House Extension Cost Calculator
Calculate accurate costs for your UK house extension project
Extension Cost Calculator
Cost Breakdown
House Extension Costs in the UK
Average Costs by Extension Type
| Extension Type | Cost per m² |
|---|---|
| Single Storey Rear | £1,800 – £3,000 |
| Double Storey Rear | £1,500 – £2,500 |
| Side Return | £2,000 – £3,200 |
| Wraparound | £1,700 – £2,800 |
Cost Factors
Size & Complexity
Larger extensions and complex designs require more materials and labour, increasing overall costs significantly.
Materials Quality
High-quality materials cost more initially but offer better durability and aesthetic appeal.
Location
London and South East typically cost 20-30% more than northern regions due to higher labour costs.
Extension Planning Guide
Before You Start
- Check if you need planning permission
- Consider party wall agreements with neighbours
- Set a realistic budget with 10-20% contingency
- Research local builders and architects
- Review building regulations requirements
Timeline & Process
Planning Phase (2-4 months)
- Initial design and planning applications
- Building regulations approval
- Detailed architectural drawings
Construction Phase (3-6 months)
- Foundation work and structural elements
- Roof installation and weatherproofing
- Internal fit-out and finishing
Money-Saving Tips
- Get multiple quotes: Compare at least 3-5 detailed quotes from different contractors
- Time your project: Avoid peak summer months when builders are busiest and most expensive
- Consider DIY elements: Handle painting, tiling, and simple finishing tasks yourself
- Use standard sizes: Stick to standard window and door sizes to avoid custom costs
- Plan utilities carefully: Minimise changes to existing plumbing and electrical systems
Frequently Asked Questions
Many extensions fall under permitted development rights, but you’ll need planning permission if your extension exceeds certain limits. Single-storey rear extensions can extend up to 6m for terraced/semi-detached houses or 8m for detached houses without planning permission, provided they meet other criteria.
A typical single-storey extension takes 10-16 weeks to complete, whilst double-storey extensions can take 16-20 weeks. Factors affecting timeline include weather, complexity, and any unforeseen complications during construction.
Well-designed extensions typically add 15-20% to your property value. Kitchen extensions and additional bedrooms tend to offer the best return on investment. However, over-developing for your area can limit value gains.
Our estimates include structural work, roofing, windows, doors, basic electrical and plumbing, and standard finishes. Additional costs may include planning fees, building control fees, and premium finishes or fixtures.
For complex or large extensions, an architect can optimise design and space usage, potentially saving money long-term. For simple extensions, a good builder with design experience may suffice. Architect fees typically range from 8-15% of the total project cost.