Total Extension Cost
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Calculate Your UK Extension Build Costs
Accurate Price Estimates for Single & Double Storey Extensions
Planning a house extension in the UK requires accurate cost estimation to budget effectively. Our extension cost calculator provides realistic price estimates based on 2026 market rates, helping homeowners and builders plan projects with confidence. Average extension costs range from £1,500 to £3,000 per square metre depending on location, quality, and complexity.
Whether you're planning a single-storey kitchen extension, double-storey addition, or side-return project, understanding base costs is essential. This guide covers all aspects of extension pricing including materials, labour, professional fees, and regional variations across the UK. For planning guidance, consult the Planning Portal.
Calculate your house extension build costs
Extension costs vary significantly based on quality of finish and location. These prices represent typical 2026 UK rates for complete extensions including all building work, but excluding kitchens, bathrooms, and professional fees which are calculated separately.
| Region | Basic (£/m²) | Standard (£/m²) | High-End (£/m²) | London Premium |
|---|---|---|---|---|
| London | £2,500 - £3,000 | £3,000 - £4,000 | £4,000 - £5,000 | Base |
| South East | £2,200 - £2,600 | £2,600 - £3,200 | £3,200 - £4,000 | +15% |
| South West | £1,800 - £2,200 | £2,200 - £2,800 | £2,800 - £3,500 | UK Average |
| Midlands | £1,600 - £2,000 | £2,000 - £2,400 | £2,400 - £3,000 | -10% |
| North England | £1,500 - £1,800 | £1,800 - £2,200 | £2,200 - £2,800 | -15% |
| Scotland | £1,500 - £1,900 | £1,900 - £2,300 | £2,300 - £2,900 | -12% |
| Wales | £1,400 - £1,800 | £1,800 - £2,200 | £2,200 - £2,700 | -15% |
Different extension types have varying cost profiles based on complexity, structural requirements, and typical size ranges. Understanding these differences helps in accurate budgeting and project planning.
Average Cost: £40,000 - £80,000
Per m²: £1,800 - £2,400
Typical Size: 20-40 m²
Duration: 10-14 weeks
Best For: Kitchen extensions, living space, dining rooms
Planning: Often permitted development
Average Cost: £90,000 - £144,000
Per m²: £1,500 - £2,200
Typical Size: 30-70 m² (total both floors)
Duration: 14-20 weeks
Best For: Adding multiple rooms, bedrooms plus living space
Planning: Usually requires planning permission
Average Cost: £30,000 - £50,000
Per m²: £2,000 - £2,800
Typical Size: 12-20 m²
Duration: 8-12 weeks
Best For: Victorian/Edwardian terraced houses, kitchen expansion
Planning: Permitted development in most cases
Average Cost: £50,000 - £100,000
Per m²: £1,900 - £2,500
Typical Size: 25-40 m²
Duration: 12-16 weeks
Best For: Open-plan living, kitchen-diners, garden rooms
Planning: Check permitted development limits
Average Cost: £70,000 - £140,000
Per m²: £1,800 - £2,600
Typical Size: 35-60 m²
Duration: 16-22 weeks
Best For: Maximising ground floor space, L-shaped additions
Planning: Usually requires planning permission
Average Cost: £45,000 - £75,000
Per m²: £1,600 - £2,200
Typical Size: 20-35 m²
Duration: 10-14 weeks
Best For: Additional bedroom, home office, en-suite
Planning: Structural assessment essential
Understanding what's included in quoted prices helps avoid budget surprises. Base extension costs typically cover structural build and basic finishes, but many items require separate budgeting.
✅ Typically Included in Base Cost:
⚠️ Usually NOT Included (Extra Cost):
Project costs scale with size, though larger extensions often benefit from economies of scale. These examples show typical 2026 costs for standard specification single-storey extensions in UK average locations.
| Size | Floor Area | Basic Spec | Standard Spec | High-End Spec |
|---|---|---|---|---|
| Small | 15 m² | £27,000 | £33,000 | £42,000 |
| Medium | 20 m² | £36,000 | £44,000 | £56,000 |
| Large | 30 m² | £51,000 | £66,000 | £84,000 |
| Extra Large | 40 m² | £68,000 | £88,000 | £112,000 |
| Very Large | 50 m² | £85,000 | £110,000 | £140,000 |
Professional services are essential for compliant, well-designed extensions. Budget 12-15% of the build cost for all professional fees including architects, engineers, and statutory approvals.
Cost: 8-12% of build cost
Services: Design, drawings, planning application, building regs
Small Project: £3,000 - £5,000
Medium Project: £5,000 - £10,000
Large Project: £10,000 - £20,000+
Note: RIBA-qualified architects recommended
Cost: £800 - £1,500
Services: Structural calculations, beam sizing, foundation design
Typical Fee: £1,200 average
Reports: Full calculations and stamped drawings
Site Visits: Usually 1-2 visits included
Required For: All extensions removing/adding load-bearing walls
Application Fee: £206 (England 2026)
Drawing Prep: £500 - £1,500 (if not included in architect fee)
Timeline: 8 weeks for decision
Success Rate: ~85% approval rate
Appeals: Additional £500+ if required
Note: Not always needed - check permitted development rights
Cost: £600 - £1,000
Services: Plans approval, site inspections, completion certificate
Inspections: 6-8 visits typical
Options: Local authority or private approved inspector
Essential: Required by law for all extensions
Certificate: Needed for future property sales
Cost: £700 - £1,500
Required: Work on shared walls/boundaries
Surveyor Fee: £700 - £1,000 for your surveyor
Neighbour's Surveyor: You may also pay for their surveyor
Process: 1-2 months before work starts
Legal Requirement: Must comply with Party Wall Act 1996
Cost: 1-2% of build cost
Services: Cost estimation, tender management, cost control
Value: Can save 10-15% on total costs
Best For: Projects over £100,000
Deliverables: Bill of quantities, cost reports
When to Use: Complex or high-value projects
Multiple variables influence final extension costs. Understanding these factors helps in making informed decisions and setting realistic budgets for your project.
📊 Key Cost Factors: