If you’re considering a house extension London, one of the first questions you’ll ask is: how much is it going to cost? In 2026, extension costs across London continue to vary depending on size, design, borough, and build complexity — but with the right planning, extending your home can still be far more cost-effective than moving.
In this detailed guide, we break down realistic 2026 costs, what affects pricing, hidden expenses to consider, and how to budget properly for your project.
If you’re just starting your research, you may also find this helpful:
👉 https://housemanage.co.uk/the-complete-guide-to-house-extensions-in-london/
Average House Extension Costs in London (2026)
As of 2026, the average cost of a house extension in London typically ranges between:
- £2,200 – £3,500 per m² for standard quality finishes
- £3,500 – £4,500+ per m² for high-spec or bespoke builds
However, London prices are generally 15–25% higher than the UK average due to labour, logistics, and property access constraints.
Here’s a rough guide by extension type:
Single-Storey Rear Extension
- 20m²: £50,000 – £75,000
- 30m²: £65,000 – £95,000
Side Return Extension (Popular in London Terraced Homes)
- 15–20m²: £45,000 – £70,000
Wraparound Extension
- 30–40m²: £80,000 – £130,000
Double-Storey Extension
- 30–40m² total: £90,000 – £160,000
Basement Extension (High Complexity)
- £4,000 – £7,000 per m²
If you want a tailored breakdown for your property, you can explore professional advice on house extension London services to better understand what applies to your situation.
What Affects House Extension Costs in London?
Every house extension London project is different. Several key factors influence your final price:
1. Size and Layout
The larger the extension, the higher the total cost — but cost per square metre can reduce slightly on bigger projects due to economies of scale.
2. Structural Complexity
Costs increase significantly if your build involves:
- Steel beams (RSJs)
- Removing load-bearing walls
- Complex roof alterations
- Underpinning or foundation reinforcement
Victorian and Edwardian London properties often require additional structural work.
3. Location Within London
Some boroughs are more expensive due to:
- Parking permits and access restrictions
- Congestion zone costs
- Planning constraints (especially conservation areas)
Central and South West London projects tend to cost more than outer boroughs.
4. Specification and Finishes
Your choices dramatically impact cost:
- Aluminium bifold doors vs uPVC
- Underfloor heating vs radiators
- Bespoke kitchen installations
- Skylights and roof lanterns
- Smart home integration
High-end finishes can easily add £20,000–£50,000 to a project.
5. Planning and Professional Fees
Beyond construction costs, don’t forget professional services.
Professional Fees You Must Budget For
In 2026, expect to allocate 10–15% of your build cost to professional fees.
These may include:
- Architect: £2,000 – £6,000+
- Structural engineer: £800 – £2,500
- Planning application: £206 (standard householder application)
- Building control: £700 – £1,500
- Party wall surveyor: £700 – £2,000 per neighbour
- Thames Water build-over agreement (if required)
London projects often require party wall agreements due to terraced housing layouts.
Planning Permission Costs in London
Many house extensions fall under Permitted Development Rights, but not all.
You may require planning permission if:
- You live in a conservation area
- Your property is listed
- You exceed permitted size limits
- You alter the front elevation
For a full breakdown of London-specific rules, read:
👉 https://housemanage.co.uk/the-complete-guide-to-house-extensions-in-london/
Planning approval timelines in London typically take 8–10 weeks, though some boroughs experience delays.
Hidden Costs Most Homeowners Forget
When budgeting for a house extension London, don’t overlook:
VAT
Most extensions are subject to 20% VAT on labour and materials.
Temporary Accommodation
If your kitchen is unusable for weeks, you may need short-term rental accommodation.
Structural Surprises
Common in London’s older housing stock:
- Drain relocation
- Asbestos removal
- Poor existing foundations
Interior Upgrades
Once extended, many homeowners also:
- Redecorate adjacent rooms
- Upgrade flooring throughout
- Install new kitchens or bathrooms
This can add £15,000–£40,000 beyond structural build costs.
How Long Does a House Extension Take in London?
Time is money — and longer builds increase labour costs.
Typical timelines:
- Design & planning: 2–4 months
- Planning approval (if required): 2 months
- Construction:
- Single storey: 10–14 weeks
- Double storey: 14–20 weeks
- Basement: 20–30+ weeks
Delays can occur due to:
- Material shortages
- Weather
- Planning conditions
- Structural amendments mid-build
A professional project-managed house extension London service can significantly reduce costly overruns.
Is Extending Still Cheaper Than Moving in London?
In most cases — yes.
Consider average London moving costs in 2026:
- Stamp duty (on £700,000 home): £22,500+
- Estate agent fees: £10,000+
- Legal fees: £2,000–£3,000
- Removal costs: £1,000–£2,000
Total moving cost: £35,000–£50,000+
That money alone could fund a substantial extension deposit.
Additionally:
- You avoid changing schools
- You remain in your neighbourhood
- You increase property value
How Much Value Does a House Extension Add in London?
A well-designed extension can increase property value by:
- 10–20% for quality rear or wraparound extensions
- 15–25% for double-storey extensions
- Even more in high-demand boroughs
Kitchen-diner extensions with large glazing are particularly desirable in London’s terraced housing market.
However, overextending beyond your street’s ceiling price can reduce return on investment. A feasibility study before committing is essential.
Ways to Reduce House Extension Costs in London
If you’re trying to control your budget, consider:
✔ Keep the Layout Simple
Rectangular builds are cheaper than complex shapes.
✔ Avoid Moving Drainage
Relocating underground pipes increases labour costs.
✔ Choose Standard-Sized Glazing
Bespoke glazing adds thousands.
✔ Plan Early
Late design changes are one of the biggest cost drivers.
✔ Use an Experienced London Contractor
London builds require logistical expertise. Hiring specialists in house extension London projects reduces risk and unexpected costs.
Explore trusted extension services here:
👉 https://extensions.housemanage.co.uk/
Example Cost Scenario (2026)
Project: 30m² rear extension in South London
Specification: Mid-range finish with bifold doors
- Build cost (30m² @ £2,800/m²): £84,000
- Architect & structural fees: £7,000
- Building control & planning: £1,200
- Party wall surveyor: £1,500
- Kitchen installation: £18,000
- Contingency (10%): £11,000
Estimated total: £122,700
This is a realistic example for 2026 London pricing.
Should You Fix or Design Around Budget First?
There are two approaches:
- Design-first approach – Create your ideal extension, then cost it.
- Budget-led approach – Define budget first, then design within it.
In London’s high-cost construction environment, a budget-led strategy is often safer.
Final Thoughts: Planning Your House Extension in London
In 2026, the cost of a house extension London project typically ranges from £50,000 to £160,000+, depending on size and specification. While prices remain high compared to the rest of the UK, extending is still often more economical than relocating — and can significantly increase your property’s value.
The key to success is:
- Realistic budgeting
- Proper planning permission checks
- Professional design input
- Experienced London-based builders
- A clear contingency fund
If you’re ready to explore your options, start by reviewing expert guidance on house extension London services and learn more from this comprehensive guide:
👉 https://housemanage.co.uk/the-complete-guide-to-house-extensions-in-london/
With careful planning, your extension can transform both your living space and your long-term investment.
