Key Takeaways
- London extension costs: £3,100-£4,300 per m² (first 10m²), then £1,850 per m² additional
- Professional fees typically add 15-25% to construction costs
- Always budget an additional 10-20% contingency for unexpected costs
- Location within London can affect costs by up to 30%
Want accurate costs for your specific project? Get a personalised estimate based on your requirements, location, and design preferences.
Get Your Project Cost EstimateWhat's in This Guide
Here's what I tell every client who asks about extension costs: In London, you're looking at £3,100-£4,300 per square metre for the first 10m², then £1,850/m² for additional space.
But that's just the starting point. After 15 years designing extensions across London, I've learned that every project tells a different story.
Last month, I completed a Victorian terrace extension in Clapham for £52,000 and a similar-sized project in Kensington for £78,000. Same size, different boroughs, completely different requirements.
London's property prices justify the premium we pay for extensions. The latest ONS data shows London property values still climbing, making well-designed extensions far more cost-effective than moving.
I've based every figure in this guide on real projects from my architectural practice. Not industry averages or outdated surveys, but actual contractor invoices, builder quotes, and final construction costs from extensions I've designed across London's 32 boroughs.
The pricing reflects July 2025 market conditions, incorporating current material costs, labour shortages, and the 15% market adjustment we've seen since post-pandemic recovery. I've also drawn on insights from colleagues at RICS and the latest Building Regulations updates to give you the complete picture.
Complete Extension Cost Breakdown by Type
📊 Where These Construction Costs Come From
Every cost below comes from my clients' actual contractor invoices in 2025. These construction prices include:
✓ Construction materials & labour ✓ 15% market adjustment ✓ London delivery premiums ✓ Current contractor rates
Not included: VAT (20%), professional fees, planning costs, or interior fit-out beyond standard specification.
Single-Storey Rear Extension
4m x 4m (16m²)
Basic to luxury finish - costs from our calculator
Double-Storey Extension
4m x 4m (32m²)
Includes structural premiums and scaffolding
Side Return Extension
6m x 3m (18m²)
Victorian terrace typical infill extension
Loft Conversion
25m² habitable space
Rear dormer with bathroom and bedroom - fixed price
What My Clients Pay Contractors (Per Square Metre)
Basic Build:
£3,100/m² base + £1,850/m² additional
Standard finishes, proven materials. Gets the job done well.
Standard Build:
£3,700/m² base + £1,850/m² additional
Quality finishes, premium elements. What most clients choose.
Luxury Build:
£4,300/m² base + £1,850/m² additional
High-end finishes, premium materials. For those who want the best.
Key point: Contractors charge full rates for the first 10m², then reduce to £1,850/m² for additional space. These prices include the 15% market adjustment we've all had to factor in since 2023.
Get Your Accurate Project Cost
These are general guidelines. For a precise estimate tailored to your specific project, location, and requirements, use our free cost calculator.
Calculate My Extension CostOther Extension Types I Design (And What They Cost)
Garage Conversion
£21,700 - £30,300
15m² typical garage
Plus: £1,850-2,500 for additional plumbing/kitchen
Wraparound Extension
£85,000 - £125,000
35m² L-shaped (rear + side)
Most popular for Victorian terraces
Basement Extension
£180,000 - £253,000
50m² typical basement
Most complex but highest value
Side Return Extension (Victorian Terrace)
Cost: £42,600 - £59,800 (18m²)
What's included: Knocking through side return, structural beam, bi-fold doors, kitchen connection.
Popular in: Clapham, Hackney, Islington, Wandsworth
First Floor Extension (Above Existing)
Cost: £51,200 - £72,800 (20m²)
What's included: Structural support, scaffolding premiums, roof modifications, internal access.
Best for: Adding bedrooms/bathroom above kitchen
What Else You'll Need to Budget For
Here's where many clients get caught off guard. The construction cost is only part of your total budget.
Professional fees, planning applications, and statutory approvals typically add another 15-25% on top. I always discuss these upfront because nobody likes budget surprises halfway through their project. You can check official fee schedules on the government planning portal and your local council's website.
| Service | Typical Cost | Notes |
|---|---|---|
| Planning Application | £462 | Standard householder application fee |
| Building Regulations | £825-£920 | Third-party approval fee (Local Authority) |
| Party Wall Notice | £1,300-£1,460 | Surveyor fees if award needed (per neighbour) |
| Structural Engineer | £1,200-£1,600 | Calculations and drawings (varies by complexity) |
| Architect Fees | £2,800-£8,500 | Design and planning service (6-8% of build cost) |
| CCTV Drainage Survey | £430 | Existing drainage condition assessment |
Helpful Resources: Find qualified professionals through RIBA (Royal Institute of British Architects) for architects, IStructE for structural engineers, and Federation of Master Builders for vetted contractors.
How London Borough Affects Your Costs
Prime Central London
+20-30%
Zones 1-2, including Kensington, Chelsea, Marylebone, Mayfair
- • Premium contractor rates
- • Limited site access
- • Strict planning requirements
- • Conservation area constraints
Inner London
+10-15%
Zones 2-3, including Islington, Clapham, Fulham, Hackney
- • Higher material transport costs
- • Competitive contractor market
- • Moderate access challenges
- • Some planning restrictions
Outer London
Baseline
Zones 4-6, including Croydon, Bromley, Barnet, Hillingdon
- • Standard contractor rates
- • Better site access
- • More flexible planning
- • Lower overhead costs
How to Save Money on Your London Extension
💰 Design Smart
- ✓Keep it rectangular: Square/rectangular extensions cost 15-20% less than complex shapes
- ✓Standard ceiling heights: Avoid double-height spaces unless essential
- ✓Flat roof vs pitched: Can save £3,000-5,000 on smaller extensions
- ✓Use our calculator: Compare options instantly
⚡ Timing Strategy
- ✓Winter builds: Contractors often discount Nov-Feb bookings by 5-10%
- ✓Early booking: Secure contractors 3+ months ahead for better rates
- ✓Material ordering: Bulk orders can save 10-15% vs piecemeal purchasing
- ✓Avoid peak times: Never start during Wimbledon/summer holidays
🔧 Value Engineering
- ✓Phased approach: Build shell first, fit-out later (saves 20-30% upfront)
- ✓Standard windows: Avoid bespoke sizes (saves £500-2,000 per window)
- ✓Reclaimed materials: Victorian-style bricks, timber beams for character
- ✓DIY finishes: Painting, simple tiling can save £2,000-4,000
⚠️ Don't Compromise On
- !Structural work: Never cut costs on foundations, beams, or structural engineers
- !Waterproofing: Proper damp proofing saves thousands in future repairs
- !Planning/Building Control: Proper approvals prevent costly demolition orders
- !Professional team: Experienced architects prevent expensive mistakes
💡 Pro Tip: The 15% Rule
Smart project management can reduce total costs by 15%. Get multiple quotes, negotiate payment terms, and maintain strict change control. Our clients typically save £8,000-15,000 on average projects using these strategies.
Calculate Your Savings PotentialHow I Help Clients Budget Realistically
The 10-20% Contingency Rule
Even with detailed planning, extension projects commonly encounter unexpected costs. Budget an additional 10-20% contingency for:
- •Structural discoveries during construction
- •Hidden drainage or utility issues
- •Planning condition requirements
- •Material price fluctuations
- •Design changes during construction
- •Temporary accommodation costs
Questions My Clients Always Ask
Still Have Questions?
Get personalized answers and detailed cost breakdowns for your specific project.
Get Expert Consultation (FREE)Making Your Extension Financially Viable
After 15 years in this business, I can tell you that London extension costs often surprise people initially. But here's what I've learned: with proper planning and honest budgeting, these projects almost always deliver exceptional value.
The secret is understanding every cost component before you start. No nasty surprises, no budget overruns. Homebuilding & Renovating magazine consistently ranks well-planned extensions as the top home improvement for investment returns.
Here's the reality: a thoughtfully designed extension typically adds more value to your London property than it costs to build. I've seen clients gain £50,000 in property value from a £35,000 extension.
Focus your budget on quality design and structural integrity. Never compromise on the foundations or professional expertise. The latest Zoopla analysis shows well-executed extensions add 15-25% to property values.
Ready to get accurate costs for your specific project? Use our free cost calculator - it's based on the same contractor data I use when advising clients on realistic budgets.
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Get an instant, personalised cost estimate for your London home extension project. Our calculator considers your specific location, requirements, and current market rates.
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