What Does a Rear Extension Cost?

Cost Per m² by Type (2026)

• Single storey rear (basic spec): £1,200–£1,600/m²
• Single storey rear (mid spec): £1,600–£2,200/m²
• Double storey rear: £1,000–£1,800/m² (lower per m² as foundations and roof are shared)
• Side return (terrace): £1,500–£2,500/m² (smaller area, higher per m²)
• Wraparound: £1,400–£2,200/m²

Example Costs

• 3m × 3m single storey (9m²): £15,000–£20,000
• 4m × 5m single storey (20m²): £24,000–£44,000
• 6m × 4m single storey (24m²): £30,000–£53,000
• 4m × 5m double storey (40m²): £40,000–£72,000

What Affects the Price?

• Location: London and South East 20–40% more than national average
• Specification: Bi-fold doors, underfloor heating, kitchen fit-out add significant cost
• Ground conditions: Clay soil may need deeper foundations
• Access: Restricted access increases labour costs
• Drainage diversions: Moving existing drains can add £2,000–£5,000

Additional Professional Fees

• Architect/designer: £2,000–£5,000
• Structural engineer: £500–£1,500
• Building Regulations application: £300–£900
• Planning application (if needed): £250
• Party wall surveyor: £700–£1,500 per neighbour

Last updated: April 2026