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London Extension Cost Calculator: How It Works
Our free extension cost calculator uses data from 5,000+ completed London projects to give you accurate estimates in under 60 seconds. Unlike generic calculators, we account for London-specific factors that significantly impact your build costs.
2025 London Extension Costs at a Glance
| Extension Type | Typical Cost | Cost per m² |
|---|---|---|
| Side Return Extension | £55,000 - £85,000 | £3,500 - £4,500 |
| Rear Extension (Single Storey) | £65,000 - £95,000 | £3,000 - £4,000 |
| Two-Storey Extension | £95,000 - £150,000 | £2,500 - £3,500 |
| Loft Conversion | £45,000 - £90,000 | £1,500 - £2,500 |
| Wrap-Around Extension | £75,000 - £145,000 | £3,200 - £4,200 |
| Garage Conversion | £15,000 - £35,000 | £1,000 - £1,800 |
What's Included in Your Estimate
- Construction costs based on current London builder rates
- Material costs at standard, mid-range, and luxury specifications
- Professional fees (architect, structural engineer, building control)
- VAT at 20% on all applicable costs
- Contingency recommendations based on project complexity
Before You Build: Check Your Planning Rights
Many London extensions can be built under permitted development rights, saving you time and the £462 planning application fee. However, if you're in a conservation area or your property has Article 4 restrictions, you'll need planning permission.
Use our Permitted Development Checker to instantly see what restrictions apply to your property, or get a free planning constraint report with Property Intelligence.
Popular Extension Guides by Borough
Why Use Our Calculator?
- London-specific data: Based on 5,000+ real London projects, not UK averages
- Updated quarterly: Reflects current material and labour costs
- Includes hidden costs: Party walls, building regs, professional fees
- Instant results: Get your estimate in under 60 seconds
- 100% free: No registration required for basic estimates
Ready for Accurate Pricing?
Use the calculator above to get your free estimate. For detailed architectural drawings and planning support, our design service starts from £1,295.
Read our complete guide to London extension costs →