Table of Contents
- What Does a House Extension Cost in London in 2026?
- Extension Types and Their Typical Price Ranges
- What Drives the Cost of a House Extension in London?
- Additional Costs to Budget For
- How London Compares to the Rest of the UK
- How to Get a More Accurate Estimate
- What You Should Expect from Your Contractor
- FAQs
- Conclusion
What Does a House Extension Cost in London in 2026?
If you've searched this question before, you'll know the answer you usually get: "it depends." That's technically true, but it doesn't help you plan a budget.
Here's a more useful starting point. In London, a house extension typically costs between £2,500 and £4,500 per square metre for the build alone. A modest single-storey rear extension of around 20 square metres will generally land between £50,000 and £90,000 once design, structural work, and finishes are included. Larger or more complex projects can push well beyond £150,000.
Those figures are broad because London's construction market shifts considerably by postcode, specification, and site conditions. A project in Fulham SW6 will price differently from one in Bromley SE6, even with an identical footprint.
This guide breaks down the numbers by extension type, explains what actually drives the cost, and covers the additional expenses most homeowners forget to account for.
Extension Types and Their Typical Price Ranges
Single-Storey Rear Extension
The most common extension in London. It typically creates a larger kitchen-diner, a more generous living space, or both.
| Size | Estimated Cost (London, 2026) |
|---|---|
| Small (15–20 m²) | £45,000 – £75,000 |
| Medium (20–30 m²) | £65,000 – £110,000 |
| Large (30–40 m²) | £95,000 – £150,000+ |
These figures assume a mid-to-high specification finish, which is standard for most SW and W London properties. Fewer bespoke elements will bring you toward the lower end. Underfloor heating, structural glazing, and high-end finishes push costs toward the upper range.
Double-Storey Extension
A double-storey extension adds significantly more floor area at a proportionally lower cost per square metre, because foundations, scaffolding, and roof work are shared across both levels.
Expect to pay £90,000 to £180,000 for a typical double-storey rear or side extension in London, depending on size and finish. The cost per square metre often drops to £2,000–£3,000 when spread across both floors, making this one of the more cost-efficient routes to adding substantial space.
Side Return Extension
Particularly common in Victorian and Edwardian terraces across SW and SE London, a side return fills the narrow alley running alongside the ground floor — typically adding 8 to 15 square metres.
Because these projects involve structural changes to the party wall and often require a new roof, the cost per square metre is higher than a straightforward rear extension. Budget £40,000 to £75,000 for a well-specified side return in London.
Wrap-Around Extension
A wrap-around combines a rear extension with a side return, creating an L-shaped addition that opens up the ground floor considerably.
These are more structurally involved and tend to cost £80,000 to £160,000 in London. The design needs to work hard to make the new space feel like a natural part of the existing house rather than something bolted on.
What Drives the Cost of a House Extension in London?
Understanding the cost variables helps you make better decisions before committing to a design.
Size and footprint
The most obvious factor. More square metres means more materials, more labour, and more time. But size alone doesn't determine your cost per square metre.
Specification and finishes
A standard kitchen extension with mid-range finishes costs far less than the same footprint fitted with bespoke joinery, large-format tiles, and a structural glass roof. Finishes can account for 20 to 40 percent of the total project cost.
Structural complexity
Removing load-bearing walls, working on sloping sites, dealing with poor ground conditions, or building close to a boundary all add cost. London's dense Victorian housing stock means structural complications are more the rule than the exception.
Glazing and natural light
Bifold doors, rooflights, and structural glass elements are popular choices in London extensions — and they add meaningful cost. A quality rooflight or set of bifold doors can add £5,000 to £15,000 depending on size and specification.
Party Wall Act requirements
If your extension affects a shared wall, you are legally required to serve notice on your neighbours. Party wall surveyor fees typically run from £700 to £2,000 per neighbour, depending on complexity.
Site access and logistics
London sites are often tight. Limited delivery access, skip placement restrictions, and road permit requirements all add to programme time and cost in ways that don't apply on easier sites.
Postcode and local labour rates
Labour costs in inner London postcodes — SW3, W10, SW6 — are consistently higher than in outer London or the commuter belt. This is a real and predictable difference, not an excuse.
Additional Costs to Budget For
Most homeowners focus on the build cost and underestimate the full project spend. These are the items that most often catch people out.
Architectural design and planning
Architect or designer fees typically run from 8 to 15 percent of the build cost for a full service. Even if your extension falls under permitted development, you still need drawings for building regulations approval.
Structural engineer
Required for any project involving load-bearing elements. Fees range from £500 to £2,500 depending on complexity.
Planning application
If your project requires planning permission, the application fee is currently £258 for a householder application in England. Add consultant time if you need a planning statement or heritage report.
Building regulations
Building control fees vary by local authority and project value. Budget £500 to £1,500 for a standard residential extension.
Party wall surveyor
As above, £700 to £2,000 per affected neighbour.
VAT
New construction on a residential property is generally zero-rated for VAT, but some elements of the work may attract the standard rate. Confirm this with your contractor and accountant before finalising your budget.
Contingency
A 10 to 15 percent contingency isn't pessimism — it's good planning. Unexpected ground conditions, hidden structural issues, and specification changes during the build are all common. Budget for them before you start, not after.
How London Compares to the Rest of the UK
London construction costs run consistently higher than the national average, typically by 20 to 40 percent. A single-storey extension that costs £55,000 in Manchester or Bristol might cost £70,000 to £80,000 for the same specification in SW London.
This reflects higher labour costs, more complex logistics, stricter local authority requirements in some boroughs, and the premium that skilled London tradespeople command.
If you're comparing quotes from contractors based outside London, be cautious. A lower headline figure from a firm without London site experience can close quickly once logistics, travel, and unforeseen complications are factored in.
How to Get a More Accurate Estimate
Generic cost guides give you a starting point. An accurate estimate requires someone to look at your specific property, your intended design, and your specification.
The fastest way to get a ballpark figure without committing to a consultation is to use an online cost estimator. MVV's instant cost estimator at themvv.co.uk gives you a project estimate based on your inputs, with no booking required. It's mobile-friendly and takes a few minutes to complete.
From there, a detailed design and cost plan requires a proper site visit and a brief. That's where the numbers become reliable enough to build a budget around.
When you do receive quotes, make sure you're comparing like for like. Check whether each quote includes:
- Full architectural design and planning support
- Structural engineering
- Building regulations sign-off
- Project management throughout the build
- A clear payment schedule tied to milestones
A quote that excludes any of these isn't a complete picture of your project cost.
What You Should Expect from Your Contractor
Budget overruns and poor communication are the two things London homeowners worry about most when planning an extension. Both are avoidable with the right contractor.
A design and build contractor who manages the project from concept to completion under one team removes the most common source of cost and programme drift: the gap between what a designer specifies and what a builder prices. When both sides sit within the same team, there's no room for that misalignment to develop.
Budget oversight shouldn't stop at the initial quote. You should receive monthly progress reports that include budget tracking alongside on-site photography — so you can see exactly where your money is going and what has been completed. That's a concrete deliverable, not a vague promise of "regular updates."
MVV works this way across all its projects, including completed extensions in Fulham SW6 and Wimbledon SW19. The single-team model, combined with monthly reporting, is built specifically to address the anxieties that most homeowners bring to a project of this scale.
FAQs
How much does a house extension cost in London in 2026?
A single-storey rear extension in London typically costs between £45,000 and £150,000 depending on size and specification. The cost per square metre ranges from approximately £2,500 to £4,500 for the build, with additional costs for design, structural engineering, and planning.
Does a house extension add value to a London property?
Generally, yes. A well-designed extension that adds usable floor area in a desirable London postcode typically adds more in property value than it costs to build. The return varies by location, extension type, and finish quality. A side return or rear extension in SW or W London that creates an open-plan kitchen-diner is particularly valued by buyers.
Do I need planning permission for a house extension in London?
Many single-storey rear extensions fall under permitted development rights and don't require a planning application. However, permitted development rules are more restrictive in conservation areas and for listed buildings — both of which are common across central and inner London. Always confirm your permitted development status with your local planning authority or a qualified designer before starting.
How long does a house extension take to build in London?
A single-storey rear extension typically takes 10 to 16 weeks on site once planning and design are complete. A double-storey or more complex extension can take 20 to 30 weeks. Design, planning, and procurement add further time before the build starts, so allow 6 to 12 months from first enquiry to completion for a typical project.
What is the cheapest type of house extension in London?
A single-storey rear extension with a straightforward footprint and a mid-range specification is generally the most cost-effective way to add space. Prefabricated or modular extensions can reduce build time and cost in some cases, but they're less common in inner London due to site access constraints.
Should I use a design and build contractor or separate architects and builders?
A design and build contractor means one team is responsible for the project from start to finish. This reduces the risk of cost and specification gaps that can appear when a designer hands over to a separate builder, and it simplifies your communication throughout. The trade-off is less independent oversight of build quality, which makes choosing a contractor with a clear track record and transparent reporting all the more important.
How do I get an accurate cost estimate for my London extension?
An online cost estimator is the quickest starting point — it gives you a ballpark figure based on your project type and size without requiring a consultation. For a figure you can actually build a budget around, you need a site visit and a detailed design brief. At that stage, a good contractor will give you a cost plan that breaks down each element of the project clearly.
Conclusion
House extension costs in London in 2026 vary widely, but the factors behind that variation are predictable. Size, specification, structural complexity, and postcode are what matter most. Budget for the full project cost — not just the build — and always set aside a 10 to 15 percent contingency.
The most reliable way to keep costs under control is to work with a team that handles design and build together, tracks your budget throughout the project, and reports back every month with real evidence of progress.
If you want a starting figure without booking a consultation, use the instant cost estimator at themvv.co.uk and get a ballpark in minutes.