A single storey extension in 2026 costs £1,500–£2,500 per m² outside London, or £2,000–£3,500/m² in London and the South East. A small 9m² extension starts from £15,000. A typical 20m² kitchen-diner extension costs £30,000–£50,000. A large 36m² extension runs £55,000–£90,000.
Cost by Size
3m × 3m (9m²)
Small utility, study, or boot room. Often permitted development. 8–10 weeks. Good if you just need one extra room.
4m × 5m (20m²)
The classic kitchen-diner extension. Big enough for a dining table, bi-fold doors, and a proper kitchen. 10–14 weeks.
5m × 5m (25m²)
Large open-plan living space. Room for a kitchen island, dining area, and sofa zone. Often the maximum under PD with prior approval.
6m × 6m (36m²)
Major ground-floor transformation. Full open-plan kitchen-living-diner. May need planning permission. 12–16 weeks build time.
3m × 8m (24m²)
Long, narrow rear extension — full width of a typical terrace. Popular in Victorian houses. More steelwork than a square plan.
4m × 8m (32m²)
Large rear extension on a detached property. Full-width kitchen-diner with serious presence. Likely needs prior approval or planning.
Rear Extension vs Side Extension
Rear Extension
The most common type. You're extending into the garden from the back wall of the house. Rear extensions benefit from the simplest structural approach — you're removing part of the rear wall and inserting a steel beam (RSJ) to support the floor above. Access for materials is usually straightforward through the garden.
Cost: £1,500–£2,500/m² · Cheapest option per square metre because the structure is simple and access is typically good.
Side Extension
Built on the side of the house, often where there's a passage or driveway. Side extensions are popular for adding a utility room, downstairs WC, or widening the kitchen. The structural work is more involved because you're often supporting the existing side wall of the house (especially if there are bedrooms above).
Cost: £1,800–£2,800/m² · 10–15% more than rear because of additional steelwork, narrower access (materials through a tight side passage), and potentially more complex roof tie-ins.
Planning note: Side extensions that face a highway almost always need planning permission. They're not covered by permitted development in most cases, and many councils have policies restricting side extensions on corners or where they affect the street scene.
Permitted Development Limits
Understanding permitted development (PD) rights can save you £462 in planning fees and 8–12 weeks of waiting. Here's what you can build without full planning permission in England:
- Semi-detached or terraced house: Up to 3m depth from the original rear wall under standard PD
- Detached house: Up to 4m depth from the original rear wall under standard PD
- Prior approval (larger home extension scheme): Semi/terraced up to 6m, detached up to 8m — but you must notify the council and neighbours get 21 days to comment
- Maximum height: 4m to the ridge, 3m to the eaves
- Must not cover more than 50% of the original garden area (including all previous extensions, sheds, and outbuildings)
- Side extensions: Must be single storey, no wider than half the width of the original house, and not face a highway
Important: PD rights can be removed or restricted by your local council through Article 4 directions, particularly in conservation areas, listed buildings, flats, and maisonettes. Always check with your council before assuming PD applies.
Regional Pricing (Per m²)
Labour and material costs vary dramatically across the UK. A builder in Manchester charges significantly less than one in Richmond. These per-m² rates are for a standard-specification single-storey extension.
What Affects the Price
Foundation conditions
Standard strip foundations: £150–£200 per linear metre. Near trees on shrinkable clay, you need deeper trenches or piled foundations (£1,500–£3,000 per pile). Poor ground can add £5,000–£15,000 to the job. A trial hole (£300–£500) before pricing avoids nasty surprises.
Roof type
Flat roof (EPDM or GRP): £80–£120/m² — cheapest and fastest. Pitched tile roof: £120–£180/m² — looks more in keeping with the house. Zinc or copper standing seam: £200–£350/m² — premium look, longer lifespan. A flat roof saves 20–30% over pitched.
Structural steelwork
A single RSJ to open up the rear wall: £800–£2,500 installed. A goalpost frame for a wide opening (4m+): £2,500–£5,000. Multiple steels for removing internal walls and supporting the first floor: £3,000–£8,000.
Glazing
Bi-fold doors (3m): £2,500–£5,000. Sliding doors (3m): £3,000–£5,500. Roof lantern: £2,500–£5,000. Multiple flat rooflights (£800–£1,500 each). The glazing package on a single storey extension often accounts for 15–20% of the total build cost.
Drainage
Building over or near a public sewer requires a build-over agreement (free, 6–8 weeks). Diverting drain runs: £1,500–£3,500. Moving the soil stack: £2,000–£4,000. Keep your extension footprint clear of drains wherever possible.
Internal specification
Basic plaster and paint: £15–£25/m². Engineered oak flooring: £50–£80/m². Polished concrete: £80–£130/m². Underfloor heating: £50–£70/m². The fit-out and finishes can account for 20–30% of the total cost.
Kitchen or bathroom inclusion
If the extension houses a new kitchen (£5,000–£20,000+) or bathroom (£3,000–£10,000), these are usually separate costs on top of the building work. A "kitchen extension" is really a building project plus a kitchen project.
Site access
Good rear access with a wide gate? Materials come straight in. No side access, narrow Victorian terrace? Everything goes through the house — add £2,000–£5,000 in labour and protection costs. A concrete pump for restricted access adds £400–£800.
Electrics and heating
Extending your central heating: £800–£2,000 (radiators or UFH). Consumer unit upgrade: £400–£700. Rewiring to accommodate the extension: £1,000–£2,500. Downlights: £40–£80 each. External lighting: £200–£600.
Time of year
Starting in winter (November–February) can save 5–10% because builders have more availability. But wet weather slows groundwork, and concrete pours below 5°C need frost protection measures, adding minor cost.
How to Save Money on a Single Storey Extension
💡 Practical cost-cutting strategies
- Stay within permitted development. Avoiding the planning application process saves £462 in fees and 8–12 weeks of waiting. Design to the maximum PD dimensions (3m semi/terraced, 4m detached) if that's enough space.
- Use prior approval for bigger extensions. If you need more depth (up to 6m semi or 8m detached), prior approval costs £120 and takes 42 days — much cheaper and faster than full planning.
- Choose a flat roof. Saves 20–30% over a pitched tile roof and takes less time. A roof lantern on a flat roof looks just as good (better, some would argue) and floods the space with light.
- Design around existing drainage. Every drain you don't move saves £1,500–£4,000. Position the extension to avoid the soil stack and main drain runs.
- Standard bi-fold sizes. A 3-panel 2.4m set is a stock size at £2,500–£4,000. Bespoke widths cost 30–50% more. Standard sizes also mean faster delivery.
- Get itemised quotes from three builders. Not just a total — line-by-line breakdowns. You'll spot which builder is overpricing steelwork and which has forgotten the drainage.
- Do your own clearance and demolition. Removing an old conservatory, clearing the site, and handling initial skip fills saves £1,000–£3,000.
- Build in the winter months. November–February: quieter diaries, better rates, faster start. The build may take a week or two longer, but the savings are real.
What Should Be in a Builder's Quote
Every quote for a single storey extension should clearly include or exclude the following:
- Groundwork and foundations — excavation, concrete, DPM, drainage connections
- Structural work — blockwork, steels, lintels, cavity insulation, wall ties
- Roof — structure, insulation, covering (flat or pitched), fascias, soffits, guttering
- Windows and doors — including bi-folds or sliders if specified
- Rooflights or lanterns — if applicable
- First and second fix electrics — sockets, switches, lights, consumer unit
- First and second fix plumbing — radiators/UFH, hot and cold supplies
- Plastering — walls and ceiling
- Decoration — mist coat and emulsion (often excluded)
- Floor screed — ready for flooring (flooring itself often excluded)
- Scaffolding, skip hire, waste removal
- Building regs compliance — though the fee may be separate
Common exclusions: Kitchen/bathroom fitting, flooring, tiling, landscaping, building regs fees, party wall costs. Clarify everything upfront.
Hidden Costs to Budget For
Building Regulations
Full plans application required for every single storey extension. Some builders include this, but many don't — always ask.
Structural Engineer
Steel beam calculations, foundation design, structural report. Required for building regs and essential before your builder starts.
Architectural Drawings
Design, planning, and building regs drawings. A simple rear extension is at the lower end. Complex layouts with structural changes cost more.
Party Wall Surveyor
Needed if digging foundations within 3m of a neighbour's property (6m if deeper than their foundations). Your neighbour can appoint their own surveyor — you pay.
Planning Application
Only needed if your extension doesn't fall under permitted development. Prior approval is cheaper at £120. Check your PD rights first.
Garden Reinstatement
New patio, fencing, re-turfing, drainage. The garden always takes a beating during a build — budget for putting it right.
Frequently Asked Questions
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