A house extension in 2026 costs between £1,500 and £3,000 per square metre depending on the type, specification, and where you are in the UK. A typical 20m² single-storey rear extension costs £36,000–£50,000. A 40m² double-storey extension costs £60,000–£100,000.
Extension Costs at a Glance
Single Storey Rear
15–25m² · £1,800–£2,500/m² · 10–14 weeks build time. The most common extension type.
Single Storey Side
12–20m² · £1,800–£2,400/m² · 10–12 weeks. Great for adding a utility or dining room.
Double Storey Rear
30–50m² total · £1,500–£2,200/m² · 14–20 weeks. Best value per m² — adds 2 rooms.
Side Return Extension
8–15m² · £2,200–£3,000/m² · 8–12 weeks. Popular on Victorian terraces.
Wrap-Around
30–50m² · £2,000–£2,800/m² · 16–24 weeks. Combines rear and side for maximum space.
Over-Structure
15–30m² · £2,000–£2,600/m² · 12–16 weeks. Building above a garage or existing extension.
Detailed Cost Breakdown by Type
Single Storey Rear Extension
The bread and butter of UK house extensions. You're extending out the back of the house, typically into the garden. A 4m × 5m (20m²) rear extension is the sweet spot — big enough to transform your ground floor without needing planning permission in most cases.
Costs: Budget £36,000–£50,000 for a 20m² rear extension at standard spec. That gets you strip foundations, blockwork walls, a flat or pitched roof, bi-fold or sliding doors, plastering, electrics, plumbing for a radiator or two, and decoration. At the higher end you're looking at underfloor heating, higher-spec glazing, and premium finishes.
What pushes the price up: Moving the soil stack (£2,000–£4,000), digging deeper foundations near trees (add 20–30%), lantern roof lights (£3,000–£6,000 each), and bi-fold doors over 3m wide (£4,000–£8,000).
Double Storey Extension
The best value extension per square metre. You're sharing foundations and roof structure across two floors, so the cost per m² drops compared to building single storey. A 4m × 5m double storey (40m² total) gives you a kitchen-diner downstairs and a bedroom with ensuite upstairs.
Costs: Expect £60,000–£100,000 for a 40m² double storey. That's roughly 50–70% more than a single storey, not double. The upper floor adds walls, a floor structure, windows, a staircase modification, and finishing — but the expensive groundwork and roof are already covered.
Planning note: Double storey extensions almost always need planning permission. Budget £462 for the application in England. You'll also need to maintain 7m from the rear boundary to the back of the extension.
Side Return Extension
Hugely popular on Victorian and Edwardian terraces where there's a narrow alley (the side return) running along the kitchen. You're typically adding 8–15m² and opening up the kitchen into an open-plan space.
Costs: Side returns cost £30,000–£55,000 despite the smaller footprint. The per-metre cost is higher (£2,200–£3,000/m²) because you still need structural steelwork, a new roof, drainage alterations, and the access for building materials through a narrow alley is awkward and slow.
Wrap-Around Extension
A wrap-around combines rear and side extensions into one L-shaped or U-shaped build. These are the big-ticket extensions that completely transform a ground floor — typically creating a huge open-plan kitchen-living-dining space.
Costs: Budget £60,000–£120,000+ for a wrap-around. The larger footprint means more foundations, more steel, and a more complex roof. But you get transformative space — 30–50m² of new living area. At the higher end, expect structural glazing, vaulted ceilings, and premium kitchen installations.
Regional Pricing (Per m², Single Storey)
Where you live has a significant impact. Labour rates, material delivery costs, and local demand all vary. Here's what you'll pay per square metre for a standard single-storey extension across the UK in 2026:
What Affects the Price
Foundation depth
Standard strip foundations cost £150–£200/m. Near trees or on clay soil, you may need piled foundations at £1,500–£3,000 per pile. A deep foundation job can add £5,000–£15,000.
Structural steelwork
Opening up the back wall to your new extension requires steel beams (RSJs). A single beam costs £800–£2,500 installed. Complex openings with multiple steels can hit £3,000–£6,000.
Roof type
A flat roof (EPDM or GRP) is cheapest at £80–£120/m². A pitched tile roof costs £120–£180/m². Zinc standing seam or green roofs push to £200–£350/m².
Glazing specification
Standard bi-fold doors (3m): £3,000–£5,000. Premium aluminium bi-folds (4m+): £5,000–£10,000. Structural glass roof panels: £800–£1,500/m². Roof lanterns: £2,500–£6,000.
Moving the drainage
If you're building over or near a public sewer, you'll need a build-over agreement (free from water company but takes 6–8 weeks). Moving the soil stack costs £2,000–£4,000. Diverting a drain run: £1,500–£3,500.
Access to the site
Rear-access only? Materials carried through the house or craned over? Restricted access adds £2,000–£5,000 to a project. A skip on the road needs a permit (£30–£80/fortnight).
Heating system
Extending your existing central heating into the new space: £800–£2,000. Underfloor heating (water): £60–£100/m². Underfloor heating (electric): £30–£50/m². New radiators: £250–£500 each installed.
Electrics & lighting
A new consumer unit upgrade: £400–£700. Rewiring to accommodate the extension: £1,000–£2,500. Downlight installation: £40–£80 per light. External lighting: £200–£600.
Internal finishing standard
Basic plaster and paint: £15–£25/m². Engineered oak flooring: £50–£80/m². Polished concrete: £80–£130/m². The fit-out can easily account for 20–30% of total cost.
Time of year
Builders are busiest March–October. Starting in winter can get you 5–10% off, but groundwork in wet weather takes longer. Concrete pours below 5°C need frost protection, adding cost.
How to Save Money on Your Extension
💡 Practical ways to reduce your extension costs
- Keep the soil stack where it is. Designing around existing drainage rather than moving it saves £2,000–£4,000 instantly.
- Choose a flat roof. A flat EPDM or GRP roof costs 30–40% less than a pitched tile roof and is faster to build.
- Go double storey if you need the space. Per m², a double storey costs 25–35% less than building the same area single storey in two separate projects.
- Do your own demolition and clearance. Removing an old conservatory or clearing the site yourself saves £1,000–£3,000 in labour and skip costs.
- Source your own materials. Buying tiles, sanitaryware, or lighting directly rather than through your builder avoids their 15–25% markup. Discuss this upfront.
- Get three detailed quotes. Not just bottom-line prices — ask for itemised breakdowns so you can compare like for like.
- Avoid design changes mid-build. Variations after work starts typically cost 20–40% more than if they'd been included from day one. Finalise everything before groundwork begins.
- Build in winter. Some builders offer 5–10% discounts during their quieter months (November–February). The build takes slightly longer but costs less.
What Should Be Included in a Builder's Quote
When comparing quotes, make sure each one covers the same scope. A proper builder's quote for a house extension should include:
- Bathroom Tiling and foundations — excavation, concrete, drainage connections
- Structural work — blockwork walls, steel beams, lintels, damp-proof course
- Roof structure and covering — joists, insulation, membrane, tiles or flat roof system
- Windows and external doors — including bi-folds/sliders if specified
- First and second fix electrics — sockets, switches, lighting, consumer unit
- First and second fix plumbing — radiators/UFH, waste pipes, water supply
- Plastering — walls and ceilings, ready for decoration
- Decoration — mist coat and two coats of emulsion (sometimes excluded)
- Flooring preparation — screed or boarding (actual flooring often excluded)
- Scaffolding — for double storey or roof work
- Skip hire and waste removal
- Building regulations compliance — though the application fee may be separate
Watch out for exclusions: Kitchen/bathroom fitting, flooring, landscaping/making good the garden, decoration, and building regs fees are commonly excluded. Always ask.
Hidden Costs to Budget For
Building Regulations
Full plans application. Required for every extension. Some builders include this, many don't. Ask.
Structural Engineer
Beam calculations, foundation design, structural drawings. Essential for building regs approval.
Architectural Plans
Design drawings and planning/building regs drawings. More complex designs cost more.
Party Wall Surveyor
Required if building within 3m of a neighbour's wall (6m for deep foundations). Your neighbour can appoint their own surveyor — at your cost.
Planning Application
Householder planning application fee in England (2026). Not needed if your extension falls under permitted development.
Making Good the Garden
New patio, fencing, turf, planting after the builders leave. The garden always takes a hammering during a build.
Frequently Asked Questions
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