Extension Cost Calculator
Estimate home extension costs per m² for single/double storey, side return and wrap-around extensions.
Last updated: April 2026 · Source: GOV.UK — Building Regulations
Estimated Total Cost
£64,280.00
| Build (20m² x £2200) | £44,000.00 |
| Kitchen | £15,000.00 |
| Professional Fees (~12%) | £5,280.00 |
| Total | £64,280.00 |
Value Added
£52,500.00
ROI
-18%
Disclaimer
This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.
How It Works
House extension costs in the UK are typically quoted per square metre of floor area. In 2026, a single-storey rear extension costs approximately £1,800–£2,800/m², while a double-storey extension ranges from £1,500–£2,400/m² (the upper floor is cheaper per m² as foundations and roof are shared). London and the South East are typically 20–40% more expensive than the Midlands or North.
The total project cost includes building work, professional fees (architect 7–12%, structural engineer 3–5%, Building Regulations application £200–£900), planning permission if required (£258 for householder applications) and finishes (kitchen, bathroom, flooring). VAT at 20% applies to all labour and materials.
Enter the extension size, type (single or double storey) and your region. The calculator provides a cost range for building work and estimates for professional fees, planning and finishes. Many single-storey rear extensions up to 6 m (detached) or 3 m (attached) fall under Permitted Development and do not need a full planning application.
Example: Single-storey rear extension, 4 m × 5 m, Midlands
- Floor area: 20 m²
- Build cost: 20 × £2,200 = £44,000 (mid-range)
- Professional fees (~12%): £5,280
- Kitchen fit-out: £8,000–£15,000
- Total estimated project cost: £57,000–£64,000 including VAT
Source: GOV.UK — Building Regulations
Frequently Asked Questions
- What does the Extension Cost Calculator do?
- Estimate home extension costs per m² for single/double storey, side return and wrap-around extensions. All calculations are performed in your browser using official UK rates and thresholds.
- Does this include waste allowance?
- Yes. Most building material calculations include a standard 10% waste allowance. You can adjust this figure depending on the complexity of your project and cutting requirements.
- Are prices accurate?
- Material prices are indicative estimates based on average UK prices. Actual costs vary by region, supplier, brand and current market conditions. Always get quotes from local suppliers.