Extension Cost Calculator

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.

UK home extension costs 2026. Single-storey rear extension: £1,800-£3,500/m². Sample 4m × 3m (12 m²) = £21,600-£42,000. Two-storey side extension: £2,200-£3,800/m². Wrap-around extension: £2,500-£4,000/m² (largest, most complex). London/SE: 30-50% premium over national average. Premium fittings (kitchen, bathroom): can double finishing costs. Average UK kitchen extension complete: £40,000-£80,000 mid-market; £80,000-£150,000+ premium.

Planning permission for extensions. Permitted development limits (no planning needed): rear extension up to 4m (terraced/semi) or 8m (detached); side extension up to half width of original house; height under 3m (single-storey) or 4m (multi-storey); not in front of property; materials match existing. Conservation areas: tighter limits (Article 4 directions). Larger extensions (over 4m terraced, 8m detached): planning required. Two-storey extensions need planning permission in most cases. Always check council before designing — refused planning costs months.

Building Regulations and structural work. Mandatory for all extensions: Building Regulations approval (Full Plans or Building Notice). Structural calculations for: load-bearing walls removed, beams over openings, foundations. Party Wall Act 1996: if attaching to or excavating within 3m of shared boundary, notify neighbour 2 months in advance. Common issues: foundation depth (clay soils need 1m+); proximity to trees (oak, willow within 1.5× tree height need deeper foundations); drainage diversions (water company involvement).

Where the money goes — typical breakdown. 30-35% structural shell (foundations, walls, roof, windows): £15,000-£25,000 typical. 15-20% fit-out (plaster, electrics, plumbing): £8,000-£15,000. 15-25% kitchen (if extending kitchen): £8,000-£20,000. 10-15% flooring + decoration: £5,000-£10,000. 5-10% professional fees (architect, structural engineer, planning, Building Control): £4,000-£8,000. 5-10% contingency: budget at least 10% extra for unforeseen issues. VAT: 20% on labour and materials in nearly all cases.

Value add vs cost. Kitchen extension: typically adds 10-20% to property value. ROI most areas: net positive return after 3-5 years. Worst ROI: over-extending small properties (£100k extension on £200k home rarely returns full cost). Best ROI: turning 3-bed into 4-bed by extension — significant value jump. Loft conversion typically better ROI than extensions (cheaper, similar value add). Always check local property values via Rightmove/Zoopla before committing — extension can't push value above local ceiling.