A home extension is one of the most significant building projects a homeowner can undertake. It is also one of the most variable in terms of cost. Two neighbours with identical houses can end up with wildly different bills depending on the type of extension, the specification, the groundwork conditions, and whether they went with a design and build firm or managed the trades themselves.
This guide cuts through the variation and gives you real numbers based on what extensions actually cost in 2026. We cover every common extension type, the costs you might not be expecting, and how to tell whether your project is a good financial decision.
A single storey rear extension costs £25,000 to £50,000. A double storey extension costs £45,000 to £90,000. A garage conversion costs £15,000 to £30,000. All figures are for the build cost and exclude architect fees, planning, and structural engineering costs.
Extension Costs at a Glance
| Extension Type | Typical Total Cost | Cost per m² |
|---|---|---|
| Single storey rear | £25,000 to £50,000 | £1,200 to £2,200 |
| Double storey rear | £45,000 to £90,000 | £1,000 to £1,800 per floor |
| Side return | £25,000 to £50,000 | £1,500 to £2,500 |
| Wrap around (L-shape) | £50,000 to £100,000+ | £1,400 to £2,400 |
| Garage conversion | £15,000 to £30,000 | £1,000 to £1,600 |
| Over garage | £35,000 to £70,000 | £1,500 to £2,500 |
Regional Price Variations
Labour rates are the primary driver of regional variation. The bricks, timber, and steel cost roughly the same wherever you are in England, but the daily rate for a builder in London is substantially higher than in the North West or Wales.
| Region | Single Storey Rear (25m²) | Double Storey (50m²) |
|---|---|---|
| London | £40,000 to £65,000 | £70,000 to £110,000 |
| South East | £35,000 to £55,000 | £60,000 to £95,000 |
| South West | £30,000 to £50,000 | £55,000 to £85,000 |
| Midlands | £27,000 to £46,000 | £50,000 to £82,000 |
| North of England | £25,000 to £42,000 | £45,000 to £75,000 |
| Scotland | £25,000 to £44,000 | £46,000 to £78,000 |
| Wales | £23,000 to £40,000 | £43,000 to £72,000 |
Breaking Down the Costs
Groundwork and foundations
Foundations are the most unpredictable element of any extension. A standard strip foundation on good ground costs £3,000 to £8,000 for a single storey extension. The moment the ground becomes difficult (made ground, tree roots, high water table, clay that shrinks and swells), the cost escalates. Raft foundations on poor ground can run £10,000 to £20,000. This is why experienced builders always price groundwork as an unknown until they break ground. Budget a 15% contingency specifically for groundwork surprises.
Structure (walls, roof, steel)
| Element | Typical Cost |
|---|---|
| External walls (blockwork, brick outer leaf) | £4,000 to £10,000 |
| Flat roof (single ply membrane) | £3,500 to £7,000 |
| Pitched roof (to match existing house) | £6,000 to £14,000 |
| Steel beam to open rear wall | £1,500 to £4,000 fitted |
| Bi-fold or sliding doors (per leaf) | £700 to £1,500 per leaf |
| Roof lantern or glazed roof | £3,000 to £8,000 |
First and second fix (internals)
| Trade | Typical Cost |
|---|---|
| Plumbing (if extending kitchen or adding bathroom) | £2,000 to £6,000 |
| Electrics (rewire extension, consumer unit) | £1,500 to £4,000 |
| Plastering and boarding (walls and ceiling) | £2,000 to £5,000 |
| Underfloor heating (wet system) | £1,200 to £3,000 for 25m² |
| Flooring (tiling, engineered wood, or LVT) | £1,500 to £4,000 |
| Decoration (painting, skimming) | £800 to £2,500 |
Professional Fees on Top of Build Costs
The build cost is not the full cost. You also need to budget for the professionals who design, approve, and certify the project.
| Fee | Typical Cost |
|---|---|
| Architect or architectural technician (design only) | £1,500 to £5,000 |
| Full project management (architect) | 5% to 15% of build cost |
| Structural engineer | £500 to £1,500 |
| Planning application fee (England) | £206 householder application |
| Building regulations application (local authority) | £750 to £1,500 |
| Party wall surveyor (if shared wall) | £700 to £2,000 |
| Energy performance certificate (if required) | £60 to £120 |
Types of Extension Explained
Single storey rear extension
The most common type of home extension. It extends the footprint of the ground floor into the back garden. Typical sizes run from 15 to 35 square metres. Most people use this space to create a larger kitchen, a kitchen diner with bi-fold doors, or a family room opening to the garden. The key design decision is the roof. A flat roof with a roof lantern is cheaper than a pitched roof that matches the existing house. Bi-fold or sliding doors opening to the garden add cost but are often the centrepiece of the project.
Double storey rear extension
A double storey extension adds space on both the ground and first floors simultaneously. The groundwork and roof costs are shared across two floors, which means the cost per square metre is lower than a single storey project. If you need both a larger ground floor room and an extra bedroom or bathroom upstairs, doing both at once is almost always cheaper than two separate projects. You will almost certainly need full planning permission for a double storey extension, and a party wall agreement if you share walls with neighbours.
Side return extension
A side return is the narrow gap between the side of a semi-detached or terraced house and the boundary fence or wall. Filling in this space with an extension can transform a small galley kitchen into a full width kitchen diner. Popular in Victorian terraces across London and northern cities, the side return extension typically adds 8 to 15 square metres but creates a disproportionate improvement in how usable the kitchen feels. The relatively small footprint means costs are comparable to a single storey rear extension despite the narrower space.
Garage conversion
If you have an integral or attached garage, converting it is the most cost effective way to add a room to your home. The structure already exists. You are insulating the walls, replacing the garage door with a window or wall, laying a proper floor, and connecting it to the house. Most garage conversions do not require planning permission and are completed in 3 to 6 weeks. The most common uses are a home office, fourth bedroom, playroom, or gym. The main limitation is ceiling height. Victorian and Edwardian garages often have generous proportions. Modern estate garages can feel cramped once converted.
Planning Permission: What You Need to Know
Many single storey rear extensions fall within permitted development rights, meaning you can build without a planning application. The limits are:
- Up to 4 metres behind the original rear wall for detached houses
- Up to 3 metres behind the original rear wall for semi-detached and terraced houses
- No more than half the area of the original garden covered by extensions and outbuildings
- Maximum height of 4 metres for a single storey extension
- Materials should match the existing house in appearance
If your extension falls within these limits, you can apply for a Lawful Development Certificate (around £103 in England) to confirm you do not need planning permission. This protects you when you come to sell the property.
If your house is in a conservation area or is a listed building, permitted development rights are significantly reduced. Always check with your local planning authority before starting design work.
Hidden Costs That Catch People Out
- Party wall agreement: If your extension involves any work to a shared wall, your neighbours must be formally notified under the Party Wall Act 1996. If a neighbour does not consent, you each need a surveyor. A non-contested party wall agreement costs £700 to £1,200. A disputed one can reach £3,000 to £5,000.
- VAT: All building work on an existing home is subject to VAT at 20%. Ensure your quotes are clear about whether VAT is included. A £40,000 build cost plus VAT is £48,000. New builds attract zero VAT, but extensions do not.
- Tree surveys: If there are trees near the build area, a structural engineer may require an arboricultural survey (£300 to £600) before confirming the foundation design.
- Temporary accommodation: Large rear extensions often make the kitchen unusable for 6 to 12 weeks. Budget £400 to £800 in takeaways and eating out costs.
- Boiler and radiator relocation: If the extension removes the boiler cupboard or a radiator on the rear wall, relocation adds £500 to £2,000.
- Drainage: If the extension falls over existing drainage, the drains must be diverted or built over with a special agreement from the water authority. This can add £2,000 to £5,000.
How Long Does a Home Extension Take?
| Stage | Typical Duration |
|---|---|
| Design and planning drawings | 4 to 8 weeks |
| Planning permission (if required) | 8 to 13 weeks |
| Tender and contractor selection | 3 to 6 weeks |
| Single storey build | 10 to 16 weeks |
| Double storey build | 16 to 24 weeks |
| Garage conversion | 3 to 6 weeks |
From the first conversation with an architect to completing the decoration, a typical single storey extension takes 6 to 12 months. Projects that require planning permission sit at the longer end of that range. If you want your extension finished for a particular date (Christmas, a new arrival), work backwards from that date and start the design process at least 9 months before.
Does a Home Extension Add Value?
Estate agents and property professionals consistently report that a well designed home extension adds 10% to 20% to a property's value. The specific return depends on the type of extension, the property value, and the local market.
| Extension Type | Typical Value Added | Best Case ROI |
|---|---|---|
| Open plan kitchen extension | 10% to 20% | Excellent. Buyers pay premium for this space. |
| Extra bedroom | 10% to 15% | Strong. Adds a bedroom to the sales listing. |
| Home office | 5% to 10% | Good in post-pandemic market. |
| Garden room or utility | 3% to 8% | Moderate. Depends on buyer priorities. |
In London and the South East, extensions tend to offer a stronger financial return because high baseline property values mean the percentage gain translates to a larger cash figure. A 15% uplift on a £600,000 property is £90,000. The same percentage on a £200,000 property is £30,000. The build cost is similar in both cases, so the return on investment is fundamentally different.
Do not expect an extension to return more than you spend in every location and property type. Over-improving a terraced house in a low value area often fails to recoup the cost because the property value is capped by its neighbourhood. Get a valuation from two or three estate agents before committing to a large project.
How to Get the Best Value on Your Extension
- Get at least three quotes from builders. Extension costs vary significantly between contractors. Three competitive quotes give you a realistic sense of the market and protect you from being overcharged.
- Use a design and build firm for a fixed price. A single contractor responsible for design, planning, and build means one point of accountability and, usually, a fixed price contract. The margin is higher than managing trades yourself, but the management overhead and risk sit with the contractor.
- Specify clearly before going to tender. Vague specifications produce vague quotes that do not compare like for like. Agree on bi-fold doors versus sliding doors, flat roof versus pitched, floor finish, and glazing type before pricing.
- Build in a 15% contingency. Groundwork surprises, hidden drainage, and structural discoveries are common. A contingency means you do not run out of money mid-project.
- Do not skip building regs. Building regulations sign off is not optional. A completed extension without a completion certificate will cause problems when you sell the property, and your mortgage lender may refuse to lend against it.
Red Flags When Hiring Builders
- Quote based on a phone call with no site visit. No experienced builder will price an extension without visiting the site. Anyone who quotes without coming to see the property is guessing.
- Full payment upfront. A deposit of 20% to 30% on a staged payment schedule is standard. Paying the full amount before work starts is never appropriate.
- No mention of planning or building regs. A builder who does not raise planning and building regulations is either unaware of the requirements or hoping to skip them. Either situation should give you pause.
- No contract. All work over a few thousand pounds should be covered by a written contract specifying the scope, price, payment schedule, start date, and completion date.
- Very low quotes. If one quote is 40% cheaper than the others, it is almost certainly missing something. Either the specification is thinner or the contingency is absent. You will usually end up paying more, not less, once the extras arrive.
A home extension costs £25,000 to £50,000 for a single storey rear addition and £45,000 to £90,000 for a double storey. Add 15% to 25% for professional fees, planning, and contingency. Do it properly, use a contract, and get building regs sign off, and a well designed extension will add genuine value to your home.
Frequently Asked Questions
How much does a single storey home extension cost in the UK?
A single storey rear extension costs £25,000 to £50,000 in the UK in 2026 for a 20 to 30 square metre addition. The cost per square metre ranges from £1,200 to £2,200 depending on specification, location, and groundwork complexity.
How much does a double storey extension cost in the UK?
A double storey extension costs £45,000 to £90,000 in the UK in 2026. The cost per square metre is lower than a single storey extension because the foundations and roof are shared across two floors.
Do I need planning permission for a home extension?
Single storey rear extensions within 4 metres of the original rear wall (3 metres for semi or terraced homes) often fall under permitted development and do not require planning permission. Double storey extensions and extensions to listed buildings or homes in conservation areas almost always require a full planning application.
How long does a home extension take?
A single storey extension takes 10 to 16 weeks to build once work starts. Factor in 4 to 8 weeks for design, up to 13 weeks for planning permission if required, and a further 3 to 6 weeks to procure a builder. The total process from start to finish typically takes 6 to 12 months.
Does a home extension add value?
A well designed home extension typically adds 10% to 20% to a property's value. A kitchen extension opening to the garden offers among the strongest returns. The financial case is strongest in areas with high baseline property values where the percentage gain translates to a larger cash figure.
What is the cheapest type of home extension?
A garage conversion is typically the cheapest way to add usable space at £15,000 to £30,000, because the external structure already exists. A single storey rear extension is the next most affordable option at £25,000 to £50,000.
What are the hidden costs of a home extension?
Hidden costs include VAT at 20% on all labour and materials, party wall surveyor fees (£700 to £2,000 if you share walls with neighbours), structural engineer fees, planning and building regulations fees, and a contingency for groundwork surprises. Budget an additional 20% to 25% on top of the build cost to cover these items.
See also: How Much Does a New Kitchen Cost in the UK? and How Much Does a Loft Conversion Cost?