House Extension Cost Calculator UK
Get an instant build-cost estimate for your extension. Based on real pricing data from 519 UK towns, cross-referenced with RICS BCIS figures. No email required — get your number in 60 seconds.
Build Your Extension Estimate
Tell us about your project in 4 quick steps. Get a realistic cost range, timeline and recommended contingency — plus an option to receive 3 local builder quotes.
The Maths Behind Your Estimate
No black-box magic. Here's exactly how we arrive at your number.
We start with the 2026 UK base £/m² for your extension type. Single-storey rear extensions average £2,000/m² across the UK.
Your chosen spec adjusts the base rate — from 0.85× for basic to 1.70× for luxury architect-led builds.
Regional labour and material costs adjust the subtotal. London +35%, North East −15%, most regions within ±10% of average.
We add ~10% for planning, building regs, architects and structural engineer fees, plus a 15% recommended contingency buffer.
What Can Push Your Price Up or Down
The biggest swings in UK extension pricing come from these five factors. A £40,000 extension and a £90,000 extension on the same street can have nearly identical footprints.
Things that increase cost ⬆
- Structural steel work — load-bearing walls or large opening spans add £3,000–£12,000+
- Bifold or aluminium sliding doors — £8,000–£20,000 vs £3,000 for a standard patio door
- Kitchen in the extension — £8,000–£40,000 on top of base build
- Underfloor heating — typically £3,000–£7,000 for 20m²
- Conservation area or listed building — planning premium of £2,000–£6,000 and restricted material choices
- Party wall agreements — £1,500–£4,000 per neighbour if needed
- Tricky access — narrow side return, no rear access, or scaffolding over a neighbour's garden
Things that reduce cost ⬇
- Permitted Development rights — no planning application needed, saves £3,000–£6,000
- No internal reconfiguration — leaving existing walls standing saves labour
- Simple rectangular shape — curves, bays and complex rooflines add 15–30%
- Flat roof with EPDM — cheaper than pitched roof with tiles by 20–40%
- Using the existing kitchen — extending the room without redoing units saves £10k+
- Building in low season — some UK builders discount 10–15% for January–February starts
- Rural Scotland, Wales, North — labour rates 15–25% below UK average
UK Extension Cost by Region (2026)
Based on a typical 20m² single-storey rear extension, standard finish.
Get Free Home Extension Quotes from Vetted Builders
BestBuilders matches you with up to 3 vetted UK extension builders. Compare real prices, check reviews, and hire with confidence — all for free.
Common Questions About the Calculator
How accurate is the estimate?
For typical UK homes, expect the mid-range figure to be within ±10–15% of what a builder will quote, provided you've chosen the right type, size, finish and region. The low and high figures capture more extreme outcomes — difficult sites, unusual designs, or unexpected structural issues. No calculator replaces a site survey — the only way to get a locked-in price is to get 3 quotes from local builders.
Does the estimate include kitchen or bathroom fit-out?
No — the base £/m² figure covers the building shell (groundworks, walls, roof, windows, doors, electrics, plumbing rough-in, and standard finishes like plaster, skirting, paint). If you're fitting a new kitchen in the extension, add £8,000 for basic, £20,000 for mid-range, £40,000+ for premium. A new bathroom adds £4,000–£15,000.
Why are London prices 35% higher?
Labour rates in London average £350–£500/day for skilled trades vs £200–£280 outside the capital. Materials have higher delivery and waste removal costs. Party wall agreements are more common. And many London builds involve Victorian terraced properties with tricky access, conservation area rules and elaborate brick matching — all of which push prices up.
What is the 15% contingency for?
Nearly every extension uncovers something unexpected — dodgy foundations, asbestos, non-compliant wiring, previously undetected damp. A 15% contingency buffer lets you absorb those discoveries without blowing the budget or making rushed decisions. Experienced project managers recommend 10% for simple single-storey builds and up to 20% for older properties or complex projects.
Do you store my data or share it with builders?
The calculator runs entirely in your browser — no data is sent to our servers unless you click "Get My 3 Free Quotes" and submit the quote form with your contact details. You can use the calculator as many times as you like, anonymously.
Where does your pricing data come from?
Our base rates come from itemised quotes flowing through the BestBuilders platform across 519 UK towns, cross-referenced against RICS BCIS (Building Cost Information Service) data and Homebuilding & Renovating's annual industry survey. Figures are reviewed quarterly and updated as UK construction material and labour indices change.
Turn your estimate into real quotes
We'll match your project with 3 vetted local builders. You compare quotes, portfolios and timelines side-by-side. No obligation, no spam.
Get 3 Free Quotes →Extension Costs by Town
Each town page shows local extension builders, town-specific cost ranges, recent project examples and nearby areas. Compare 3 free quotes from vetted local extension builders.
How Our Extension Cost Calculator Works: Inputs, Method, Accuracy
Our 2026 UK extension cost calculator uses verified Q2 2026 per-m² rates from RICS BCIS, the Federation of Master Builders, and our own 519-town builder quote dataset. The output is a realistic budget range — not a single fixed price — because real builder quotes for any given specification typically span ±20-30%. Below is how each input affects the final figure.
| Input | Effect on Estimate | Why It Matters |
|---|---|---|
| Size (m²) | Linear scaling | Bigger extensions have lower per-m² cost (foundations + roof spread over more area) |
| Single vs two-storey | Two-storey adds 50-70% to single-storey cost but delivers 2× floor area | Two-storey is cheaper per m² but more expensive total + longer build |
| Region (postcode) | ±20-40% multiplier vs UK average | Inner London adds 35-50%, North East subtracts 10-15% |
| Specification tier | Budget / mid / premium tier multipliers | Affects materials, openings, finishes — not structure |
| Wraparound or side-return | +15-25% vs simple rear extension | Complex roof junctions + dual elevations cost more |
| Includes kitchen relocation | +£8,000-25,000 | New kitchen install + plumbing/electrical move |
Calculator output is a planning-stage estimate. For builder-confirmed pricing always get 3 fixed-price quotes from FMB or TrustMark-registered builders with itemised scope. Typical accuracy of calculator vs final tender: ±15% for clear-spec single-storey rear extensions; ±25% for complex projects.
5 Most Common UK Extension Scenarios with 2026 Calculator Outputs
Below are five common UK extension scenarios with the verified Q2 2026 calculator outputs and what they typically cost. Use these as benchmarks before commissioning detailed quotes.
| Scenario | Calc Output (Mid-Range) | Build Time | Property Value Add |
|---|---|---|---|
| 3m × 4m single-storey rear (Midlands) | £25,000-34,000 | 6-10 weeks | £22,000-32,000 (90-95% ROI) |
| 5m × 4m single-storey rear (South-East) | £48,000-58,000 | 8-12 weeks | £40,000-55,000 |
| 4m × 4m two-storey rear (Yorkshire) | £62,000-78,000 | 10-14 weeks | £60,000-95,000 (100-130% ROI) |
| Side return on Victorian terrace (London) | £28,000-44,000 | 8-12 weeks | £40,000-65,000 (110-150% ROI) |
| Wraparound on detached (Greater London) | £72,000-105,000 | 12-16 weeks | £75,000-110,000 (95-110% ROI) |
All scenarios assume standard spec, brick/block construction, basic openings (no bi-folds), no kitchen relocation, no wall removal. Add £8,000-25,000 for kitchen relocation, £4,000-9,000 for bi-fold doors, £3,000-6,000 for wall removal. Source: RICS BCIS Q2 2026 + 519-town builder quote data.