Cost of Building a Home Extension in London: 2024 Estimates

London Extension Cost Ranges (per square meter)

Costs vary widely by specification, location, and contractor. Below are ballpark figures for common project types (including VAT, professional fees, and basic finishes).

Single-Storey Rear Extension

• Small (≤15 m²): £20,000–£35,000
• Medium (15–30 m²): £35,000–£50,000
• Large (>30 m²): £50,000–£70,000

Double-Storey Rear Extension

• Small: £40,000–£60,000
• Medium: £60,000–£80,000
• Large: £80,000–£100,000

Side Return Extension

• Small (≤10 m²): £15,000–£25,000
• Medium (10–20 m²): £25,000–£40,000
• Large (>20 m²): £40,000–£60,000

Loft Conversion

• Basic (no bathroom): £30,000–£50,000
• With bathroom: £50,000–£70,000
• Luxury: £70,000–£90,000

Basement Conversion

• Small (≤50 m²): £60,000–£80,000
• Medium (50–100 m²): £80,000–£100,000
• Large (>100 m²): £100,000–£120,000+

Key Cost Drivers

• Structural complexity (steelwork, underpinning)
• Quality of finishes (flooring, glazing, joinery)
• Planning/Building Regulation fees
• Party Wall agreements
• Site access constraints in dense London boroughs

Next Steps

Engage an architect or surveyor for a feasibility study and detailed quote. Obtain at least three contractor bids. Budget a 10–15% contingency.