House extension cost in London and Essex (2026)
A house extension in London and Essex costs between £2,500 and £4,500 per square metre in 2026. For a typical single storey rear extension that works out at somewhere around £65,000 to £120,000 once design, structure and VAT are counted in.
Per square metre rates are the only sensible way to compare extension prices, and they behave in a way that catches people out. A double storey extension costs less per square metre than a single storey, because the foundations and the roof are paid for once and serve two floors. The total bill is higher. The value for money is better.
Below are the 2026 figures for each extension type, what drives your quote up or down, and a worked example with every cost visible rather than just the build price. We design and build extensions across North London, Brentwood, Chingford and Essex, so these rates run 20 to 40 percent above the national averages you will see quoted elsewhere.
House extension cost by type
| Extension type | Typical size | Cost per m² | Typical project cost |
|---|---|---|---|
| Single storey rear | 15 to 25 m² | £2,500 to £4,000 | £55,000 to £110,000 |
| Side return | 8 to 15 m² | £2,800 to £4,200 | £45,000 to £90,000 |
| Double storey | 30 to 60 m² total | £2,000 to £3,000 | £90,000 to £200,000 |
| Wraparound (L-shaped) | 25 to 40 m² | £2,600 to £4,000 | £90,000 to £180,000 |
Figures cover the shell build, windows, doors, electrics, plumbing, plastering and decoration. They exclude VAT, the kitchen fit-out and professional fees unless stated.
Why the rate per square metre falls as the extension grows
Every extension carries a set of costs that barely move whether the footprint is 12 square metres or 40. Site setup, scaffolding, the structural engineer's involvement, the party wall process, connecting into existing drainage, and the builder's own overhead all sit in that category.
Spread those fixed costs across a small side return and they weigh heavily on the rate. Spread them across a double storey extension and they almost disappear into the total. That is the whole reason a side return can be the most expensive thing on the table per square metre while being the cheapest project overall.
It is also why extending twice, five years apart, costs considerably more than doing the larger job once.
What drives your final quote
Size and number of storeys. A double storey extension is cheaper per square metre than a single storey, because the foundations and roof are shared. The total project cost is higher because you gain more floor area.
Type of extension. A side return usually means removing an external wall and infilling a narrow void beside the house, which brings structural work and awkward access. A wraparound is the largest single storey addition a standard plot will take and combines a rear and a side return in one build.
Specification. Standard finishes hold you near the bottom of the range. Bi-fold or sliding doors, a glazed roof lantern, underfloor heating and a fitted kitchen at the higher end will push you towards the top of it, and a roof lantern alone can add £3,000 to £8,000.
Ground conditions and foundations. Poor ground, nearby trees or a drain running under the proposed footprint can mean deeper or reinforced foundations. Trees are the common one in London and Essex, where clay soil shrinks and swells. Foundations near a mature tree can need to go 2.5 metres or deeper.
Scaffolding and structure. Scaffolding runs £1,500 to £3,000. Structural steels and engineer fees are additional and rise sharply where you are removing a large section of the rear wall to open the space up.
Party wall agreements. Terraced and semi-detached homes usually need a party wall surveyor for each affected neighbour. Two attached neighbours means two processes, and if a neighbour appoints their own surveyor you pay for that as well.
A worked example
A 20 square metre single storey rear extension on a semi-detached house in North London, opening the kitchen into a dining area, mid-range specification.
The build comes in at £70,000 at £3,500 per square metre. Architectural design and planning drawings add £3,200. Structural calculations add £1,400. Building Regulations add £900. A party wall surveyor for the one attached neighbour adds £1,600. Scaffolding adds £2,200. The kitchen units and appliances, which no builder's quote includes, add £14,000. VAT at 20 percent applies to the build and most services.
The total sits close to £112,000 against a headline build figure of £70,000.
Nothing there is an overrun or a surprise cost. Every line is predictable at the point you start. The problem is that most homeowners budget against the build number, because that is the number they were quoted.
What the price covers, and what sits on top
A design and build quote typically covers architectural design, structural calculations, Building Regulations, the construction itself, electrics, plumbing, plastering and decoration.
Budget separately for VAT at 20 percent, a planning application where one is needed at £2,000 to £5,000, kitchen or bathroom units, high-end finishes, and any party wall surveyor.
Ask any builder to state plainly whether their figure includes VAT and professional fees. Two quotes £20,000 apart are often the same job, with one of them quietly excluding both.
Planning permission, prior approval and Building Regulations
Many single storey rear extensions fall under Permitted Development if they stay inside the size limits: 3 metres deep on a terraced or semi-detached house, 4 metres on a detached, with a maximum height of 4 metres.
You can go further under the Larger Home Extension scheme, up to 6 metres on a terrace or semi and 8 metres on a detached house, by applying for prior approval. Your council notifies the neighbours, and if none of them object within 21 days the extension is allowed. It is faster and cheaper than a full application, and it is underused.
Double storey extensions, wraparounds, front extensions and anything in a conservation area normally need full planning permission. Budget 8 to 12 weeks and £2,000 to £5,000 for that stage before the build can begin.
Every extension needs Building Regulations approval regardless of which route above applies to it.
How long a house extension takes
Single storey rear extensions typically take 12 to 16 weeks on site. Double storey extensions take 20 to 26 weeks because of the extra floor, the scaffolding and the additional structural work. Side returns fall between the two, depending on how much of the existing wall comes out.
Add design and planning time before any of that starts. From first conversation to finished room, a straightforward single storey extension is realistically a six to nine month project, and a double storey closer to a year.
Will an extension add value to your home?
An extension adds usable floor area, and done well it adds value. The check that decides it is your street's ceiling price.
Find what the largest, best-finished houses on your road have actually sold for in the last year. If your current value plus the full cost of the extension pushes you past that number, you are unlikely to get all of it back at resale. If it lands comfortably below, the extension is on solid ground financially as well as practically.
In most London and Essex family areas, a well-designed extension that adds a bedroom, a larger kitchen-diner, or both, tends to hold its value. The projects that struggle are the ones that take the whole garden or push a modest terrace well past what the street supports.
House extension cost: frequently asked questions
How much does a single-storey extension cost in London?
A 20 m² single-storey rear extension typically costs £65,000 to £120,000 all-in at mid-range specification in 2026, including professional fees and VAT.
How much does a side return extension cost?
Between £45,000 and £90,000 in London and Essex, adding 8 to 15 square metres. Side returns often involve removing an external wall, which adds structural cost.
Is a double-storey extension cheaper than a single-storey?
Per square metre, yes, because the foundations and roof are shared across two floors. The total cost is higher because you gain more floor area, but it is one of the most cost-efficient ways to add substantial space.
Do I need planning permission for an extension?
Many single-storey rear extensions fall under Permitted Development. Double-storey, wraparound and front extensions usually need planning permission. All extensions need Building Regulations approval.
How long does an extension take to build?
Single-storey extensions take 12 to 16 weeks and double-storey extensions 20 to 26 weeks, plus design and planning time beforehand.
Planning an extension in London or Essex?
Mammoth Builds designs and builds single storey, side return, double storey and wraparound extensions under one roof. Request a free consultation and quote today.
