The short answer
A UK single-storey extension in 2026 costs in the range of £1,800 to £3,000 per m² for the build, with the size, region, and specification doing most of the work. A small 10 m² kitchen extension runs £22,000–£30,000 build cost, a 20 m² standard rear extension runs £44,000–£55,000, and a larger 25–30 m² family-room extension runs £55,000–£80,000. Add design fees, VAT, and contingency, and a 20 m² build typically lands at £55,000–£70,000 all-in.
For the full national house extension cost guide — including rear, side, wraparound, and double-storey — see Pillar 3: UK House Extension Cost Guide 2026: Price by Type, Size, and Region. This guide is the single-storey-specific quick version.
2026 single-storey extension cost by size
| Floor area | Typical 2026 use case | Build cost (ex VAT) | All-in cost (inc VAT, fees, contingency) | |------------|-----------------------|--------------------:|------------------------------------------:| | 10 m² | Small kitchen side return or rear lean-to | £22,000–£30,000 | £28,000–£38,000 | | 15 m² | Mid-sized kitchen-diner | £30,000–£42,000 | £38,000–£54,000 | | 20 m² | Standard rear kitchen-diner (3-bed semi) | £44,000–£55,000 | £55,000–£70,000 | | 25 m² | Larger kitchen-family room (3-bed semi) | £55,000–£65,000 | £70,000–£85,000 | | 30 m² | Big family-room or wraparound | £60,000–£80,000 | £80,000–£110,000 |
The build cost column is the builder's fixed-fee quote (ex VAT). The all-in column adds design fees (typically 10% of build), VAT at 20% on the build, and a 10% contingency for hidden costs. The all-in number is the right figure to budget against.
What a 2026 single-storey extension quote includes
A 2026 fixed-fee single-storey extension quote from a builder usually includes:
- Design and structural engineering (architect drawings + structural engineer's beam and foundation design).
- Building Regulations application (Full Plans or Building Notice).
- Groundworks, foundations, and drainage, including any new soil-pipe runs and connection to the existing sewer (subject to a build-over agreement if needed).
- External walls, insulation, roof structure, and roof covering, designed to the relevant U-value targets in Approved Document Part L.
- Windows, doors, and rooflights, including bi-fold doors if specified.
- First-fix M&E for electrics, plumbing, and heating, including any new radiators or underfloor heating loops.
- Plastering, joinery, and decoration to a "builders’ finish".
It usually does not include:
- The architect's full design fee — usually 7–12% of build cost for full service, or 3–7% for design-only.
- The topographical survey — see below.
- The kitchen fit-out (cabinets, worktops, appliances) — typically £15,000–£40,000 for a 20–30 m² kitchen.
- Bi-fold doors — typically £3,000–£6,000 supply and install for a 3-panel aluminium set.
- External landscaping (patio, turf, planting) to finish the garden side of the extension.
VAT is usually standard-rated at 20% for extension work on an occupied home. A small subset of projects — extensions on a residential property that has been empty for two years or more — can be zero-rated under VAT Notice 708.
Single-storey vs double-storey: the cost difference
A double-storey extension is typically 1.5× to 1.8× the cost of a single-storey extension for the same footprint, not 2×. The reason is that the second storey shares the foundations, groundworks, roof structure, and some of the first-fix M&E, so the marginal cost of adding a second storey is much less than the cost of building a second equivalent single-storey extension beside it.
In 2026, the per-m² build cost for a single-storey extension is roughly £2,000–£2,500/m² at the national mid-range, while a double-storey extension comes in at £1,500–£2,400/m² of total floor area (counting both floors). For a 40 m² double-storey extension (20 m² × 2 floors), the build cost is roughly £60,000–£95,000, against the £40,000–£50,000 for a 20 m² single-storey at the same footprint.
A double-storey extension almost always needs a structural engineer's design for the new first-floor joists, the additional load on the foundations, and the beam positions over the bifold doors below. The structural engineer's fee is typically 1–2% of build cost.
Regional cost variations
The same 20 m² single-storey extension that costs £44,000 in the Midlands might cost £55,000 in Outer London and £66,000 in Inner London. The regional bands are:
| Region | 2026 multiplier (vs UK mid-range) | Indicative 20 m² build cost | |--------|-----------------------------------:|----------------------------:| | Inner London | ×1.30–1.50 | £57,000–£75,000 | | Outer London | ×1.15–1.25 | £50,000–£62,000 | | South East (outside London) | ×1.05–1.15 | £46,000–£60,000 | | Midlands (baseline) | ×1.00 | £44,000–£55,000 | | North West / Yorkshire | ×0.90–0.95 | £40,000–£52,000 | | North East / Wales | ×0.85–0.95 | £37,000–£52,000 |
The full regional table is in Pillar 3: UK House Extension Cost Guide 2026.
How long does a single-storey extension take?
A typical 2026 single-storey rear or side extension takes 10–12 weeks on site from the start of groundworks to practical completion. Allow an extra 8–12 weeks at the front for design, planning (if needed), and tender, so a realistic end-to-end timeline is 20–30 weeks for a typical PD project.
Do I need planning permission for a single-storey extension?
Most single-storey rear extensions on a UK house in 2026 are permitted development under Class A of GPDO 2015 Schedule 2 Part 1, with the 4 m / 6 m / 8 m depth limits depending on house type. A terraced house is limited to 4 m, a semi-detached to 6 m, and a detached to 8 m. Semi-detached and detached houses go through the larger-home prior approval process — a 42-day LPA check on neighbour amenity and highways only, not a full planning application.
The full PD rules, including the height, eaves, coverage, and materials conditions, are covered in Do You Need Planning Permission for an Extension? UK 2026 Rules.
A single-storey extension that needs full planning typically takes 8–12 weeks longer than a PD build, with the £548 planning fee (from 1 April 2026) and the architect's full planning drawings pack on top.
Where a topographical survey fits in
A single-storey extension cannot be designed or quoted accurately without a topographical survey of the existing site. The architect needs to know the levels of the garden, the boundary positions, the drainage runs, and any trees that might affect the foundation design. A topographical survey for a typical house extension in 2026 costs in the region of £800–£1,200 + VAT for a small residential site.
A single-storey extension that turns out to be 100 mm over the depth limit, or 50 mm above the height cap, can be caught at design stage if the architect has an accurate topographical survey and a measured building survey to work from. A £1,500 survey is the difference between a PD build and a forced full planning application at £548 plus 8 weeks of delay.
icelabz provides RICS-compliant topographical and measured building surveys across London and the South East, with deliverables in 2D CAD and (optionally) Revit BIM and point cloud. Contact us for a fixed-fee quote and a typical 10–15 working-day turnaround.