because you pay once for the groundwork and roof. A 60m² two storey extension typically costs £108,000 to £180,000 for the build. Use the calculator to compare a two storey figure against a single storey one for the same footprint.
Rear house extension cost
A rear extension is built off the back of the house and is the most popular way to open up a kitchen or living area. As a single storey build the calculator prices it at £1,800 to £3,000 per m². A small 15 to 20m² rear extension starts around £30,000 to £45,000 in the North and £55,000 to £75,000 in London for the build.
Side house extension cost
A side extension, often a side return, fills the narrow strip of land beside the house and is common in terraced and semi detached homes. Side returns tend to be small, often 5 to 12m², but the rate per m² can sit at the higher end of the single storey range because of the tight access and the work needed to tie the new roof into the existing one. Enter the side return area and the calculator gives a realistic figure for the smaller footprint.
Front house extension cost
A front extension adds space at the front, such as a larger hallway, a porch or an extra room. It follows single storey rates in the calculator, but with one key difference: a front extension almost always needs planning permission, since permitted development rarely covers extensions facing a road. Add the planning fee and a possible planning consultant to your budget.
Ground floor house extension cost
A ground floor extension is any single storey addition at ground level, whether rear, side or wrap around. The calculator prices it at the single storey rate of £1,800 to £3,000 per m², adjusted for finish and region. Most kitchen and family room extensions fall into this group.
Wrap around house extension cost
A wrap around extension combines a rear and a side extension into one L-shaped build, giving the most ground floor space in a single project. Because it is larger and more complex, it sits at the upper end of the single storey range and the total is higher simply because of the bigger footprint. Enter the combined rear and side area and the calculator works out the full figure.
Kitchen extension cost
A kitchen extension is priced as a single storey extension for the shell, at £1,800 to £3,000 per m², with the kitchen itself added on top. A new kitchen with units, worktops, plumbing, gas and electrics usually adds £15,000 to £35,000. When you calculate a kitchen extension, budget the build and the fit out as two separate lines, since the per m² rate does not include the kitchen.
Conservatory house extension cost
A conservatory is usually the cheapest way to add a room, since it is mostly glazing rather than full masonry. Typical UK prices, based on a common 3m x 3m size, are:
| Conservatory type | Typical cost |
|---|---|
| Lean-to | £9,000 to £14,250 |
| Victorian | £12,000 to £17,500 |
| Edwardian | £10,500 to £18,000 |
| Orangery | £30,000 to £40,000 |
Prices run from around £5,000 for a small uPVC lean-to up to £40,000 or more for a large bespoke orangery, and each extra square metre adds roughly £1,000 to £1,500. A wooden framed conservatory costs more than a uPVC one.
Timber house extension cost
A timber frame extension uses factory made structural wall panels, erected on site and then clad in brick, render or timber so it matches the house. The calculator prices timber frame at £1,700 to £2,500 per m², usually 10 to 20 percent below a comparable brick and block build. For a 30m² single storey rear extension, that puts timber frame at roughly £54,000 to £84,000 against nearer £72,000 to £96,000 for brick at the same specification. The main advantage is speed, since the frame goes up in days and reaches watertight faster, which cuts labour time.
Wooden house extension cost UK
A wooden extension can mean two things, and it helps to separate them when you calculate the cost. The first is the timber frame structure covered above, which sets the per m² rate. The second is a wooden clad finish, where timber boarding is fixed to the outside for a natural look. Cladding is a finish choice that changes the appearance rather than the frame, so it affects the finish level in the calculator rather than the structural rate. A wooden clad, timber frame extension in the UK sits in the same £1,700 to £2,500 per m² band, with the cladding pushing it towards the upper end.
House extension cost calculator by property type
The kind of house you have shapes what you can build and how much it costs, mainly through access and party wall agreements rather than the build rate itself. A terrace, a semi and a detached home each come with different constraints, so the same size extension can carry different real world costs. The calculator prices the build from your chosen type and size, and the notes below help you allow for the property specific extras.
Terraced house extension cost
Terraced homes usually extend at the rear or through a side return, since there is no room to the side of the plot. Party wall agreements with the neighbours on both sides are common, and access for materials is often tight, both of which can nudge the figure up. The build still follows single storey rates in the calculator, so set your type and size and add a little extra for access.
Semi detached house extension cost
A semi detached home can usually take a rear extension, a side extension into the gap beside the house, or a two storey build. There is more room to work than in a terrace, and a party wall agreement is only needed on the shared side. The calculator prices it at the standard rate for whichever type you choose.
Detached house extension cost
A detached property gives the most freedom, with room to extend at the rear, to either side or across two storeys, and often no party wall agreement at all. The wider choice means your total depends more on the type and size you pick than on any access restriction. Try a few combinations in the calculator to compare them.
House extension cost calculator by size
Size is the single biggest driver of your estimate, so it helps to see how the cost changes from a small extension up to a large multi bedroom build. The calculator works from the floor area you enter, which is a more reliable guide than the number of rooms, since two extensions with the same room count can differ a lot in square metres. The sections below give typical figures by size and scope so you know what to expect before you calculate your own.
Small house extension cost
A small extension of 15 to 20m² is enough for a larger kitchen or a single family room. As a single storey build it starts around £30,000 to £45,000 in the North and Wales and £55,000 to £75,000 in London for the build, before VAT and fit out. Small extensions can carry a higher rate per m², because fixed costs are spread over less floor area, which the calculator reflects.
2 bedroom house extension cost
Adding two bedrooms usually means a two storey extension or a first floor addition, since bedrooms sit upstairs in most homes. The cost depends on the floor area you add rather than the number of rooms, so a two bedroom extension is calculated at the £1,900 per m² two storey rate across the combined floor area, which runs into the £100,000 range for a full two storey build. A loft conversion is often a cheaper route to extra bedrooms, at around £1,200 to £2,000 per m².
3 and 4 bedroom house extension cost
A three or four bedroom extension means a large two storey build, or a two storey extension combined with a loft conversion. Because these add a lot of floor area, the calculator total climbs well into six figures, priced at the two storey rate across the full area. At this scale it is worth pricing a two storey extension and a loft conversion separately in the calculator, since combining a smaller extension with a loft can deliver the same number of bedrooms for less than one very large extension.
Average house extension cost
There is no single average, because size, type and region all pull the figure around. As a broad picture, a typical single storey extension lands between £30,000 and £90,000 for the build depending on size and finish, and a typical two storey extension between £108,000 and £180,000. The per m² method the calculator uses is far more reliable than any headline average, which is why it works from your actual dimensions rather than a fixed number.
Building house extension cost: basic shell vs full build
When you calculate a house extension cost, it helps to know whether you are pricing a shell or a finished room. A basic or shell build gives you the structure, roof and weatherproofing but leaves the inside unfinished, at around £1,200 to £1,700 per m². A full build hands you a finished, usable room at the standard rates above. Choose based on whether you plan to fit out the inside yourself or later.
House extension cost by region
Location is one of the biggest single factors in your estimate, which is why the calculator asks for your region.
London and South East house extension cost
London and the South East run 20 to 40 percent above the national average, driven by higher labour rates, party wall surveyors and more complex access. A single storey extension here is priced at £2,400 to £3,800 per m², and often at the top of that band in inner London. Set the region to London and the calculator applies these higher rates.
North, Midlands, Wales and Scotland house extension cost
Rates ease outside the South East. The calculator prices the North of England and Wales at £1,600 to £2,400 per m² and the Midlands at £1,800 to £2,800 per m². Scotland generally sits nearer the lower end too. The same 25m² extension can cost noticeably less in the North than in London, which is why region is built into every estimate.
Extra costs the calculator adds on top
The per m² figure covers the build and basic finishes. The calculator lists these extras separately so your estimate stays honest.
Planning permission cost
Many extensions fall under permitted development and need no application, but limits apply and front extensions usually do need permission. If you need a householder planning application in England, the fee is £528 under the schedule that took effect on 1 April 2025. The fee in Wales is lower. Confirm the current fee on GOV.UK and with your local council before you apply.
Building regulations cost
Almost every extension must meet building regulations for structure, insulation, drainage and fire safety. Inspection fees vary by council, usually £500 to £1,200. This is separate from planning permission and applies even when planning is not needed.
VAT, professional fees and party wall costs
Most extension work is subject to VAT, so check with your builder how it applies to your quote. Architect and structural engineer fees are extra where drawings and calculations are needed. If you build against or near a shared wall, a party wall agreement and surveyor may be required. A new kitchen or bathroom fit out is also separate from the build. Set aside a contingency of 10 to 15 percent on top of the calculator total for surprises, which are more common in older homes.
Does a house extension add value?
A well designed extension can add value to your home, sometimes more than it costs to build, though this depends on your location, the finish and how well the new space works. The practical way to judge it is to run your plans through the calculator, then compare the estimated build cost against what similar extended homes in your area sell for. If the likely uplift comfortably clears the build cost and fees, the numbers make sense. If it is close, the extension may still be worth it for the extra space, just not as an investment.
How to reduce your house extension cost
If the calculator figure is above your budget, there are sensible ways to bring it down:
- Keep the footprint simple, since square and rectangular builds cost less than complex shapes.
- Choose a standard finish rather than premium, then upgrade later.
- Consider a timber frame, which is usually cheaper and faster than brick.
- Reuse the existing layout where you can, to avoid moving drains and services.
- Get three quotes and compare them line by line against your estimate.
- Build under permitted development where possible, to save the planning fee and time.
How to use your house extension cost estimate
Treat the calculator result as a planning tool, not a fixed quote:
- Check it against your budget and any borrowing.
- Get at least three written quotes from local builders and compare them to the estimate.
- Ask each builder exactly what is included, especially VAT, fit out and fees.
- Keep a contingency for the unexpected.
If quotes come back well above the estimate, it usually points to a higher specification, a difficult site or regional rates, all worth understanding before you commit.
How we calculate these house extension costs
The calculator and the figures on this page are built from current UK cost data for 2026, drawn from established sources including the Federation of Master Builders, Checkatrade and the HomeOwners Alliance for build rates, and from GOV.UK and local council fee schedules for planning and building control. Rates are set per square metre by extension type, finish and region, then combined with your inputs to produce a range rather than a single number, because real quotes always vary. The figures are a budgeting guide, and we recommend confirming any cost that matters with written quotes and official sources before you commit.
House extension cost calculator FAQ
How much does a house extension cost in the UK?
Most single storey extensions cost £1,800 to £3,000 per m² for the build, so a 20m² extension is roughly £36,000 to £60,000 before VAT and fees. The calculator gives a figure for your exact size, finish and region.
How accurate is a house extension cost calculator?
It gives a realistic budgeting range based on typical UK rates, not a fixed quote. Your final cost depends on your builder, your site and your finish, so confirm the estimate with written quotes.
Is the calculator free to use?
Yes. The house extension cost calculator is free and gives an instant estimate. You can enter your details at the end to save the result and get matched with tradespeople.
Is a two storey extension cheaper than a single storey one?
Per square metre, usually yes, because the roof and foundations are shared across two floors. The total is still higher because you are adding more space.
Do I need planning permission for an extension?
Not always. Many extensions fall under permitted development, but size, height and position limits apply, and front extensions usually do need permission. Check with your local planning authority before starting.
Which is cheaper, timber frame or brick?
Timber frame is usually 10 to 20 percent cheaper on a like for like single storey build and much faster to erect, though the gap narrows once foundations, glazing and fit out are added.
Does the estimate include VAT?
The per m² figures usually exclude VAT and fit out. The calculator shows extras separately, and you should ask your builder to confirm how VAT applies.
How do I calculate the cost of my extension per m2?
Measure the length and width of the new space, multiply them for the area in m², then multiply by the rate for your region. The calculator does this automatically when you enter your size.
Will an extension add value to my home?
A well designed extension can add value, sometimes more than it costs to build, depending on location, finish and how well the space works.
Figures on this page are current UK market ranges for 2026 and are intended as a budgeting guide. Prices vary by region, specification and site conditions. Planning and building control fees are official figures that should be confirmed on GOV.UK and with your local council before you apply.