How much does a rear house extension cost in the UK?
For a standard single storey rear house extension with a good quality finish, most UK homeowners pay between £1,800 and £3,000 per square metre for the build itself in 2026. That per square metre figure is the most reliable way to budget, because it scales cleanly with the size of the room you are adding off the back of the house.
| Project | Typical size | Rough cost (build only, before VAT and fees) |
|---|---|---|
| Small rear extension | 15 to 20m² | £30,000 to £60,000 |
| Medium rear extension | 20 to 30m² | £45,000 to £90,000 |
| Large rear extension | 30m² and above | £60,000 to £115,000 |
These are build costs. VAT, professional fees, planning, building control and any kitchen or bathroom fit out sit on top, and we cover those further down. Every site is different, so treat these as a starting budget and confirm with written quotes from local builders.
Rear extension cost per square metre
The clearest way to price a rear extension is per square metre, then adjust for storey, finish and area. Builders quote this way because it lets you compare projects of different sizes on the same basis. For a single storey rear build, expect £1,800 to £3,000 per m² in 2026; for a two storey rear build, roughly £2,000 to £2,800 per m² across both floors.
To use the rate, measure the rough depth and width of the new space, multiply them to get the area in m², then multiply that by the rate for your area. The rear extension cost calculator applies this same rate automatically and adds the extras, so you get a realistic total rather than just the headline build figure.
How much does a 3m rear extension cost?
A 3m deep rear extension is one of the most common single storey builds, since 3m is a typical permitted development depth for many homes. The cost depends on how wide it is. A 3m deep extension across a 4m wide rear wall gives 12m², and at a 3m wide span gives roughly 9m². As a 2026 guide:
| Size | Depth x width | Rough cost (build only, before VAT and fees) |
|---|---|---|
| 3m x 3m | 9m² | £18,000 to £27,000 |
| 3m x 4m | 12m² | £22,000 to £36,000 |
| 3m x 5m | 15m² | £27,000 to £45,000 |
So a 3m rear extension typically costs somewhere between £18,000 and £45,000 for the build, depending on width, finish and area. Add VAT, fees and any kitchen fit out on top.
Rear extension cost by area of the UK
Location is one of the biggest factors in a rear extension cost, so it helps to budget against your own area rather than a single national figure. As a guide for 2026:
| Area | Cost per m² (single storey rear) |
|---|---|
| North of England | £1,600 to £2,400 |
| Midlands | £1,800 to £2,800 |
| Scotland and Wales | £1,600 to £2,400 |
| London and the South East | £2,400 to £3,800 |
A rear extension cost in London or the South East typically runs 20 to 40 percent above the national average, driven by higher labour rates and tighter access. The North, Scotland and Wales tend to sit at the lower end. These are build rates only, before VAT and fees.
Cost per m² by finish: basic, standard and premium
Finish level is a major swing in the per m² price. As a rough guide for a single storey rear build in 2026:
| Finish level | Cost per m² | What it usually means |
|---|---|---|
| Basic | £1,700 to £2,200 | Blockwork, warm flat roof, standard windows, painted plaster |
| Standard | £2,000 to £2,800 | Pitched or upgraded flat roof, a rooflight, decent glazed doors, underfloor heating |
| Premium | £2,900 to £3,800 | Structural glazing, large span doors, high end finishes throughout |
A basic rear extension keeps the cost down with a simple footprint and standard materials. A premium build costs more per m² but can add more value where it suits the house and the area.
Rear extension cost by type in the UK
Rear extensions come in a few forms, and the type you choose changes the cost. Here is how the main options compare in the UK in 2026, from a simple single storey build to a two storey or a wrap around rear and side.
Rear single storey extension cost UK
A rear single storey extension is the most common and most affordable rear build, following the £1,800 to £3,000 per m² range. A small 15 to 20m² single storey rear extension starts around £30,000 to £45,000 in the North and £55,000 to £75,000 in London for the build. Single storey rear extensions are also the most likely to fall under permitted development, which can save the cost and delay of a planning application. For a full breakdown, see the single storey extension cost calculator.
Rear double storey extension cost UK
A rear double storey extension adds space on two floors off the back of the house, typically a larger room downstairs and a bedroom or bathroom above. It costs roughly £2,000 to £2,800 per m² across both floors, which is cheaper per square metre than a single storey build because the foundations and roof are shared, but higher in total because you are adding twice the space. A double storey rear build usually needs planning permission rather than permitted development. For a full breakdown, see the double storey extension cost calculator.
Rear and side extension cost UK
A rear and side extension combines a rear extension with a side return into one L-shaped build, giving the most ground floor space in a single project. It is popular for creating large open plan kitchen diners. Because it is bigger and more complex, with more roof and wall to tie into the existing house, it sits at the upper end of the per m² range, and the total is higher simply because of the larger footprint. In the calculator, choose wrap around for this type.
Rear kitchen extension cost UK
A rear kitchen extension is a rear build with the added cost of a fitted kitchen. The building work follows the usual per m² rates, and the units, worktops and appliances sit on top. As a guide, the fit out adds anywhere from £5,000 for a modest kitchen to £25,000 or more for a high end one, plus the plumbing, gas and electrical work. A medium 20m² rear kitchen extension often lands between £45,000 and £70,000 all in, depending on the specification you choose.
Rear extension terraced house cost
A rear extension on a terraced house works a little differently from one on a detached home. Because a terraced house is attached on both sides, you will usually need a party wall agreement with the neighbours on each side, which adds surveyor cost and time. Access is often tight, with materials carried through the house or along a narrow side passage, which raises labour costs. On the upside, a single storey rear extension on a terraced house frequently falls under permitted development, within the depth limits, so it can often avoid a full planning application. As a guide, a rear extension on a terraced house follows the same per m² rates as any single storey rear build, with the party wall and access factors pushing the final figure up.
What affects the cost of a rear extension
Two rear extensions of the same size can come back with very different quotes. The main things that move the price are:
| Cost driver | Effect on price |
|---|---|
| Storey count | A single storey rear build costs less in total; a double storey rear build is cheaper per m² but higher overall. |
| Size in m² | The single biggest factor. Larger builds cost more overall, though the price per m² often eases as the project grows. |
| Finish and specification | A basic build sits far below a premium build with bi-fold doors, underfloor heating and a fitted kitchen. |
| Rear or rear and side | A plain rear extension is the most straightforward. A rear and side build carries more roof and wall work. |
| Area of the UK | London and the South East run 20 to 40 percent above the national average. |
| Roof type | A flat roof is usually cheaper than a pitched roof of the same footprint. |
| Kitchen or bathroom fit out | A new kitchen can add £15,000 to £35,000. |
| Structural steel | Wide openings between the house and the new space need steel beams, which add cost. |
| Ground and access conditions | Poor soil, sloping plots or tight access push up groundwork and labour. |
Labour usually accounts for around 35 to 50 percent of the total, with materials making up much of the rest.
Rear extension cost breakdown: extra costs to budget for
The per m² figure covers the build and basic finishes. These extras are usually separate, and together they make up the full rear extension cost breakdown:
- VAT. Most extension work is subject to VAT, so check with your builder how it applies to your quote.
- Planning permission. Single storey rear extensions often fall under permitted development and need no application, but a double storey rear extension usually needs planning permission. If you need a householder application in England, the fee is £548 from 1 April 2026. Scotland and Wales fees differ. Confirm the current figure on GOV.UK before you apply.
- Building regulations. Almost every rear extension must meet building regs for structure, insulation, drainage and fire safety. Inspection fees vary by council, usually £500 to £1,200. You can read the official rules at GOV.UK building regulations approval.
- Professional fees. Architect and structural engineer fees are extra where drawings and calculations are needed, often 7 to 15 percent of the build cost.
- Party wall surveyor. If you build against or near a shared wall, a party wall agreement and surveyor may be required, more likely on terraced and semi detached homes.
- Kitchen or bathroom fit out. A new kitchen can add £15,000 to £35,000.
- Contingency. Set aside 10 to 15 percent for surprises, especially in older homes.
Do I need planning permission for a rear extension?
Not always. Many single storey rear extensions fall under permitted development and need no planning application, provided they stay within the depth, height and position limits. As a general guide, single storey rear extensions up to a set depth for terraced and semi detached homes, and a greater depth for detached homes, can often be built without permission. A double storey rear extension usually goes beyond permitted development and needs a full application. The rules are tighter if you are in a conservation area or an Area of Outstanding Natural Beauty, or if your home has Article 4 restrictions. Always check with your local planning authority before you start. You can read the official guidance at GOV.UK planning permission.
Does a rear extension add value?
A well designed rear extension can add value, sometimes more than it costs to build, though this depends on your location, the finish and how well the new space works. A rear extension that opens the kitchen into a bright family space tends to appeal to buyers, while one that eats too much garden or blocks light can work against you. As a rough guide, a good rear extension in the right home adds in the region of 10 to 20 percent to the value, but the figure varies widely by area, so treat it as a bonus rather than a guaranteed return.
How the rear extension cost calculator works
The rear extension cost calculator turns the per m² method into a single figure for your project. You choose:
- Extension type, starting on rear. Switch to double or two storey for a two storey rear build, or wrap around for a rear and side build.
- Size in m², or use the length and width helper if you do not know your area.
- Finish level, from basic through standard to premium.
- Region, since area moves the figure by 20 to 40 percent.
It then shows a low to high total for the build, the per m² rate used, and a separate line for extras like planning, building control and fit out. Showing those add ons separately keeps the estimate honest, so the rear extension cost calculator gives you a realistic figure rather than hiding the extras inside one number. Enter your details at the end to save your estimate and get a copy.
For other extension types, see the main house extension cost calculator homepage, or the single storey extension cost calculator and double storey extension cost calculator pages if you are weighing up building out against building up.
Frequently asked questions
How much does a rear extension cost per square metre in the UK?
Most single storey rear extensions cost £1,800 to £3,000 per m² for the build in 2026, higher in London and the South East. A two storey rear build is roughly £2,000 to £2,800 per m² across both floors.
How much does a 3m rear extension cost?
A 3m deep rear extension typically costs £18,000 to £45,000 for the build, depending on its width, finish and area, before VAT and fit out.
Do I need planning permission for a rear extension?
Not always. Many single storey rear extensions fall under permitted development within the depth limits, but a double storey rear build usually needs an application. Conservation areas have tighter rules. Check with your local planning authority before starting.
Is a single storey or double storey rear extension better value?
Per square metre, a double storey rear build is usually 10 to 15 percent better value because the foundations and roof are shared. A single storey rear build costs less in total, since you are adding less space.
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 to your project.
How long does a rear extension take to build?
Most single storey rear extensions take around 8 to 16 weeks on site, and a two storey rear build around 16 to 26 weeks, depending on size, finish and access.
Figures on this page are current UK market ranges for 2026 and are intended as a budgeting guide. Prices vary by area, 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.