An extension is the most popular way to add serious space to a UK home. It is also one of the most expensive projects you will ever take on, and the one where costs spiral fastest if you do not pin things down early.
This guide gives you the real numbers for 2026. Not the vague "it depends" answers you get from most sites, but actual prices broken down by extension type, size and spec level so you can plan a realistic budget before you speak to a builder.
A single storey extension costs £1,800 to £3,000 per m2 in 2026. A double storey costs £1,650 to £2,800 per m2. For a typical 25 m2 single storey rear extension, expect to pay £45,000 to £80,000 all in. A double storey of the same footprint runs £70,000 to £140,000.
Extension Costs by Type
| Extension Type | Cost per m2 | Typical Size | Total Cost Range |
|---|---|---|---|
| Single storey rear | £1,800 to £3,000 | 20 to 30 m2 | £40,000 to £80,000 |
| Single storey side return | £2,000 to £3,200 | 8 to 15 m2 | £20,000 to £45,000 |
| Wrap around (rear + side) | £2,000 to £3,200 | 25 to 40 m2 | £55,000 to £110,000 |
| Double storey rear | £1,650 to £2,800 | 40 to 60 m2 total | £70,000 to £160,000 |
| Double storey side | £1,650 to £2,800 | 30 to 50 m2 total | £55,000 to £130,000 |
These prices include the build itself: groundworks, foundations, brickwork, roof, windows, doors, first and second fix electrics and plumbing, plastering and basic decoration. They do not include the kitchen or bathroom you might put in the new space, professional fees, or the extras covered below.
Single Storey Extensions
Rear Extension
The classic back of house extension. Most people knock through from the existing kitchen or dining room to create an open plan kitchen diner that opens onto the garden. Sizes vary from a modest 15 m2 to a generous 30 m2+.
| Spec Level | Cost (25 m2) | What's Included |
|---|---|---|
| Budget | £45,000 to £55,000 | Standard brick, flat or pitched roof, uPVC windows, basic kitchen, standard flooring |
| Mid range | £55,000 to £70,000 | Brick with render detail, bi fold doors, vaulted ceiling, mid range kitchen, LVT flooring |
| Premium | £70,000 to £90,000+ | Architect designed, structural glazing, bespoke kitchen, underfloor heating, porcelain tiles |
The biggest cost driver on a rear extension is the span. If you want a fully open plan space without a support post in the middle, you need a steel beam (RSJ). A standard steel costs £1,500 to £3,000 including fitting. Longer spans need bigger steels and cost more.
Side Return Extension
Common on Victorian and Edwardian terraced houses with a narrow alleyway down the side. You are typically adding 8 to 15 m2 by filling in that wasted side passage and opening it up into the kitchen. Because the footprint is smaller, the total cost is lower, but the cost per m2 can be higher because you still need foundations, structural work and a roof for a relatively small space.
Budget £20,000 to £45,000 for a side return. Most come in at £25,000 to £35,000.
Wrap Around Extension
A wrap around combines a rear extension with a side return to create an L shaped extension that maximises the ground floor space. This is the premium option for anyone who wants a large open plan kitchen, dining and living area.
Expect to pay £55,000 to £110,000 depending on size and spec. The cost per m2 is similar to a standard rear extension, but you are covering more ground so the total is higher.
Double Storey Extensions
A double storey extension gives you roughly double the floor space for 50% to 60% more money than a single storey. The reason it is not simply double the price is that you are sharing the cost of foundations, roof and professional fees across two floors.
| Spec Level | Cost (25 m2 footprint / 50 m2 total) | What's Included |
|---|---|---|
| Budget | £70,000 to £95,000 | Standard build, uPVC windows, basic finish upstairs and down, standard bathroom |
| Mid range | £95,000 to £130,000 | Better windows, bi folds on ground floor, mid range bathroom, good quality kitchen |
| Premium | £130,000 to £170,000+ | Architect designed, aluminium windows, underfloor heating both floors, premium bathroom, bespoke kitchen |
A double storey extension almost always needs planning permission. It also typically requires a Party Wall agreement if you are semi detached or terraced. Factor in 3 to 5 months for the planning and paperwork before building starts.
The Costs Everyone Forgets
The build cost is only part of the story. Here are the extras that push the real total higher:
| Cost | Typical Price | Notes |
|---|---|---|
| Architect / designer | £3,000 to £10,000 | 7% to 15% of build cost. Some charge fixed fees. |
| Structural engineer | £500 to £1,500 | Structural calculations for steels, foundations, etc. |
| Planning application | £258 | Standard householder application in England. |
| Building regulations | £500 to £1,500 | Building control fees for inspections and sign off. |
| Party Wall surveyor | £1,000 to £2,500 per neighbour | You pay for both your surveyor and your neighbour's if they appoint one. |
| Kitchen (in new space) | £5,000 to £30,000 | Not included in build costs. See our kitchen costs guide. |
| Bathroom (upstairs in double storey) | £3,000 to £12,000 | Not included in build costs. See our bathroom costs guide. |
| Landscaping (making good the garden) | £1,000 to £5,000 | Builders will leave the garden in a state. Budget for this. |
| Skip hire | £250 to £400 per skip | Expect 2 to 4 skips on a typical extension project. |
| Temporary kitchen | £200 to £500 | Microwave, kettle and a worktop in the spare room for 2 to 3 months. |
A realistic total budget for a 25 m2 single storey rear extension with a mid range kitchen, all professional fees and making good is £75,000 to £100,000. For a double storey of the same footprint with a bathroom upstairs, budget £110,000 to £160,000.
Planning Permission
Whether you need planning permission depends on the type and size of extension:
Permitted Development (no planning needed)
- Single storey rear extensions up to 3 metres (attached houses) or 4 metres (detached)
- Under the Larger Home Extension scheme, you can go up to 6 metres (attached) or 8 metres (detached) with a prior approval notification to the council
- Maximum height of 4 metres
- Single storey side extensions up to half the width of the original house
- Materials must match the existing house
Planning Permission Required
- Double storey extensions (almost always)
- Extensions that exceed the permitted development limits above
- Extensions closer than 2 metres to a boundary that are taller than 3 metres
- Properties in conservation areas, listed buildings, AONB
- Flats and maisonettes (no permitted development rights)
The planning application fee in England is £258. Decisions take around 8 weeks for a standard application, sometimes 12 weeks if there are objections or it goes to committee.
The Party Wall Act
If you share a wall with a neighbour (semi detached or terraced house), you will almost certainly need to serve a Party Wall notice before starting an extension. The same applies if you are digging foundations within 3 metres of a neighbouring building, or within 6 metres if your foundations go deeper than their existing ones.
The process:
- Serve a written Party Wall notice at least 2 months before you plan to start work
- Your neighbour has 14 days to respond
- If they consent in writing, you are good to go with no surveyors needed
- If they dissent or do not respond, both sides appoint a surveyor (or agree on a single surveyor)
- The surveyors draw up a Party Wall Award that records the condition of the neighbour's property and sets out how the work should be carried out
- You pay for everything, including your neighbour's surveyor fees
Budget £1,000 to £2,500 per affected neighbour. If you have neighbours on both sides (terraced house), that could be £2,000 to £5,000 just for the Party Wall process.
How Long Does an Extension Take?
| Phase | Single Storey | Double Storey |
|---|---|---|
| Design and drawings | 3 to 6 weeks | 4 to 8 weeks |
| Planning permission (if needed) | 8 to 12 weeks | 8 to 12 weeks |
| Building regulations | 4 to 6 weeks | 4 to 8 weeks |
| Party Wall notices | 8 weeks minimum | 8 weeks minimum |
| Build time on site | 10 to 14 weeks | 14 to 20 weeks |
From the moment you pick up the phone to the day you move furniture into the new space, expect 6 to 10 months for a single storey extension and 8 to 12 months for a double storey. The biggest delays are usually planning decisions and finding a builder with availability.
Extension vs Loft Conversion vs Moving House
Not sure if an extension is the right move? Here is how the main options compare:
| Option | Typical Cost | Space Gained | Disruption |
|---|---|---|---|
| Single storey extension | £45,000 to £80,000 | 20 to 30 m2 | High (3 to 4 months) |
| Loft conversion | £35,000 to £60,000 | 15 to 25 m2 | Medium (5 to 7 weeks) |
| Moving house | £15,000 to £30,000 in fees | Varies | Very high (chain dependent) |
Moving house is deceptively expensive once you add stamp duty, estate agent fees, solicitor costs and removal costs. For many homeowners, extending is the better financial decision even if the build cost seems high. Check our full home renovation costs guide for the bigger picture on all your options.
How to Save Money on an Extension
- Stay within permitted development. Avoiding a planning application saves you time, money and stress. Design your extension to fit within the PD limits if possible.
- Go double storey if you need both floors. The cost per m2 is lower and you get double the space for 50 to 60% more money.
- Simple roof design. A flat roof with a parapet wall is cheaper than a pitched roof. It also creates a cleaner, more modern look.
- uPVC over aluminium. Aluminium windows and bi fold doors look great but cost 50% to 100% more than uPVC equivalents. Good quality uPVC still looks smart.
- Get three builder quotes. Prices vary 20% to 40% between builders for the same job. Always compare and check references.
- Be your own project manager. Hiring an architect to do full project management adds 5% to 10% to the total. If you are organised, you can manage the trades yourself and save thousands.
- Talk to your neighbours early. A friendly conversation before serving a Party Wall notice often leads to a written consent within 14 days, saving you £1,000+ in surveyor fees per neighbour.
Does an Extension Add Value?
A well done extension typically returns £1.20 to £1.50 for every £1 you spend, adding 10% to 20% to your property value. The return depends on where you live, the quality of the build, and whether the new space solves a genuine problem (like a small kitchen or missing bedroom).
The best value extensions are the ones that fix an obvious shortcoming. A three bedroom house with a tiny kitchen that gets extended into an open plan kitchen diner will see a bigger percentage uplift than a five bedroom house that adds a home cinema.
If your extension includes a new kitchen, see our kitchen costs guide for detailed pricing. You will also need a plumber for the kitchen and any new bathrooms.
A house extension in 2026 costs £1,800 to £3,000 per m2 for single storey and £1,650 to £2,800 per m2 for double storey. The total cost for a typical project is £45,000 to £80,000 for a single storey and £70,000 to £160,000 for a double storey, before kitchens and bathrooms. Add 10% to 15% for professional fees, and keep a 10% contingency fund for surprises. Get three quotes, check your permitted development rights, and talk to your neighbours before you do anything else.
Frequently Asked Questions
How much does a single storey extension cost in the UK?
A single storey extension costs £1,800 to £3,000 per square metre in 2026. For a typical 20 m2 rear extension, that works out at £40,000 to £65,000 including groundworks, structure, roofing, windows, first and second fix, and basic finish. Add 10% to 15% for professional fees and building regulations on top.
Is a double storey extension worth the extra cost?
Usually yes. A double storey extension costs roughly 50% to 60% more than a single storey but gives you double the floor space. You are sharing the cost of foundations, roof and professional fees across two floors, which makes the cost per square metre lower. A well done double storey extension also adds more value to your property than a single storey.
Do I need planning permission for a house extension?
Many single storey rear extensions fall under permitted development and do not need planning permission. The limits are 3 metres for terraced and semi detached houses, or 4 metres for detached houses (doubled under the larger home extension scheme with prior approval). Double storey extensions, side extensions over one storey, and extensions that are closer than 2 metres to a boundary almost always need planning permission. The application fee in England is £258 and decisions take around 8 weeks.
What is the Party Wall Act and does it apply to me?
The Party Wall etc Act 1996 applies in England and Wales when your building work involves a shared wall with a neighbour, building on a boundary, or excavating within 3 or 6 metres of a neighbouring building depending on the depth of your foundations. You must serve a Party Wall notice at least 2 months before starting work. If your neighbour agrees in writing within 14 days, you can proceed without appointing surveyors. If they dissent or do not respond, both sides appoint surveyors and you pay for both. Budget £1,000 to £2,500 per affected neighbour.
How long does a house extension take to build?
A single storey extension typically takes 10 to 14 weeks to build once on site. A double storey takes 14 to 20 weeks. Before building starts, allow 2 to 5 months for design, planning permission (if needed), building regulations and Party Wall notices. From first conversation to completion, expect 6 to 10 months for a single storey and 8 to 12 months for a double storey.