How Much Does a House Extension Cost in London in 2026?
A single-storey extension in London costs £2,800–£4,500 per m² in 2026, so a 25 m² rear extension is typically £70,000–£130,000 before VAT, fees and the kitchen. What moves the number, and what to budget.
A house extension in London costs £2,800 to £4,500 per square metre for the build in 2026, which puts a typical 25 m² single-storey rear extension at £70,000 to £130,000 before VAT, professional fees and the kitchen. High-specification and architect-led projects in inner boroughs run to £5,500 per m² and beyond. Those are market ranges, not quotations — but they are the right numbers to start a budget with, and the rest of this guide explains what pushes a project towards the top or bottom of them.
2026 cost ranges by extension type
The table below is compiled from published 2026 cost guides (sources at the end) adjusted for what we see on North London sites. Figures exclude VAT at 20%, professional fees and kitchens or bathrooms unless stated.
| Extension type | Typical size | Typical build cost (2026) | Notes |
|---|---|---|---|
| Single-storey rear | 20–30 m² | £70,000 – £130,000 | The commonest project; usually permitted development up to 3 m / 4 m depth |
| Side return | 10–15 m² | £55,000 – £95,000 | Highest cost per m² — full structural opening and glazed roof over a small area |
| Wraparound (rear + side) | 30–45 m² | £100,000 – £180,000 | Needs planning permission; two elevations opened on steel |
| Double storey | 40–60 m² total | £120,000 – £220,000 | About 50–60% more than a single storey of the same footprint, for double the area |
| Kitchen extension (any of the above) | — | Add £15,000 – £45,000 | Units, worktops, appliances, fitting |
| Bathroom or WC within the extension | — | Add £6,000 – £15,000 | Drainage is the variable |
What is inside the build cost
A builder's price for an extension typically covers: demolition and site set-up (hoarding, scaffold, skips); excavation and foundations; drainage alterations; the masonry or timber-frame superstructure; structural steel; the roof and rooflights; external doors and windows; first-fix electrics and plumbing; insulation and plasterboard; plastering; screed and floor finishes; second-fix electrics, heating and sanitaryware; decoration; and making good the junction with the existing house. What it does not usually include is the kitchen, appliances, landscaping beyond the extension's footprint, VAT, and professional fees.
The seven things that move the price
1. Ground conditions and trees
North London sits on shrinkable clay. Building control's foundation tables require deeper footings near trees — often 1.5 m to 2.5 m, sometimes piled — and a large oak in a neighbour's garden can add £5,000–£15,000 to the groundworks. This is the single biggest source of "unexpected" cost and the first thing we look at on a site visit.
2. Structure
Every extension that opens the back of the house needs steel. One beam across the rear wall is routine; a goalpost frame to open a corner for a wraparound, or steels picking up a chimney breast, adds design time and tonnage. Budget £4,000–£15,000 for steelwork and installation on a typical project.
3. Glazing
Bi-fold and sliding doors range from about £1,500 per linear metre for standard aluminium to £4,000+ for slim-frame systems with solar-control glass. A 5 m opening can therefore cost £8,000 or £20,000. Rooflights are £1,500–£6,000 each fitted; a fully glazed side-return roof £8,000–£25,000.
4. Drainage and services
Moving the kitchen means moving the soil stack and often the boiler. If a public sewer runs under the extension, Thames Water's build-over process applies (£300–£1,500 in fees plus a CCTV survey). A consumer-unit upgrade for a modern kitchen is common: £800–£1,500.
5. Specification
The same 25 m² box can be finished for £70,000 or £130,000. Under-floor heating (£100–£180/m²), porcelain floors that run outside (£90–£200/m² fitted), bespoke joinery and layered lighting are where the upper end of the range comes from. None of it is wrong — but decide it at feasibility, not on site.
6. Access and location
A terrace with no side access, a controlled parking zone that needs suspensions for skips, or a conservation area that dictates materials all add cost. Inner boroughs are 10–20% dearer than Barnet and Harrow for the same work.
7. Whether the house is lived in
Working around a family adds dust-sealing, temporary kitchens and sequencing constraints. Most rear extensions can be done with you in the house; a wraparound combined with a renovation usually cannot, and moving out for three months is often cheaper than the extra weeks on site.
Fees and approvals to add on top
| Item | Typical cost (2026) |
|---|---|
| Measured survey and feasibility | £900 – £2,500 |
| Planning drawings and application management | £2,000 – £4,500 |
| Householder planning application fee (England) | £258 |
| Lawful Development Certificate (if permitted development) | £129 |
| Building Regulations drawings and specification | £2,000 – £5,000 |
| Structural engineer's calculations | £900 – £2,500 |
| Building control fees (full plans + inspections) | £900 – £1,800 |
| Party wall surveyors (if a neighbour dissents) | £1,500 – £4,000 per neighbour |
| Thames Water build-over agreement (if a sewer is affected) | £300 – £1,500 |
As a rule of thumb, design, engineering and approvals come to 8–12% of the build cost. On a design-and-build contract these are charged as fixed-fee stages rather than as a separate architect's percentage.
A worked example: 25 m² rear kitchen extension on a 1930s semi in Barnet
- Build cost at £3,400/m²: £85,000
- Kitchen, mid-range, supplied and fitted: £22,000
- Design, structural engineer, Building Regulations, Lawful Development Certificate: £8,500
- Party wall surveyor (one neighbour dissented): £2,000
- Contingency at 10%: £8,500
- Subtotal: £126,000. VAT at 20% on the build, kitchen and fees: £25,200
- Total budget: c. £151,000 — of which £117,500 is committed at contract and the rest is contingency and VAT
That is a realistic all-in figure for a good kitchen extension in outer North London in 2026. Read it against the "from £35,000" figures some builders advertise: those are for a shell, before VAT, before the kitchen, before fees, and before the ground has been looked at.
Will it add value?
In North London's family-house streets a well-designed kitchen extension usually adds at least what it costs, but not always: the ceiling price of the street, how much garden is left, and whether the layout works matter more than the square metres. We give a view on this at feasibility, and our guide does every extension increase property value? goes into the exceptions.
How to get an accurate figure
Have the site looked at before the drawings are commissioned. A feasibility visit that considers the ground, the structure, the drainage and the planning route turns the ranges above into an estimate you can plan around, and it costs a fraction of what a redesign costs later. Our house extensions page explains the planning rules and process; the enquiry form asks for the details we need to give a first view.
Sources
Frequently asked questions
How much does a single-storey rear extension cost in London in 2026?
Typically £2,800–£4,500 per m² for the build in North London, so a 20 m² extension is roughly £60,000–£95,000 and a 30 m² extension £85,000–£135,000, before VAT (20%), professional fees (8–12%) and the kitchen (£15,000–£45,000).
Is the cost per square metre lower for a bigger extension?
Slightly. Fixed costs — the structural opening, roof, drainage connection, scaffold, glazing — are spread over more metres, so a 35 m² extension usually costs less per m² than a 12 m² side return, even though it costs more in total.
Do extension costs include VAT?
Most builders quote excluding VAT and most extension work is standard-rated at 20%. The exceptions are certain conversions and work to listed buildings, which can be reduced- or zero-rated. Always ask whether a figure includes VAT.
How much contingency should I allow for an extension?
10% on a straightforward extension to a 1930s house; 15% on Victorian and Edwardian houses or where the ground is unknown. Unused contingency is not spent.
What is the cheapest type of extension?
Per square metre, a double-storey extension is the most cost-efficient because roof and foundations are shared across two floors. In absolute terms, a simple single-storey rear extension under permitted development with a flat roof and standard glazing is the cheapest project.
Want a range for your own project?
Project cost calculatorAbout this guide. Written by the Milkov & Son Construction team and reviewed by Stoyan Milkov, Director & Project Manager. Cost figures are North London market guides for the year stated and not quotations; planning and Building Regulations summaries are general and should be confirmed for your property. About the company.
