How Much Does a House Extension Cost in London?
Typical 2026 costs per square metre for single storey, side return and two storey extensions, the factors that move the price, and a calculator to get a first budget in seconds.
In short: at a standard specification in 2026, a single storey rear extension in London typically costs around £2,200 to £3,400 per m² to build, a side return around £2,400 to £3,600 per m², and a two storey extension around £2,000 to £3,000 per m² across both floors. A typical 20 m² single storey extension lands around £45,000 to £70,000 before VAT, fit-out and professional fees. These are indicative market ranges, not quotations; the real number for your house comes from a measured design. Send us your plans and we will put engineering and build figures against them.
Typical London extension costs in 2026
The figures below are indicative build costs at a standard specification, including labour and materials for the structure, envelope and standard finishes, and excluding VAT, kitchen and bathroom fit-out, and professional fees. London sits well above national averages, and inner London terraces with restricted access sit above outer London semis.
| Extension type | Basis | Indicative cost |
|---|---|---|
| Single storey rear extension | Standard specification, per m² of new floor area | £2,200 to £3,400 per m² |
| Side return extension | Typically tighter access and more party wall interface | £2,400 to £3,600 per m² |
| Two storey extension | Per m² across both floors; roof and foundations shared | £2,000 to £3,000 per m² |
| 3 m deep rear extension (12 to 15 m²) | Typical London terrace width, standard specification | £30,000 to £50,000 |
| 20 m² single storey extension | Standard specification build cost | £45,000 to £70,000 |
| Structural engineering package | EMA Structures fee: calculations and drawings for a rear extension, correct as of July 2026 | £1,400 to £3,500 + VAT |
Indicative ranges for budgeting only, reviewed periodically. Build costs are always site-specific and should be confirmed by written quotations from your builder; the EMA structural engineering fee is confirmed by written quotation before work begins.
Extension cost calculator
Pick the type of extension, enter the floor area and choose a specification level to get an indicative 2026 London build cost range. If you know the footprint, multiply depth by width to get the area, a 3 m deep extension across a 4.5 m terrace is 13.5 m².
Indicative build cost: £33,000 to £51,000
plus structural engineering package: £1,400 to £3,500 + VAT
Excludes VAT, kitchen and bathroom fit-out, professional fees, party wall costs and site-specific items. Indicative only, not a quotation.
What the ranges include, and what they do not
The build cost ranges cover the core construction: excavation and foundations, the structural shell, roof, glazing at a standard level, first and second fix, and standard finishes. The items that most often surprise people sit outside them:
- VAT at 20% on a standard builder's invoice.
- Kitchen and bathroom fit-out. Most rear extensions are kitchen projects, and the kitchen itself can run from a few thousand pounds to well beyond the cost of the shell.
- Professional fees. Architecture, structural engineering, planning applications and Building Control typically add around 10 to 15% of the build cost.
- Party wall costs. Works on or near a shared wall usually engage the Party Wall etc. Act 1996, with surveyor fees to match.
- Glazing upgrades. Large sliders and structural glass move the price quickly; they are the most common reason a "standard" extension prices as premium.
What moves the cost up or down
- Access. A terrace with no side access means everything, spoil out and materials in, moves through the house. That is time, and time is money.
- Ground conditions and drains. Shallow drains crossing the footprint, trees near the foundations or made ground all change the foundation design. Where the ground is uncertain, trial pits settle it before the design is finalised, which is far cheaper than finding out during the dig.
- How much of the back wall you remove. A doorway is simple; removing the whole rear wall and a return means goalpost steel frames, moment connections and heavier foundations. Our guide to load-bearing wall removal explains what is involved.
- Storeys. Foundations and roof are needed once whether you build one floor or two, which is why two storey extensions usually cost less per square metre.
- Specification. Underfloor heating, rooflights, structural glass and premium finishes each nudge the rate; together they define whether you are at the bottom or top of the range.
The structural engineering side
Every extension needs structural design: foundations, beams over the new openings back into the house, and calculations for Building Control. At EMA Structures a rear extension structural package costs £1,400 to £3,500 + VAT depending on size and complexity, correct as of July 2026 and confirmed by written quotation. The full fee breakdown for our services is in our guide to what a structural engineer costs in London. If you are weighing an extension against digging down, our basement and underpinning cost guide covers below-ground rates, and if the loft is on your list, see dormer vs rooflight loft conversions.
Getting a real number
Ranges get budgets moving, but decisions need real figures. Send us your drawings, photos or a sketch of what you want and we will advise on the structural scope, quote the engineering package in writing, and where the project suits our design-and-build approach, price the works themselves.
All figures on this page exclude VAT unless stated, are indicative and correct as of July 2026, and are not quotations. Build costs vary with site conditions and specification and should be confirmed by written quotations. EMA Structures engineering fees are confirmed by written quotation before work begins.
Extension cost questions
How much does a house extension cost in London?
In 2026, a single storey rear extension in London typically costs around £2,200 to £3,400 per square metre to build at a standard specification, so a typical 20 m² extension lands around £45,000 to £70,000 before VAT, fit-out and professional fees. Side return extensions usually run slightly higher per square metre because of access, and two storey extensions are usually cheaper per square metre because two floors share one roof and one set of foundations. Every project is site-specific, so real numbers come from a measured design and written quotations.
How much does a 3 metre rear extension cost?
A 3 metre deep rear extension across the typical width of a London terrace is roughly 12 to 15 square metres. At typical 2026 London rates that is around £30,000 to £50,000 of build cost at a standard specification, before VAT, kitchen or bathroom fit-out and professional fees. Access, ground conditions and the amount of structural opening back into the house move the figure in both directions.
Is a two storey extension cheaper per square metre?
Usually, yes. The most expensive parts of an extension, foundations, groundworks and the roof, are needed whether you build one storey or two. Adding the second storey reuses both, so the cost per square metre across the whole extension typically falls, often to around £2,000 to £3,000 per square metre in London compared with £2,200 to £3,400 for single storey. The total cost is higher, but each square metre works harder.
What does the extension cost calculator not include?
The calculator estimates the core build cost only. It excludes VAT, kitchen and bathroom fit-out, professional fees such as architecture, structural engineering and Building Control, party wall surveyor costs, and site-specific items such as drainage diversions or difficult ground. Treat the output as a budgeting starting point, not a quotation.
Do I need a structural engineer for an extension?
Yes. An extension needs foundation design, beams over new openings back into the house, and structural calculations for Building Control. At EMA Structures a rear extension structural package costs £1,400 to £3,500 plus VAT depending on size and complexity, correct as of July 2026 and confirmed by written quotation.
Planning an extension?
Send us your drawings or a sketch of what you want and we will quote the structural package in writing, and advise on the build.