House Extension Cost Calculator UK
Get an instant estimate of your house extension cost in the UK. Our free extension cost calculator works for single storey rear, kitchen, side return, wrap around and double storey extensions, with prices adjusted for London and the South East. Whether you need a kitchen extension cost calculator, single storey extension cost calculator, side return extension cost calculator or general home extension calculator — this one tool covers all UK extension types.
Want a full breakdown by extension type? See our complete House Extension Cost Guide →
Use this extension calculator for any project type
This house extension calculator is designed to give you a fast cost estimate for the most common UK extension types:
- Single storey rear extension — the most popular type, typically 9 m² to 48 m²
- Kitchen extension — single storey with kitchen fit-out included as add-on
- Side return extension — narrow infill for Victorian and Edwardian terraces
- Wrap around extension — combines rear and side return into an L-shape
- Double storey extension — bigger build, lower cost per m²
- Flat roof or pitched roof — both supported in the spec settings
- Timber frame extension — eco-friendly alternative with faster build time
Simply enter your dimensions, pick the extension type and specification level, and the extension cost estimator returns a low–high price range.
How to use this calculator
- Enter your extension area in square metres. Use the floor area, not the perimeter. Typical single storey extensions range from 9 m² (3m × 3m) to 48 m² (6m × 8m).
- Select your additional works — structural changes, kitchen fit-out, bathrooms, bi-folds and high-spec finishes are all costed separately.
- Get your estimate range — the calculator returns a low–high range based on current 2026 London market rates. For UK regions outside London, expect 20–40% lower.
- Request a fixed quote — for an accurate figure, our team will visit the site and provide a detailed fixed-price quote within 5 working days.
What's included in the estimate
The calculator covers all standard build costs for a house extension:
- Foundations and groundworks
- Structural walls, steel beams and roof
- Windows, doors and glazing (standard spec)
- First and second fix electrics
- Plumbing and heating connections
- Plastering and internal finishes
- Standard flooring
- Architectural drawings and building control fees
Not included (because they vary widely): premium kitchen units, high-end appliances, specialist bathrooms, structural glass, bespoke joinery and any planning application fees (£258 if needed).
6 factors that may change your final price
The calculator gives a strong starting estimate, but your actual quote will be influenced by:
- Location — London and the South East are 20–40% above the UK national average; the Midlands and North are 0–10% below.
- Ground conditions — clay soils, high water tables or proximity to large trees can require piled foundations, adding £2,000–£5,000.
- Site access — if materials must be carried through the house instead of delivered to the rear, add a 5–10% premium.
- Specification level — bi-fold doors, lantern roofs, underfloor heating and structural glass all add significant cost. Mid-range vs high-spec can double the per-m² price.
- Existing services — drain diversions can cost £1,500–£4,000 depending on complexity.
- Planning route — staying within permitted development saves £258 and 8 weeks. Falling outside PD means a full planning application.
For a deep dive into each of these factors, plus typical cost ranges for every extension type, see our House Extension Cost Guide.
Typical house extension costs at a glance
To sanity-check the calculator's output, here are typical 2026 UK price ranges per extension type:
| Extension type | Typical cost per m² | Typical total |
|---|---|---|
| Single storey rear | £1,200 – £2,200 | £30k – £55k |
| Kitchen extension | £1,500 – £2,500 | £38k – £75k |
| Side return | £1,400 – £2,200 | £15k – £35k |
| Wrap around | £1,300 – £2,400 | £50k – £95k |
| Double storey | £1,000 – £1,800 | £50k – £90k |
Read the full price guide for each type →
Frequently asked questions
How accurate is this house extension calculator?
The extension cost calculator UK returns a low–high range based on current 2026 London market rates for standard build specifications. Real projects typically fall within this range, but final cost depends on site-specific conditions like ground type, access and finish level. For an exact fixed-price quote, contact our team.
Can I use this as a kitchen extension cost calculator or for other extension types?
Yes. The same tool works as a kitchen extension cost calculator, side return extension cost calculator, single storey extension cost calculator and for double storey, wrap around, flat roof and timber frame extensions. Just pick the type in the form and the calculator adjusts pricing accordingly. Kitchen extensions include the build cost — fitted kitchen units are an optional add-on.
Does the calculator include kitchen units, bathrooms and appliances?
No. The calculator covers the build shell and full fit-out (foundations, structure, electrics, plumbing, basic finishes). Kitchen units cost an extra £5,000–£25,000 depending on specification; bathrooms add £6,000–£15,000.
What's the difference between extension shell cost and full build cost?
The extension shell is the basic envelope — foundations, walls, roof, windows and doors — typically 60–70% of the total. The full build cost calculator result includes the shell plus first-fix electrics, plumbing, plastering and basic finishes ready for decoration. For shell-only estimates, expect roughly 30–40% lower than the calculator's range.
Are these London prices or UK average?
The default output is calibrated for London and the South East. For other UK regions, expect 20–40% lower. We're working on regional toggles for the calculator.
Do I need planning permission?
Many house extensions fall under permitted development rights — particularly single storey rear extensions up to 3 metres (attached houses) or 4 metres (detached). The Larger Home Extension scheme allows up to 6 or 8 metres respectively under prior approval. The calculator assumes a standard PD-compliant project.
Can I get a fixed-price quote based on this extension estimate?
Yes. After running the calculator, request a free consultation. Our team will visit the site and provide a detailed fixed-price quote within 5 working days, with no hidden costs.
Get an exact fixed-price quote
The calculator gives a strong starting figure. For an exact number tailored to your home, location and specification, book a free consultation, we'll visit, measure and provide a detailed fixed-price quote within 5 working days.
Gen is managing director and chief of digital strategy at houseUP. She has a background in information security and product management in tech startups.
houseUP is a construction company in London, specialised in high end residential and commercial projects.
