Single Storey Extension Cost in the UK (2026 Guide)
The cost of a single storey extension in the UK usually sits between £30,000 and £70,000, depending on size, finish, structure and glazing. A sensible rule of thumb is £1,800 to £3,000 per m², with most homes falling in the mid-range of that scale.
This guide breaks the costs down in simple steps. You will see what you are likely to spend, what each part of the build includes, and how the structural design affects your final budget.
Single Storey Extension Cost Breakdown (Quick Answer)
| Size | Typical Cost | Notes |
|---|---|---|
| Small 3m x 3m | £16,000 to £28,000 | Simple rooms, small opening |
| Medium 4m x 5m | £35,000 to £55,000 | Typical kitchen extension |
| Large 6m+ | £55,000 to £90,000+ | More glazing and steelwork |
Cost per m²:
- Basic finish: £1,800 to £2,200 per m²
- Standard finish: £2,200 to £2,700 per m²
- High-end finish: £2,800 to £3,000+ per m²
If you want a fast estimate, take your floor area and multiply by £2,200. That gives you a realistic mid-range figure.
Extension Cost per Square Metre Explained
Cost per m² changes depending on layout complexity, steelwork, glazing and finishes.
Low end (£1,800 per m²)
- Straightforward layout, small opening, simple roof, minimal drainage work
Mid range (£2,200 to £2,700 per m²)
- Kitchen installations, larger structural opening, better insulation and finishes, standard glazing
High end (£2,800 to £3,000+ per m²)
- Wide bi-folds or sliders, roof lights or roof lanterns, high-spec flooring and heating, complex steelwork
The more structure you alter, the higher the cost.
Shell Only Extension Cost
A shell only extension is one of the most commonly searched cost questions — and one of the most misunderstood. Here is exactly what it includes and what it costs.
What a shell only extension includes
- Foundations and footings
- Brickwork or blockwork walls
- Roof structure and covering
- Basic floor slab
- External openings (door and window frames, not fitted)
No plastering, no electrics, no internal walls, no finishes.
Shell only extension cost: £1,200 to £1,700 per m²
A 4m x 5m shell only extension would typically cost £24,000 to £34,000.
Shell only vs full build cost
A full build single storey extension adds plastering, flooring, electrics, plumbing, windows and doors, and internal walls on top of the shell. This brings the total to £1,800 to £3,000 per m² — roughly 40–70% more than shell only. Most homeowners choose a full build for their house extension.
Structural Calculations for a Single Storey Extension
One of the most commonly searched questions on this page: "how much do structural calculations cost for a single storey extension?" Here is the full answer.
What structural calculations include
Structural calculations for a single storey extension are a professional PDF pack that proves the design is safe and meets Building Regulations Approved Document A. The pack typically covers:
- Steel beam sizing and padstone design for the opening between old and new build
- Foundation specification — footing depth, width, bearing capacity and reinforcement
- Roof structure — rafter or flat roof beam design
- Connection details for your builder to work from
- Deflection checks to ensure the structure does not sag
The pack goes to Building Control before any steelwork is installed. Building Control inspectors check the calculations and inspect the steelwork on site before it is covered.
Structural calculation cost for a single storey extension
- Structural calculations: £350 to £650
- Structural drawings: £250 to £450
- Total structural engineer fee: £500 to £900 for a standard single storey extension
More complex extensions with wide steel frames, retaining walls or unusual ground conditions will sit at the upper end. A standard rear extension with a single RSJ opening is at the lower end.
How Deep Do Footings Need to Be for a Single Storey Extension?
Footing depth is one of the most important — and most frequently asked — questions about a single storey extension. Get it wrong and Building Control will reject the foundations.
Standard footing depths by soil type
| Soil Type | Minimum Depth | Notes |
|---|---|---|
| Non-clay (sandy, gravel) | 450mm | Building Regs minimum (Approved Document A) |
| Clay soils | 750mm to 900mm | Shrink-swell risk increases depth |
| London / expansive clay | 900mm to 1,000mm+ | NHBC guidance, common across SE England |
| Near trees (clay) | 1,500mm to 2,500mm+ | Depth depends on tree species and distance |
Why footing depth matters for cost
Deeper footings mean more excavation and more concrete. Going from 450mm to 1,000mm depth can add £1,500 to £3,500 to the foundation cost for a standard extension, depending on length and width. Trees near the proposed extension are the biggest cost driver — root zones can push required footing depth to 2m or more.
Strip foundations vs raft foundations
Most single storey extensions use strip foundations — a continuous strip of concrete under load-bearing walls. On very soft ground or where differential settlement is a risk, a structural engineer may specify a raft foundation instead. A raft covers the full footprint and costs more but provides more uniform support.
Costs by Extension Type
Rear Extension Cost
Most common and usually the easiest. £1,900 to £2,600 per m². Good for kitchens, dining rooms and living rooms.
Side Extension Cost
Often tighter access and more drainage issues. £2,000 to £2,800 per m². You may also need a party wall agreement.
Wraparound Extension Cost
Two extensions meeting at a corner. More steelwork, more roof complexity. £2,400 to £3,200+ per m².
Single Storey Kitchen Extension Cost
A kitchen extension is the most popular type of single storey extension in the UK. It almost always involves removing a load-bearing wall to open the kitchen onto the new space — which adds structural engineering costs on top of the build.
What a single storey kitchen extension typically costs
- Build cost: £28,000 to £45,000 for a 4m x 5m extension
- Kitchen units and appliances: £8,000 to £20,000
- RSJ beam for wall opening: £900 to £2,000
- Structural engineer fees: £500 to £900
- Bi-fold or sliding doors (optional): £2,000 to £5,000
Total typical range: £40,000 to £65,000 for a medium-sized single storey kitchen extension with a standard kitchen fit-out.
The structural element of a kitchen extension
The opening between the existing kitchen and the extension is almost always load-bearing. A structural engineer calculates the correct steel beam size, specifies the padstones and designs the foundation for the new extension. This calculation pack goes to Building Control before the builder can install the steel or start the foundation work. Kitchen extension structural calculations are typically £500 to £800. See our extension structural engineering guide for full details.
Pitched Roof Single Storey Extension
Single storey extensions can have either a flat roof, a pitched roof or a roof lantern. Each has different structural requirements and costs.
Pitched roof vs flat roof cost
| Roof Type | Extra Cost vs Flat | Notes |
|---|---|---|
| Flat roof | Baseline | Cheapest to build, common on rear extensions |
| Pitched roof (lean-to) | +£3,000 to £6,000 | More materials, higher wall, planning often preferred |
| Pitched roof (hipped or full) | +£5,000 to £12,000 | Most complex, required in some conservation areas |
| Roof lantern | +£2,500 to £6,000 | On flat or pitched roof, significant light gain |
Structural implications of a pitched roof extension
A pitched roof on a single storey extension creates different structural loads than a flat roof. Rafters push outward at the eaves — a structural engineer must design the wall plate connection and often a ring beam or padstone detail to resist this thrust. The wall height also increases for a pitched roof, adding masonry costs and sometimes requiring a different foundation depth.
Pitched roof single storey extension structural calculations cover all of this and are included in the standard extension calculation fee of £500 to £900.
Planning and pitched roofs
Many local authorities and conservation areas require a pitched roof rather than flat on a single storey extension. Always check with your local planning office before committing to a flat roof design — a change later costs significantly more than designing pitched from the start.
Structural Costs in a Single Storey Extension
Structural work is one of the biggest cost drivers. Here is a full breakdown.
Steel Beams (RSJs)
Typical steel beam: £900 to £1,600 supplied and installed. Large spans or box frames: £3,000 to £6,000+.
Padstones and Support
Most steel needs padstones: £150 to £350.
Structural Engineer Fees
Calculations: £350 to £650 | Drawings: £250 to £450. You need these for Building Control approval.
Building Control Fees
Usually £300 to £650 depending on location.
What Affects the Cost of a Single Storey Extension
1. Size and footprint
Bigger space = more materials, more labour.
2. Steelwork
Larger openings cost more because of design and installation.
3. Ground conditions and footing depth
Soft ground or clay means deeper foundations. Trees near the extension push footing depth significantly deeper — see the footings section above for detail.
4. Roof design
Flat roofs are cheaper. Pitched roofs cost more. Roof lights increase the price.
5. Glazing
Bi-folds or sliders can add £1,500 to £4,000.
6. Drainage work
Moving drains: £1,000 to £3,000.
7. Finishing
Plastering, flooring, plumbing and electrics: £8,000 to £15,000 on average.
Real UK Examples
Small 3m x 3m Rear Extension
- Build: £16,000 to £22,000
- Steel: £900
- Finishing: £4,000
Total: £20,000 to £27,000
Medium 4m x 5m Kitchen Extension
- Build: £28,000 to £38,000
- Kitchen: £8,000 to £15,000
- Steel: £1,200 to £2,000
Total: £40,000 to £55,000
Large Wraparound
- Build: £38,000 to £60,000
- Glazing: £4,000 to £8,000
- Steel frame: £3,000 to £6,000
Total: £55,000 to £90,000+
How to Reduce Extension Costs
- Keep the design simple
- Avoid oversize structural openings
- Use a standard roof shape
- Keep drainage in the same location
- Choose mid-range finishes
- Finalise the structural layout early — late changes to beam positions cost money
Do You Need Planning Permission?
Many single storey extensions fall under permitted development if you stay within maximum depth, maximum height and boundary limits. You usually need permission for wraparounds, conservation areas, large kitchen-diner extensions and anything higher than the current roofline. Check the Planning Portal for exact limits.
FAQs
How much does a single storey extension cost in the UK?
Usually between £30,000 and £70,000, depending on size and specification. A mid-range 4m x 5m extension costs £40,000 to £55,000.
How much is a single storey extension per m²?
Between £1,800 and £3,000 per m². Standard finish is £2,200 to £2,700.
Does a kitchen extension cost more?
Yes. Kitchens add £8,000 to £20,000 on top of the build for units and appliances.
How much do structural calculations cost for a single storey extension?
Typically £500 to £900 in total for calculations and drawings. Building Control will not approve steelwork without them.
How deep do footings need to be for a single storey extension?
At least 450mm in non-clay soils. 750mm to 900mm in clay. 1,000mm or more in London clay or expansive soils. Near trees the depth can reach 1,500mm to 2,500mm. A structural engineer specifies the exact depth as part of the calculation pack.
Do I need a structural engineer for a single storey extension?
Yes. Building Control will not approve steelwork without signed structural calculations from a qualified engineer.
Conclusion
A single storey extension usually costs between £30,000 and £70,000, with most of the price coming from structure, glazing and finishing. Footing depth, roof type and the size of the structural opening are the biggest cost variables after total floor area. Clear design decisions and early structural planning help you keep the project on budget.
If you need structural drawings and calculations, we provide fast turnaround times across the UK. See our extension structural engineering page for more detail.