House Extension Costs Explained
Complete guide to budgeting for house extensions with cost per square metre for single and double-storey extensions.
James Mitchell
Complete Guide to House Extension Costs in the UK
A house extension is a major home improvement project that can significantly increase your living space and property value. Understanding the costs involved is essential for effective planning and budgeting.
Average Extension Costs by Type
| Extension Type | Cost per m² | Total Cost Range | Typical Size |
|---|---|---|---|
| Single storey rear | £1,200 - £1,800 | £15,000 - £35,000 | 15-25m² |
| Double storey rear | £1,800 - £2,400 | £35,000 - £75,000 | 30-50m² |
| Side return | £1,500 - £2,200 | £20,000 - £45,000 | 15-30m² |
| Wrap-around | £1,600 - £2,000 | £50,000 - £100,000 | 40-70m² |
Detailed Cost Breakdown
Foundation and Structural Work (25-30%)
- Foundations: £80-£150 per m²
- Steel beams: £1,500-£4,000
- Brickwork: £25-£45 per m²
- Blockwork: £15-£25 per m²
Roofing (15-20%)
- Pitched roof: £60-£120 per m²
- Flat roof: £40-£80 per m²
- Roof tiles/slates: £20-£80 per m²
- Guttering: £15-£30 per linear metre
Services Installation (20-25%)
- electrical work: £2,000-£4,000
- plumbing: £1,500-£3,500
- Heating extension: £1,000-£2,500
- Ventilation: £500-£1,000
Internal Finishing (25-30%)
- Plastering: £8-£15 per m²
- Flooring: £20-£100 per m²
- Kitchen fitting: £8,000-£25,000
- Decorating: £1,000-£3,000
Planning Permission and Building Regulations
Permitted Development Rights
Many extensions can be built under permitted development:
- Single storey: Up to 6m (semi-detached) or 8m (detached)
- Double storey: Up to 6m maximum projection
- Height restrictions: Must not exceed original roof height
When Planning Permission is Required
- Extensions exceeding permitted development limits
- Conservation areas or listed buildings
- Article 4 directions in local area
- Previous extensions using up allowances
Building Regulations
All extensions require building regulations approval:
- Building control fees: £500-£1,200
- Structural calculations: £400-£1,000
- SAP calculations: £200-£500
Regional Cost Variations
- London: 35-45% above national average
- Home Counties: 25-30% above average
- Major cities: 10-20% above average
- Rural areas: 5-15% below average
Return on Investment
Extensions typically add value as follows:
- Kitchen extension: 70-80% of cost recovered
- Additional bedroom: 60-75% of cost recovered
- Extra bathroom: 50-65% of cost recovered
- Open plan living: 65-80% of cost recovered
Professional Team Required
- Architect: £2,000-£8,000 (design and planning)
- Structural engineer: £800-£2,500
- building contractor: Main build costs
- Project manager: 5-10% of total cost
Timeline Expectations
- Planning stage: 2-4 months
- Single storey build: 2-4 months
- Double storey build: 4-6 months
- Complex extensions: 6-9 months
Cost-Saving Tips
Design Efficiency
- Rectangular shapes are most cost-effective
- Avoid complex roof designs
- Minimise the number of corners
- Use standard door and window sizes
Specification Choices
- Choose cost-effective materials
- Standard rather than bespoke fittings
- Plan electrics and plumbing efficiently
- Consider phased construction
Conclusion
A house extension is a significant investment that requires careful planning and budgeting. Understanding the various cost components and factors helps ensure your project delivers maximum value for money.
Ready to explore extension options? Get a quote to get detailed quotes from experienced extension builders in your area.
