Cost of Extensions UK 2026: Full Price Guide with Real Examples
The cost of extensions in the UK varies enormously depending on size, type and finish quality. This guide gives you clear, realistic numbers for 2026 based on real domestic projects, broken down by extension type, floor area and build quality. All figures reflect current 2026 labour rates, material costs and groundwork prices.
Whether you want the cost of extensions for a small rear kitchen addition or a large two storey wraparound, this is the complete 2026 reference. Every section includes both ex VAT and inc VAT figures so you can budget accurately from the start.
Cost of Extensions UK: Average Prices in 2026
The cost of extensions in the UK in 2026 ranges from £32,000 to £155,000 for single storey work depending on size and quality. Two storey extensions typically cost between £120,000 and £195,000. These figures include materials and labour at current 2026 rates.
Quick summary: cost of extensions by type
- Single storey extension cost: £1,900 to £3,300 per m²
- Double storey extension cost: £2,000 to £3,200 per m²
- First floor extension over garage: £50,000 to £82,000
- Rear extension (terraced home): £32,000 to £46,000
- Shell only extension cost: £1,300 to £1,900 per m²
Cost of extensions per m² in 2026
| Extension Type | Cost per m² (ex VAT) | Cost per m² (inc VAT) |
|---|---|---|
| Basic build | £1,900 to £2,200 | £2,280 to £2,640 |
| Standard build | £2,200 to £2,700 | £2,640 to £3,240 |
| Premium build | £2,700 to £3,300 | £3,240 to £3,960 |
Single Storey Extension Cost UK 2026
The single storey extension cost is the most common type of home addition in the UK. Single storey extensions add space to the ground floor without the complexity or cost of building upwards. The single storey extension cost in 2026 starts from around £30,000 for a small rear addition and rises to £150,000 or more for a large premium build.
Small single storey extension cost (15 to 20m²)
£30,000 to £48,000 ex VAT
£36,000 to £57,600 inc VAT
Medium single storey extension cost (25 to 30m²)
£50,000 to £81,000 ex VAT
£60,000 to £97,200 inc VAT
Large single storey extension cost (40 to 50m²)
£95,000 to £150,000 ex VAT
£114,000 to £180,000 inc VAT
Cost of Extensions by Size (2026)
The cost of extensions varies significantly with floor area. Here are typical 2026 costs for the most common extension sizes, at standard build quality including labour, materials and groundworks.
| Extension Size | Typical Cost (ex VAT) | Typical Cost (inc VAT) |
|---|---|---|
| 3m x 3m (9m²) | £18,000 to £25,000 | £21,600 to £30,000 |
| 3m x 4m (12m²) | £24,000 to £33,000 | £28,800 to £39,600 |
| 3m x 5m (15m²) | £30,000 to £42,000 | £36,000 to £50,400 |
| 4m x 4m (16m²) | £32,000 to £45,000 | £38,400 to £54,000 |
| 4m x 5m (20m²) | £40,000 to £55,000 | £48,000 to £66,000 |
| 5m x 5m (25m²) | £50,000 to £68,000 | £60,000 to £81,600 |
| 6m x 3m (18m²) | £36,000 to £50,000 | £43,200 to £60,000 |
| 6m x 4m (24m²) | £48,000 to £65,000 | £57,600 to £78,000 |
| 20m² | £38,000 to £54,000 | £45,600 to £64,800 |
| 30m² | £57,000 to £81,000 | £68,400 to £97,200 |
| 50m² | £95,000 to £135,000 | £114,000 to £162,000 |
All figures are for single storey rear or side extensions at standard build quality. Kitchen and bathroom fitting costs are additional. The cost of extensions with premium finishes or complex roof designs will be higher.
Shell Only Extension Cost 2026
The shell only extension cost is significantly lower than a fully finished build because you receive the watertight external structure without internal finishes. Shell only extension cost in 2026 runs from £1,300 to £1,900 per m² ex VAT, compared to £1,900 to £3,300 per m² for a full turnkey extension.
The shell only approach suits homeowners who want to manage internal trades themselves, or who need to phase spending. Many builders offer shell only pricing as a separate contract.
What is included in a shell only extension cost
- Foundations and groundworks
- External walls (brick or block)
- Roof structure and covering
- External doors and windows (openings only, or basic units)
- Basic concrete floor slab
What is not included in a shell only extension cost
- Internal plaster and dry lining
- Insulation to walls, floor and roof
- Plumbing and heating
- Electrics and consumer unit
- Internal doors and ironmongery
- Flooring and decoration
- Kitchen or bathroom fitting
Typical shell only extension cost by size
| Size | Shell Only Cost (ex VAT) | Shell Only Cost (inc VAT) |
|---|---|---|
| 15m² | £19,500 to £28,500 | £23,400 to £34,200 |
| 20m² | £26,000 to £38,000 | £31,200 to £45,600 |
| 30m² | £39,000 to £57,000 | £46,800 to £68,400 |
| 50m² | £65,000 to £95,000 | £78,000 to £114,000 |
Double Storey Extension Cost 2026
The double storey extension cost works out better value per square metre than two separate single storey builds because the foundations and roof are shared between both floors. Typical double storey extension cost in 2026 for a 60m² build is £120,000 to £195,000 ex VAT.
£144,000 to £234,000 inc VAT for a 60m² double storey extension at standard to premium quality.
First Floor Extension Cost 2026
A first floor extension above an existing garage or ground floor room avoids major groundwork costs because the foundations are already in place. The first floor extension cost in 2026 for a 20 to 25m² addition is typically £50,000 to £82,000 ex VAT.
£60,000 to £98,400 inc VAT for a first floor extension at standard quality.
Cost of Extensions: Structural Engineering Fees
The cost of extensions always includes a structural engineering fee. Building Control requires structural calculations before approving any extension, and this fee is in addition to the build cost. Most homeowners budget for this separately when planning the overall cost of their extension.
Structural calculations for a single storey extension
A complete structural design pack for a single storey extension in 2026 costs:
- £550 to £900 for a straightforward rear extension
- £800 to £1,200 for a more complex layout with multiple beams
- £1,200 to £1,500 for wraparound or L-shaped extensions
The pack covers beam design, foundation checks, roof structure and all Building Control documentation. See our full guide to structural engineering for extensions.
Steel beam design costs for extensions
The cost of steel beam design within an extension structural pack is typically:
- £250 to £450 for a single beam over a new opening
- £350 to £600 for complex or compound beams
- £600 to £1,200 for a full steel package with multiple beams
Foundation depth and groundwork costs
Most single storey extension footings need to be at least 1m deep in normal ground conditions. In clay soils this can increase to 1.5m or more. The structural engineer checks foundation requirements as part of the extension calculations. Groundwork costs are typically £3,000 to £8,000 for a standard single storey extension.
Side Extension and Wraparound Extension Cost 2026
Side extension cost
£1,900 to £3,000 per m² ex VAT
£2,280 to £3,600 per m² inc VAT
Wraparound extension cost
Wraparound extensions combine a rear and side extension into one L-shaped addition. The wraparound extension cost is higher per m² because of the extra steelwork at the corner and the more complex roof structure.
£2,200 to £3,400 per m² ex VAT
£2,640 to £4,080 per m² inc VAT
Rear Extension Cost for Terraced and Semi-Detached Homes
Rear extensions are the most common type of home extension in the UK. Most terraced and semi-detached houses have a straight rear boundary that suits a simple rectangular extension. The rear extension cost depends almost entirely on floor area and finish quality.
- Small rear extension 15m²: £32,000 to £46,000
- Medium rear extension 30m²: £62,000 to £92,000
- Large rear extension 50m²: £105,000 to £155,000
Cost of Extensions with Kitchens and Bathrooms
The cost of extensions that include a new kitchen or bathroom includes fitting costs on top of the build price. These are the main additions that push the cost of extensions above the standard per m² rate.
Kitchen extension cost additions
Add £16,000 to £38,000 for a full kitchen fit-out including:
- Kitchen units and worktops
- Plumbing for sink and dishwasher
- Electrics and lighting circuits
- Gas supply if required
- Possible boiler relocation
Bathroom extension cost additions
Add £6,000 to £10,000 for plumbing, sanitary ware, tiling and waste connections.
Loft Conversion and Garage Conversion Costs (2026)
While not extensions in the traditional sense, loft and garage conversions are common alternatives when homeowners are planning the cost of extensions against other options. The loft conversion cost in 2026 runs from £52,000 to £100,000 ex VAT depending on type. Whether a dormer or hip to gable conversion, steel beams are always required and are included in the structural engineer fee.
- Loft conversion: £52,000 to £100,000 ex VAT (£62,400 to £120,000 inc VAT)
- Garage conversion: £9,000 to £22,000 ex VAT (£10,800 to £26,400 inc VAT)
- Basement extension: £110,000 to £175,000 ex VAT (£132,000 to £210,000 inc VAT)
Hidden Costs That Affect the Cost of Extensions
The cost of extensions can increase significantly if any of the following apply to your project. It is worth checking each one before finalising your budget.
- Access restrictions: Tight side access for materials increases labour time and cost.
- Deep foundations: Clay or poor soil conditions can require foundations 1.5m deep or more.
- Drainage diversions: Moving a drain run adds £1,500 to £5,000. Relocating a manhole can add £2,000 to £4,500.
- Party wall agreements: If the extension is within 3m of a neighbour's foundation you may need a Party Wall Agreement, which costs £800 to £2,000 per neighbour.
- Steelwork: Large open-plan spans and bi-fold door openings need heavier beams which increase both the structural engineering fee and the steel supply cost.
- Internal finishes: Bespoke kitchens, underfloor heating and premium glazing can add £15,000 to £40,000 to the overall cost of extensions.
How to Reduce the Cost of Extensions in 2026
- Keep the footprint rectangular to complex shapes increase costs.
- Avoid unnecessary structural changes to fewer beams means lower engineering and build costs.
- Use a shell only contract and manage internal trades separately.
- Get structural calculations early to late changes to structural layouts are expensive.
- Build during winter if possible to some builders offer lower rates in slower periods.
Planning and Building Control Costs for Extensions
These fees are separate from the build cost and need to be added to any cost of extensions budget.
- Planning permission: £206 for most domestic applications in England (extensions under Permitted Development are free).
- Building Control fee: £600 to £1,200 depending on your local council or private approved inspector. See our Building Control guide.
- Structural calculations: £550 to £1,500 depending on the scope and complexity of the extension.
- Architect or designer fees: £1,500 to £5,000 for planning drawings depending on the size of the project.
Real UK Cost of Extensions Examples (2026 Projects)
Example 1: 20m² rear kitchen extension (standard finish)
- Build cost: £48,000 ex VAT
- Kitchen fit-out: £18,000
- Structural calculations: £650
- Building Control fee: £700
Total cost of extensions project: £67,350 inc VAT
Example 2: 30m² side extension (standard finish)
- Build cost: £72,000 ex VAT
- Plumbing and electrics: £6,000
- Windows: £3,200
- Structural calculations: £750
Total cost of extensions project: £98,340 inc VAT
Example 3: 50m² wraparound extension (premium finish)
- Build cost: £140,000 ex VAT
- Extra steelwork: £6,000
- Kitchen fit-out: £20,000
- Structural calculations: £1,200
Total cost of extensions project: £202,440 inc VAT
FAQ: Cost of Extensions UK 2026
What is the average cost of extensions in the UK in 2026?
The average cost of extensions in the UK in 2026 is £1,900 to £3,300 per m² for a single storey build at standard quality. A typical 20m² rear extension costs £40,000 to £55,000 ex VAT including labour, materials and groundworks but excluding kitchen or bathroom fitting.
How much does a 20m² extension cost in 2026?
Between £40,000 and £66,000 ex VAT depending on build quality. A basic 20m² extension costs from £38,000. A premium 20m² extension can reach £66,000 or more.
What is the shell only extension cost per m² in 2026?
The shell only extension cost is £1,300 to £1,900 per m² ex VAT. This covers foundations, walls, roof and basic openings but does not include internal finishes, plumbing, electrics or kitchen and bathroom fitting.
Is a double storey extension cheaper than two single storey extensions?
Yes. The cost of extensions on two floors is lower per m² than two separate single storey builds because the foundations and roof are shared. A double storey extension typically costs £2,000 to £3,200 per m² compared to £1,900 to £3,300 for a single storey.
Do I need a structural engineer for an extension?
Yes. Building Control requires structural calculations before approving any extension. The structural engineer designs the beams, checks the foundations and produces the full Building Control calculation pack. The fee is typically £550 to £1,500 and should be added to any cost of extensions budget. Get a structural engineer quote early in the process.
How long does a single storey extension take to build?
Most single storey projects take 8 to 14 weeks from groundworks starting to practical completion. Complex layouts or weather delays can extend this timeline.
Which parts of an extension cost the most?
Kitchens, bathrooms, bi-fold glazing and steelwork for large open-plan spans are consistently the most expensive elements. Groundworks in clay or poor soil conditions can also add significant cost.
Cost of Extensions: Next Steps
The cost of extensions in 2026 is well within reach for most UK homeowners with careful planning and clear budgeting. Use the figures in this guide as a realistic starting point, then get fixed quotes from builders and a fixed-fee structural engineer calculation pack before committing to a contract.
If you need structural calculations, beam design or foundation advice for your extension, SECalcs provides fixed-fee structural engineering from £550 for a complete extension pack. Get a free quote today.