House Extension Cost: A Complete UK Guide

A comprehensive guide to house extension costs in the UK, covering single storey, two storey, side and rear extensions with typical price ranges.

Understanding the cost of a house extension helps you budget realistically and avoid nasty surprises. Costs vary significantly depending on the type of extension, your location, specification and site conditions.

Average Extension Costs

These are typical cost ranges for 2024/25. London and the South East will be at the higher end; Northern England, Wales and Scotland at the lower end.

• Single storey rear extension (15-20 sqm): £30,000-£60,000
• Single storey side extension (10-15 sqm): £25,000-£50,000
• Two storey rear extension (30-40 sqm total): £50,000-£100,000
• Wraparound extension (rear + side): £40,000-£80,000
• Kitchen extension with bi-fold doors: £40,000-£75,000

Cost Per Square Metre

A useful rule of thumb for budgeting:

• Basic specification: £1,500-£2,000 per sqm
• Mid-range specification: £2,000-£2,800 per sqm
• High specification: £2,800-£3,500+ per sqm
• London premium: add 30-50% to the above figures

What's Included in the Build Cost

• Foundations and groundwork
• Structural walls, steel beams, roof
• Windows and external doors
• Insulation, damp proofing
• First fix electrics and plumbing
• Plastering, screeding
• Second fix (sockets, switches, radiators)
• Basic decoration

What's Usually Extra

• Kitchen fitting: £5,000-£25,000+
• Bathroom fitting: £3,000-£10,000
• Flooring: £30-£100 per sqm
• Bi-fold or sliding doors: £3,000-£8,000
• Underfloor heating: £50-£80 per sqm
• Landscaping and making good the garden: £2,000-£10,000

Professional Fees

• Architect / designer: £1,500-£6,000 (planning + building regs drawings)
• Structural engineer: £500-£2,000
• Planning application fee: £258 (householder)
• Building control: £400-£900
• Party wall surveyor: £700-£1,500 per neighbour (if applicable)

Factors That Increase Cost

• Poor ground conditions - clay, tree roots, or high water table requiring deeper or engineered foundations
• Difficult access - narrow side passages or terraced houses with no rear access
• Services diversions - moving drains, gas pipes, or overhead cables
• Structural complexity - large openings, long spans, or load-bearing wall removal
• High specification - premium materials, bespoke joinery, smart home systems

How to Keep Costs Down

• Keep the design simple - rectangular footprints are cheaper than complex shapes
• Use standard window and door sizes where possible
• Choose a flat roof (cheaper than pitched) if design allows
• Avoid moving the kitchen or bathroom if possible - plumbing relocation adds cost
• Get detailed drawings and a clear specification before asking builders to quote - vague briefs lead to expensive surprises