Two neighbours on the same Victorian terrace can spend wildly different amounts on what looks like the same extension. One pays £40,000, the next pays £85,000 — for a build of similar size. The difference comes down to 11 cost factors that aren't always obvious upfront.
This guide walks through every variable that materially affects the price of a home extension in London in 2026, so you can read a builder's quote, understand why the numbers are what they are, and spot where a cheap quote is hiding extras.
1. Type of extension
The biggest single cost driver. A simple single-storey rear extension on a flat-fronted Victorian terrace might land at £40-55k. A wrap-around (rear + side-return combined) on the same property is £60-85k. A double-storey extension doubles foundation work, structural complexity, and roof reconfiguration — typically £90-150k. Glazed box extensions with frameless structural glass are a category of their own — easily £100k+ for even a modest footprint.
2. Foundations and ground conditions
London soil varies enormously. Clay-heavy areas in north and west London (Hampstead, Highgate, Chiswick) typically need deeper foundations — sometimes piled — because clay shrinks and swells with moisture. Sandy gravel in parts of the south can take standard strip foundations. If you have a tree within influencing distance (about 1× tree height for hardwoods, 0.5× for softwoods) the engineer will usually specify deeper foundations to avoid heave damage. Cost difference: standard strip foundations might be £4-6k for a small extension; piled foundations can be £15-25k+.
3. Structural opening — how much wall is coming down
If you're connecting the extension to the existing house with a single 2m opening, the structural steel and supporting work is modest. If you're opening up the entire rear wall into a 6m or 8m clear span (typical for an open-plan kitchen-diner extension), you need substantial RSJs (rolled steel joists), pad-stones, and temporary propping during the work. The steel alone for a wide span can run £3-5k; design, install, fire-protect and box-in adds another £3-5k.
4. Roof type
A flat roof on a single-storey extension is the cheapest option (EPDM rubber or fibreglass GRP, typically £2-4k for a small extension). A pitched roof matching the existing house adds £4-8k. A flat roof with a large skylight or roof-lantern adds £2-5k depending on size. A fully glazed roof (architectural glass) easily adds £8-15k. The skylight choice alone can swing the budget by several thousand.
5. Glazing
Bi-fold doors, sliding doors, frameless slim-frame doors, fixed glazed panels — all dramatically affect cost. A standard 3-panel bi-fold (around 3m wide) starts at £3,500 supplied; premium aluminium slim-frame sliders can hit £12,000+ for the same opening. Triple glazing adds 30-40% to the glass cost. Big format glass needs cranes for installation on London terraces with narrow rear access, which adds further cost.
6. Finishes and fittings
An open-plan kitchen-diner extension means you're also paying for a new kitchen inside it. A budget kitchen install might be £8-12k for cabinets and worktops. A bespoke kitchen with stone worktops, integrated appliances and feature lighting can easily be £25-50k+. Flooring choice (engineered oak vs polished concrete vs porcelain tiles), wall finishes, and lighting design all stack up.
7. Services — plumbing, electrical, heating
Moving the kitchen often means relocating the gas main, water supply, soil pipe, and electrical consumer unit. If the extension changes the location of the boiler or hot water cylinder, expect £2-5k extra. Underfloor heating in the new room adds £1,500-3,500 for a typical extension. Adding a new MVHR (mechanical ventilation with heat recovery) unit — increasingly required by Building Regs for airtight extensions — adds £2-4k installed.
8. Building Regulations + Party Wall agreements
Building Regs approval and inspection fees are typically £600-1,200 for an extension. If your property shares a wall with neighbours (most London terraces and semi-detached homes), you need to serve Party Wall notices under the Party Wall Act 1996. If the neighbour dissents, you each appoint a surveyor and split the cost — typically £1,500-3,000 per surveyor. Some borough planning fees apply if you need full planning permission rather than relying on permitted development.
9. Planning route — permitted development vs full planning
Most single-storey rear extensions on London terraces fall under permitted development rights, meaning no full planning permission is required (just a prior-approval notification for the larger PD route). Double-storey, side, and conservation-area extensions usually need full planning. Planning application fees are £258 in 2026 for a household application, but the indirect cost is bigger: full planning means architectural drawings to a higher standard, sometimes a planning consultant (£1,500-4,000), and 8-12 weeks of waiting before you can start.
10. Access constraints — the London-specific factor
This is where London prices diverge from the rest of the UK. If your house is end-of-terrace with side access, materials can be wheelbarrowed straight to the rear garden — efficient. If you're mid-terrace with no side access, every brick, every bag of cement, every roof tile has to be carried through your house. That can add 15-25% to the labour cost. Add in parking suspensions for skips and deliveries (£40-100 per day per bay across most London boroughs), and access alone can swing a quote by £5-15k.
11. Finish quality and design
Two extensions can have identical footprint, identical glass, identical kitchen — and differ £30k in price based on detailing quality. Premium tile alignment, perfectly mitred timber joints, level concrete falls in the patio, properly-detailed flashing around the skylight, hidden cable runs and zoned LED lighting — these are the things that separate a £45k extension from a £75k extension. The labour hours are the difference.
Realistic 2026 London cost ranges
| Extension type | Typical cost (London 2026) | Build time |
|---|---|---|
| Single-storey rear (3m × 5m) | £40,000-65,000 | 10-14 weeks |
| Side-return (Victorian terrace, 3m × 5m) | £45,000-70,000 | 10-14 weeks |
| Wrap-around (rear + side combined) | £60,000-100,000 | 14-18 weeks |
| Double-storey rear (3m × 4m × 2 floors) | £90,000-150,000 | 16-22 weeks |
| Glazed box extension (premium) | £100,000-180,000+ | 16-24 weeks |
Frequently asked questions
Why are London extensions more expensive than the rest of the UK?
Access constraints (narrow rear access on most terraces), parking suspensions for materials and skips, higher London labour rates, and the prevalence of clay soils needing deeper foundations all push London prices roughly 20-30% above national averages.
Can I save money by acting as project manager myself?
In theory yes, in practice rarely. You'd need to procure 6-10 different trades, schedule them correctly, troubleshoot when one delays the next, and handle Building Control yourself. Most homeowners who try end up spending more (and gaining months of stress) than if they'd hired one builder to manage everything.
How much should I budget for contingency?
10-15% on top of the contract quote is standard. Older properties especially throw up surprises (rotten timber, asbestos in old insulation, lead pipework needing replacement) that no quote can fully predict until the work opens up.
Is VAT included in extension quotes?
Usually quotes are given net of VAT and 20% is added at the end. Some quotes show inc-VAT prices — always check. New-build extensions might qualify for zero-rate VAT in specific cases (Listed Buildings, charity buildings); standard residential extensions are full-rate.
How accurate are online extension cost calculators?
Useful for ballpark figures but rarely accurate for London. Most calculators use national average labour rates which don't reflect London premiums, access issues, or borough-specific planning quirks. Treat them as £/m² starting points, then add 20-30% for London.
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