Extension Cost Calculator UK
Planning a house extension and trying to work out what it'll actually cost? This free UK extension cost calculator gives you a realistic 2026 price range in seconds. Enter your floor area, choose single or double-storey, pick your finish level and region, and tick the extras that apply — a new kitchen, a bathroom, bi-fold doors, professional fees. You'll get a low-to-high range, a most-likely figure, and a cost per square metre you can sanity-check quotes against. The figures use typical UK market rates and BCIS-style regional variation, so a job in the North East is priced differently from one in London. It's a planning estimate, not a quote — but it'll stop you being blindsided when the builder's number lands.
For double-storey, enter the total floor area across both storeys.
New footprint of the extension — length × width of the new floor area.
Regional labour and material rates can swing the total by 25%+ between London and the North East.
Adds ~£6k–£18k
Adds ~£3k–£6.5k
Adds ~£2.5k–£7k
Adds ~£2k–£4.5k
Architect, structural engineer, planning application and building control.
Enter your extension size and options to see an estimated cost.
How to use it
- Enter the floor area of the new extension in square metres (length × width of the new footprint). For a double-storey, enter the total floor area across both storeys.
- Choose the type — single-storey, double-storey, or wrap-around (L-shaped).
- Pick a finish level. Basic is a functional shell with standard fittings; Standard is the typical mid-market spec most homeowners choose; High covers premium finishes, high glazing ratios and bespoke joinery.
- Select your region — regional labour and material rates swing the total by 25%+ between London and the North East.
- Tick any extras and fees, then hit Calculate.
The two inputs that move the number most are area and region — get those right first.
What drives the cost — spec explained
| Finish level | What it typically means | Single-storey £/m² (UK avg) |
|---|---|---|
| Basic | Blockwork shell, standard plaster, basic sanitaryware and flooring, minimal glazing | ~£2,000 |
| Standard | Mid-market kitchen/bathroom-ready shell, cavity insulation to current Part L, French/patio doors, tiled or LVT floors | ~£2,500 |
| High | Large glazing, underfloor heating, high insulation values, bespoke joinery, premium finishes | ~£3,200 |
Figures are the UK national average before regional adjustment. A double-storey extension costs more in total but usually less per m², because you get two floors under one roof and one set of foundations.
A bigger extension is often cheaper per square metre than a small one — fixed costs (kitchen, glazing, fees) are spread over more floor area.
Regional variation
Building costs are not uniform across the UK. This calculator applies a regional multiplier in line with the pattern published by the Building Cost Information Service (BCIS): London and the South East run above the national average, while the North East, Wales, Scotland and Northern Ireland typically run below it. The same 25 m² kitchen extension can differ by £15,000–£20,000 on location alone.
Always price to your region, not to a national headline figure you saw online.
Worked example — Aisha's Leeds kitchen extension
Building a 25 m² single-storey rear extension in Leeds (Yorkshire & the Humber), standard spec, with a new kitchen, bi-fold doors, and professional and statutory fees included.
- Shell/build: 25 m² × £2,500/m² × 0.93 (regional) = £58,125 typical
- Extras: new kitchen (~£10,000) + premium glazing (~£4,000) = £14,000
- Fees: professional at 12% (~£6,975) + statutory ~£1,058 = £8,033
- Typical total: ~£80,200 — with a realistic range of £65,000 to £106,700, or about £3,200/m².
That per-m² figure is high because the kitchen and glazing are a big fixed spend on a relatively small footprint. On a 40 m² extension with the same fit-out, the per-m² figure would fall sharply.
On small extensions, the fit-out — not the shell — is what pushes the per-m² rate up.
UK planning & regulations
Most single-storey rear extensions fall under Permitted Development (the GPDO 2015, as amended) and don't need full planning permission, though a Larger Home Extension may need Prior Approval. Any extension must comply with the Building Regulations — notably Approved Document A (structure), Part L (energy efficiency), Part B (fire) and Part P (electrical). If you're building within 3–6 m of a neighbour's boundary or party wall, the Party Wall etc. Act 1996 may apply. Domestic extension work is standard-rated for VAT at 20% (VAT Notice 708) — extensions are not zero-rated like new builds, so factor that into any ex-VAT quote.
Budget for fees and VAT up front — they're routinely left out of the headline "£X per m²" figures.
Frequently Asked Questions
Related tools
- Loft Conversion Cost Calculator UK — 2026 dormer, mansard, rooflight and hip-to-gable rates by region
- Driveway Cost Calculator UK — block paving, resin, gravel, tarmac or concrete drive costs
- Concrete Volume Calculator — volume for the extension foundations and footings
- Brick Calculator UK — bricks and mortar for the new walls
Guides worth reading
- How Much Does an Extension Cost in the UK? (2026 Guide) — spec levels, regional variation, fees, VAT, and the per-m² figure to stop trusting
Disclaimer: This Extension Cost Calculator UK is a planning and budgeting estimate only. It uses typical 2026 UK market rates and regional averages, and does not account for ground conditions, drainage diversions, access constraints, existing-structure surprises, or your specific specification. It is not a quotation and does not replace a costed tender from a qualified builder or a cost consultant. Always obtain written quotes before committing.