Extension Cost in London 2026: What You'll Actually Pay
What does a home extension cost in London in 2026? From £36,000 to £170,000 depending on size and spec - with trade-by-trade breakdowns and borough-level detail.
A home extension in London costs more than almost anywhere else in the UK. You already knew that. What you probably don't know is how much more, exactly where the premium hits hardest, and what you can do about it.
In 2026, a single-storey rear extension in Greater London runs between £2,600 and £4,300 per square metre (m²) - compared to a national average of £2,000–£3,300/m². For a typical 4m × 5m (20m²) extension, that puts you at £52,000–£86,000 all-in with standard finishes. Go larger or higher-spec and you're into six figures.
Here's the full picture.
Quick cost summary
For a single-storey rear extension in London with standard finishes:
| Size | Budget spec | Standard spec | High spec |
|---|---|---|---|
| 3m × 4m (12m²) | £36,000–£43,000 | £44,000–£52,000 | £56,000–£72,000 |
| 4m × 5m (20m²) | £52,000–£65,000 | £68,000–£86,000 | £90,000–£115,000 |
| 5m × 6m (30m²) | £78,000–£96,000 | £96,000–£120,000 | £125,000–£170,000 |
These include everything - groundworks, structure, first fix (plumbing, electrics, and plastering done before your kitchen or bathroom goes in), fit-out, and decoration. They don't include kitchen or bathroom supply if you're fitting those separately.
For double-storey extensions, add roughly 50–70% to the single-storey figures. You're getting twice the floor area but sharing foundations and the roof structure.
Costs based on BCIS regional benchmarks (Greater London) and MyBuildAlly quote analysis, Q1 2026. Your actual costs will vary based on specification, access, and local labour rates.
How London compares to the national average
London carries a ×1.15–1.30 multiplier over UK-wide averages. That's not the same everywhere - it depends on the trade and where in London you're building.
| Cost factor | National average | London | Premium |
|---|---|---|---|
| General builder (day rate) | £200–£280 | £250–£380 | +25–35% |
| Electrician (day rate) | £200–£300 | £280–£400 | +30–40% |
| Plumber (day rate) | £200–£300 | £260–£380 | +25–30% |
| Groundworker (day rate) | £180–£260 | £240–£350 | +30–35% |
| Skip hire (8-yard, 2 weeks) | £280–£380 | £400–£600 | +40–60% |
| Scaffolding (8 weeks) | £1,200–£2,000 | £1,800–£3,200 | +50–60% |
Why London costs more
It's not just labour rates. A few things pile up:
Congestion zone and ULEZ (Ultra Low Emission Zone) charges. Tradespeople working in central or inner London face daily charges of £15 (congestion charge) plus £12.50 (ULEZ) for non-compliant vehicles. Most pass this straight on to you. For a 12-week build, that's £2,000–£3,000 in access charges alone.
Restricted site access. Tight Victorian terraces, limited parking, narrow rear alleys, and no crane access means more manual labour. Materials that would be craned in on a suburban site get carried through the house bag by bag.
Party wall agreements. A party wall is the shared wall between your property and your neighbour's - and in London, almost every extension triggers party wall obligations. Terraced houses share walls on both sides, and even semi-detached properties with side returns will need an agreement. Budget £700–£1,500 per neighbouring property - and that's if they don't appoint their own surveyor, which they're entitled to do at your expense.
Skip permits. Most London boroughs won't let you place a skip on the road without a permit (£30–£80). In some boroughs, you can't get a permit at all and need to use grab lorries instead - which cost more per load.
Material delivery windows. Many boroughs restrict delivery times to avoid rush-hour disruption - which means your builder may need to pay for early-morning premium slots, and that cost gets passed to you. It also means longer unloading times and tighter scheduling all round.
Trade-by-trade breakdown for London
Groundworks and foundations - 15–20% of total
London's clay soil is the single biggest cost variable. Most of Greater London sits on London Clay, which shrinks and swells with moisture. That means deeper foundations - typically 1.0–1.5 metres minimum, compared to 0.6–0.9m elsewhere. Deeper foundations mean more digging, more concrete, and a bigger bill.
If you're near trees (and in London, you usually are), those foundations go deeper still. Tree roots can cause clay shrinkage up to 1.5 times the tree's height from the trunk. A tree survey might be needed before you even start digging - and if there's a protected tree nearby, that survey is compulsory.
| Foundation type | Cost per linear metre | When it's needed |
|---|---|---|
| Strip foundation (concrete trenches dug along your walls) | £180–£280 | Stable ground, no trees nearby |
| Deep strip | £250–£400 | Clay soil, moderate tree influence |
| Piled foundation (deep concrete columns drilled into the ground) | £400–£700 | Poor or soft ground, significant tree root influence, basements nearby |
Budget: £5,000–£15,000 for a 20m² extension in London.
Structural shell - 25–30%
Walls, roof, steelwork, and glazing. The structural steel for opening up to the existing house is a big line item - a single steel beam (RSJ) can cost £1,500–£4,000 supplied and fitted, depending on the span.
Bi-fold doors (the large folding glass doors that open up the back of your house) or sliding doors are popular in London rear extensions and add £4,000–£10,000 to this section. For a full breakdown of extension costs by component, see our kitchen extension cost guide.
Budget: £14,000–£35,000 for a 20m² extension.
First fix - 15–18%
First fix covers the plumbing, electrics, gas, and plastering that happen before your kitchen or bathroom goes in. London electricians and plumbers charge higher day rates than anywhere else in the UK, and this stage is labour-intensive. For a detailed look at what electrical work should cost and what to watch for, see our electrician quote breakdown.
Underfloor heating is increasingly standard in London extensions - at £60–£90/m² for wet systems, it's not a huge add at first fix stage but costs significantly more to retrofit.
Budget: £8,000–£16,000 for a 20m² extension.
Fit-out - 20–30%
This is where your specification choices have the biggest impact. The structural cost of a budget extension and a premium one is almost identical - it's the kitchen, bathroom fittings, worktops, and flooring that create the gap.
| Component | Budget | Mid-range | Premium |
|---|---|---|---|
| Kitchen units (supply only) | £3,000–£6,000 | £7,000–£14,000 | £18,000–£35,000+ |
| Worktops | £600–£1,500 | £2,000–£4,000 | £4,500–£10,000 |
| Appliances | £2,000–£3,500 | £4,000–£7,000 | £8,000–£18,000+ |
| Flooring (per m²) | £30–£60 | £60–£120 | £120–£250+ |
Second fix and decoration - 10–15%
Second fix is the finishing touches - final connections for sockets, switches, and taps, plus tiling, painting, and snagging (the walkthrough where you spot anything that needs fixing). Allow £4,000–£7,000 for a standard 20m² extension - more if you're tiling large areas or installing feature lighting.
Planning and building regulations in London
Permitted development
Most single-storey rear extensions in London fall under permitted development (your automatic right to make certain changes without applying for planning permission) - so no planning application needed. The limits are:
- Depth: Up to 3m from the original rear wall (attached houses) or 4m (detached)
- Height: Maximum 4m (or 3m within 2m of a boundary)
- Coverage: Extension plus any outbuildings can't cover more than 50% of the garden
- Materials: Must be similar in appearance to the existing house
Where London gets complicated
Conservation areas. A conservation area is a protected zone where the council controls what you can change about your property's appearance. Large swathes of London are covered - Islington alone has over 30. In a conservation area, your permitted development rights are more limited. Side extensions, cladding changes, and any alteration visible from a public road typically need planning permission.
Article 4 directions. Some boroughs have issued Article 4 directions - council rules that remove your automatic right to extend without planning permission. If your property is covered by one, you'll need a full planning application even for work that would normally be permitted. Check with your borough's planning department before assuming anything.
Basement extensions. Basement excavation has become extremely popular in high-value London boroughs (Kensington, Chelsea, Westminster). These always need full planning permission, a structural engineer, and often a basement impact assessment. Costs start at £200,000 and can exceed £500,000 - a completely different world from a standard rear extension.
Building regulations are mandatory for any extension, regardless of whether you need planning permission. Building control (the council inspection that makes sure your build meets safety standards) costs £600–£1,200 in London.
Planning application costs
For a full walkthrough of the planning process, timelines, and what counts as permitted development, see our planning permission guide for 2026.
If you do need a full planning application:
| Item | Cost |
|---|---|
| Householder planning application | £258 |
| Architectural drawings | £1,500–£4,000 |
| Structural engineer | £500–£1,500 |
| Party wall surveyor (your side) | £700–£1,200 |
| Tree survey (if protected or nearby trees are affected) | £400–£800 |
| Design and access statement | Included in architect fees |
Finding a builder in London
Good builders in London are booked months in advance. Start looking at least 3–4 months before you want work to begin.
Get at least three written quotes. Not estimates - fixed-price quotes with a detailed scope of works. If a builder won't put it in writing, walk away.
Check credentials. Look for Federation of Master Builders (FMB) membership, relevant trade body memberships - NICEIC (the national register of approved electricians) for electrics, Gas Safe (the official register of qualified gas engineers) for gas work - and public liability insurance of at least £2 million. Our guide on how to check your builder's work covers what to inspect at each stage.
Ask for recent references. Specifically ask to see a project of similar size and type to yours, ideally in your borough. Better still, visit a completed project in person.
Use MyBuildAlly's free cost calculator to get a baseline before comparing quotes. Knowing what a 20m² extension should cost in your postcode means you can spot outliers immediately - whether that's a builder who's too cheap (and will cut corners) or one who's loading the price because they don't really want the job.
What to watch for in London
Party wall surprises
Your neighbours have 14 days to respond to a party wall notice. If they object (or simply don't reply), you both need surveyors - and you pay for theirs. In London, where terraced houses share walls on both sides, that's potentially two sets of surveyor fees on top of your own. Budget for the worst case: £2,500–£4,500.
Thames Water build-over agreements
If your extension sits over or near a public sewer, you'll need a build-over agreement from Thames Water. This involves a CCTV survey of the sewer (£300–£500) and can add 4–8 weeks to your timeline while you wait for approval. Worth checking early - this one catches a lot of people out.
Structural movement
Older London properties - Victorian and Edwardian terraces especially - often have existing structural issues that only show up once you start building. Subsidence, failed lintels (the beams above your windows and doors), and cracked party walls can add £3,000–£10,000 in remedial work. A pre-build structural survey (£400–£700) is money well spent.
Access for waste removal
If your property has no rear access, all demolition waste goes through the house. Some builders price this separately; others include it but underestimate the time involved. Clarify in the quote.
Is your quote right for London?
London extension quotes vary wildly. We regularly see £20,000+ differences between the cheapest and most expensive quotes for the same project - and the cheapest isn't always the best.
The real question isn't "is this cheap?" - it's "does this quote cover everything, and is the price fair for what's included?" That's exactly what MyBuildAlly helps with. Upload your builder's quote and we'll benchmark it against London-specific data, flag any gaps in the scope, and highlight the line items that need a second look.
If you're also considering other projects, our South East building costs guide covers the commuter belt where prices are lower but still above the national average.
Sources
- BCIS Average Building Prices - regional cost benchmarks for Greater London, Q1 2026
- Spon's Architects' and Builders' Price Book 2026 - material and labour rate reference
- Planning Portal - permitted development, conservation areas, and Article 4 directions
- Thames Water - Build Over - build-over agreement requirements
- FMB (Federation of Master Builders) - trade accreditation standards
- Analysis of residential construction quotes submitted to MyBuildAlly, January–March 2026