How Much Does a House Extension Cost in London in 2026?
A London extension can look simple on paper, then turn into a much bigger number once you add steelwork, drainage, party wall matters, VAT, finishes, and the kitchen you fell in love with.
For 2026, a sensible planning range for a London house extension is £2,500 to £4,500 per m² for many single-storey projects, with complex or high-spec builds often landing higher. A small rear extension may start around £60,000 to £90,000, while a larger kitchen-diner extension can easily move into the £100,000 to £180,000+ range.
That’s a wide spread, I know. But it’s more useful than a fake fixed price. London homes vary wildly, from Victorian terraces in Romford to semi-detached homes further out, and the hidden details make a huge difference.
What a London house extension is likely to cost in 2026
Here’s a practical way to think about the numbers. These are planning ranges, not quotes, because every site and specification changes the final bill.
Extension type | Typical 2026 London planning range | What it usually includes |
Basic single-storey rear extension | £2,500 to £3,500 per m² | Structure, roof, basic glazing, standard finishes |
Mid-range kitchen-diner extension | £3,200 to £4,500 per m² | Better glazing, heating, electrics, kitchen prep, nicer finishes |
High-spec or complex extension | £4,500 to £6,500+ per m² | Large openings, premium glazing, bespoke details, difficult access |
Two-storey extension | £2,800 to £4,500+ per m² | More structure and compliance, but better value per m² in some cases |
Side return extension | £3,500 to £5,500+ per m² | Often tight access, drainage changes, steelwork, party wall issues |
For a quick example, let’s say you’re planning a 24 m² single-storey rear extension in London.
At a mid-range cost of £3,500 per m², the build alone could be around:
24 m² x £3,500 = £84,000
But that probably isn’t the final project cost. You still need to allow for design fees, surveys, planning or lawful development paperwork, Building Regulations, structural calculations, finishes, flooring, kitchen, contingency, and VAT if applicable.
So that £84,000 build can become a £105,000 to £140,000 project once the real-world extras are added.
If you’re trying to work out house extension cost early on, the safest approach is to separate the build cost from the full project budget. They’re not the same thing.
Why London extensions cost more than people expect
London isn’t just expensive because it’s London. A few very specific things push extension costs up.
Access can slow everything down
A house with a driveway and side access is easier to build on than a terrace where materials have to come through the hallway. If skips can’t sit nearby, waste removal costs more. If materials need carrying by hand, labour takes longer.
That matters because building work is priced on time, risk, and complexity, not just square metres.
Older homes hide surprises
Many London homes were built long before modern drainage, insulation, electrics, and structural standards. Once work starts, a contractor may find shallow foundations, old pipework, cracked drains, uneven floors, or walls that need more support than expected.
This doesn’t mean the property is bad. It just means older buildings need careful investigation.
Steelwork can be a big part of the bill
Open-plan kitchen-diners are popular, especially for families who want one space for cooking, eating, play, and guests. But removing a rear wall normally means installing structural steel beams.
The larger the opening, the more design work, labour, lifting, fire protection, and making good you’ll need.
London labour and logistics cost more
Good trades are in demand. Parking, congestion, restricted working hours, and tight streets all add friction. A specialist contractor will price these risks into the quote, because they affect how long the job really takes.
The costs people forget to include
The build quote is only one part of the budget. If you leave the extras out, your numbers will look comforting at the start and painful later.
Here are the main costs to allow for.
Design and professional fees
For many projects, you’ll need some or all of the following:
Measured survey
Architectural drawings
Structural engineer calculations
Planning application or lawful development certificate
Building Regulations drawings or notes
Party wall surveyor fees, if neighbours are affected
Drainage survey, if building near or over drains
As a broad guide, professional fees often land around 8% to 15% of the build cost, depending on how much help you need and how complex the project is.
Planning and permissions
Some extensions fall under permitted development, but not all. The UK government's Planning Portal explains that permitted development rights have limits, including height, depth, materials, and location rules. Flats, maisonettes, listed buildings, conservation areas, and previously extended homes often need extra checks.
For peace of mind, many homeowners apply for a lawful development certificate even when the work appears to be permitted development. It can help when selling the house later.
Building Regulations
Planning decides whether you’re allowed to build the extension. Building Regulations deal with how safely and properly it’s built.
They cover things like:
Foundations
Structure
Fire safety
Insulation
Ventilation
Drainage
Electrics
Glazing
Energy performance
Building control fees vary by project, but they should always be in the budget.
Party wall matters
If your extension affects a shared wall, boundary, or nearby excavation, the Party Wall etc. Act 1996 may apply. This comes up a lot in terraced and semi-detached London homes.
If neighbours agree, the process can be simple. If surveyors are needed, costs rise. Either way, don’t leave it until the week before digging starts.
VAT
Most home extension work is charged at the standard VAT rate. Some exceptions exist for specific types of work, but you should assume VAT applies unless a qualified professional confirms otherwise.
That one line can change the budget a lot. A £100,000 quote excluding VAT becomes £120,000 including VAT at a 20% rate.
Fixtures, fittings, and finishes
This is where budgets quietly stretch.
A simple utility sink and laminate worktop are very different from a large kitchen, rooflights, underfloor heating, engineered flooring, fitted storage, and large sliding doors.
Even if the shell is modest, finishes can push the final number up fast.
Example budgets for common London extensions
Let’s put the numbers into real-life shapes. These examples are only rough, but they’re useful when deciding whether your plans match your budget.
Small rear extension for more family space
A couple with a baby on the way might want a compact rear extension to create a dining area and better pram storage.
Size Around 15 to 20 m²
Likely build range £45,000 to £90,000
Full project budget Often £65,000 to £115,000 once fees, VAT, and finishes are added
This can work well when the design is simple, the structure is straightforward, and the kitchen stays more or less where it is.
Side return kitchen extension
This is common in Victorian and Edwardian terraces. It fills the narrow strip beside the kitchen to create a wider, brighter room.
Size Around 12 to 25 m²
Likely build range £50,000 to £130,000
Full project budget Often £80,000 to £170,000+
Side returns can be surprisingly expensive per square metre because they often involve drainage, party walls, steelwork, and awkward access.
Larger rear kitchen-diner extension
This is the classic “we need the house to work properly” project. It suits growing families, couples who host often, and homeowners who want a better connection to the garden.
Size Around 25 to 40 m²
Likely build range £90,000 to £180,000+
Full project budget Often £120,000 to £230,000+
The big swing factor is the specification. A simple kitchen and standard glazing keeps costs down. Large rooflights, big glass doors, underfloor heating, and bespoke joinery push it up.
How to keep the budget under control
You don’t need to pick the cheapest route. You need to avoid expensive surprises.
Start with the real budget, not the dream version
Before drawings go too far, decide what you can actually spend. Then keep a contingency of 10% to 15% if the project is straightforward, and more if the house is older or complicated.
If every penny is already assigned before work starts, the project is too tight.
Keep the shape simple
Corners, unusual roof shapes, large openings, and lots of glazing usually add cost. A clean rectangular extension is often better value and easier to build.
Simple doesn’t mean boring. Good proportions, natural light, and practical storage often make a bigger difference than expensive details.
Decide on finishes early
Builders can price more accurately when they know what they’re building. “Kitchen to be confirmed” leaves a lot of room for assumptions.
Choose the big-ticket items early:
Kitchen layout
Flooring type
Doors and windows
Rooflights
Heating
Lighting
Bathroom or utility fittings, if included
A detailed quote makes it easier to compare options fairly.
Check what’s excluded
A cheaper quote may simply leave more out. Look for exclusions such as decorating, flooring, drainage works, kitchen installation, waste removal, Building Regulations fees, structural steel, and making good.
Ask for clarity before you sign. It’s much easier to discuss exclusions at the quote stage than halfway through the build.
Choose experience over the lowest number
A good extension specialist should ask awkward questions early. That’s a positive sign. They’ll want to know about access, drains, party walls, structure, finishes, and how the home will be used day to day.
If someone gives a firm price after a five-minute look around, be careful.
If you’d like a clearer number for your own property, you can request a tailored extension quote here.
FAQ
How much does a house extension cost in London per square metre?
For 2026 planning, many London extensions sit around £2,500 to £4,500 per m². Simple projects may be lower, while high-spec or complex extensions can go above £6,500 per m².
Is a side return extension more expensive than a rear extension?
Often, yes, on a per-square-metre basis. A side return can involve tight access, drainage changes, party wall work, and steel beams, even though the added floor area may be modest.
Do I need planning permission for a house extension?
Not always. Some extensions fall under permitted development, but rules depend on the property, size, location, and previous changes. Conservation areas, flats, and listed buildings usually need more care.
How long does a London extension take?
A straightforward single-storey extension might take around three to six months on site, depending on size, weather, materials, and complexity. Design, permissions, and party wall matters can add weeks or months before building starts.
What’s the best way to compare quotes?
Compare the scope, not just the total. Check whether VAT, structural steel, drainage, decorating, flooring, kitchen works, waste removal, and building control costs are included.
The simplest way to think about your 2026 budget
If you’re planning a London extension, start with a realistic per-square-metre range, then add the costs that sit around the build: design, engineering, permissions, party wall matters, VAT, finishes, and contingency.
A sensible early budget is rarely the smallest possible number. It’s the number that lets you build properly, make decisions calmly, and avoid stopping halfway through because key items were missed.
For most homeowners, that means getting the scope clear, pricing the whole project, and leaving breathing room for the surprises London houses love to hide.
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