How much does a London
extension cost in 2026?
The honest number.
Side-return, rear, wraparound, double-storey or basement — the real 2026 figures we use on client quotes. Built into a 60-second calculator so you can price your project without a callback.
- Every figure on this page is produced by the same pricing engine as our calculator and our written estimates. Nothing here is typed in by hand.
- Outer-London base rate, VAT included, at the finish grade named in each row. Your postcode moves the figure — the zone table shows by how much.
- Area is the finished internal floor area. On a double-storey extension that is the total across both floors, so 30 m² here is a 15 m² footprint.
- Foundation depth is the one line we cannot see before we dig. It is listed under what is subject to final design, and it is the reason we survey before quoting.
- A band, not a quotation. We hold it to ±7% only after a measured survey — see the precision ladder below.
Pick your build. See the price.
Every figure inclusive of design, build, site set-up, the risk allowance and VAT. The specification at each end of a range is named in the row — a band running from the smallest plainest job to the largest and richest is not one product's spread, and calling it one is how a page teaches a careful buyer that its numbers move.
| Type | Cost inc VAT | What it is |
|---|---|---|
| Side-return infill12–20 m² · builder's to balanced finish | £31,680–£69,600£2,640/m² – £3,480/m² | The alley beside the back addition is roofed and brought inside, usually to widen a galley kitchen into a room. Small area, but a full set of trades — which is why the per-m² figure is higher than the total suggests.Planning · Permitted development in most cases. Party wall award almost always required. |
| Single-storey rear extension18–30 m² · builder's to balanced finish | £51,840–£111,600£2,880/m² – £3,720/m² | Straight out the back across the full width. Foundations, a steel to open the rear wall, a flat or pitched roof, and a glazed rear elevation. The most commonly built extension in London.Planning · Permitted development up to 3 m on a terrace or semi, 4 m detached — or 6 m / 8 m under prior approval. |
| Large rear / kitchen-diner30–45 m² · balanced to signature finish | £111,600–£237,600£3,720/m² – £5,280/m² | The same build at the size where the kitchen, dining and living space merge. The extra cost is rarely the shell — it is the glazing run, the joinery and the underfloor heating that a room this size attracts.Planning · Usually prior approval or full planning. Structural calculations for a wider opening. |
| Wrap-around (rear + side return)28–45 m² · builder's to balanced finish | £90,720–£183,600£3,240/m² – £4,080/m² | Rear and side-return in one build. Two elevations, two roof junctions and a more complicated drainage picture, so the per-m² rate sits above a plain rear extension of the same area.Planning · Full planning in most boroughs. Party wall awards on both boundaries. |
| Double-storey rear30–50 m² · builder's to balanced finish | £93,600–£198,000£3,120/m² – £3,960/m² | Ground and first floor together — typically a kitchen below and a bedroom or bathroom above. Area is measured across both floors, so 30 m² here is a 15 m² footprint.Planning · Full planning permission. Heavier foundations and a longer party wall process. |
Each range runs from the smallest typical job at the lower finish grade to the largest at the upper one — the specification at each end is named in the row, because a range whose ends are different products is not a range for one product.
Basements are priced individually.
This page used to carry a basement row. It is gone, because we could not stand behind the number: a dig-out is priced by its underpinning sequence, its retention design and its waterproofing strategy, and none of those scale with floor area the way the builds above do. Two basements of identical size on the same street can differ by more than the cost of a rear extension.
What we can tell you before any money changes hands: whether your house is a candidate, what the party wall position looks like with the neighbours you actually have, and what the structural engineer will need to see. That conversation is free, and it is a more useful thing to have than a range with a three-fold spread.
The same finish, five sizes.
One finish grade the whole way down, so the only thing changing is floor area. Hold a competing quote against the per-m² column: a figure well below ours usually means a thinner scope rather than a thinner margin, and the rate card further down is how you find out which.
| Size | Floor area | Cost inc VAT | Per m² | On site |
|---|---|---|---|---|
| Side returnWidens a galley kitchen | 15 m² | £55,800 | £3,720/m² | 6 wks |
| Modest rear3 m out, full width | 20 m² | £74,400 | £3,720/m² | 9 wks |
| Standard rear4 m out — kitchen and dining | 28 m² | £104,160 | £3,720/m² | 12 wks |
| Large rear6 m out — open-plan family room | 38 m² | £141,360 | £3,720/m² | 16 wks |
| Wrap-aroundRear plus side return in one build | 45 m² | £167,400 | £3,720/m² | 19 wks |
One finish grade down the whole ladder, so the only thing changing between rows is area. The per-m² column is flat by design: our rate is per square metre, so the total scales with the size you give us and nothing else. That is the column to check a competing quote against.
The same extension, three finish grades.
Identical foundations, identical steel, identical roof. What moves is the glazing, the floor, the joinery and the heating — and on an extension that is where two otherwise identical shells stop being comparable.
Standard cavity build, flat EPDM roof, off-the-shelf bifolds or French doors, radiators, painted finish, client-supplied kitchen fitted by us.
Rooflight or lantern, underfloor heating throughout, engineered floor, designed lighting, quartz worktops, 2VP-supplied kitchen and glazing.
Structural glazing, sliding doors at width, stone or sintered worktops, bespoke joinery run, scene-controlled lighting, natural stone or large-format porcelain.
The same job at 28 m², three finish grades. Everything structural is identical across all three — what changes is what you touch, stand on and look at. It is the cheapest lever you have, and the one most quotes never show you.
Every stage of a 28 m² rear extension, largest first.
The actual stage allocation our written estimates carry. Joinery leads it, ahead of the steel and the groundworks, which surprises most people — the shell is the cheap half of an extension and the room inside it is the expensive one.
| Stage | Ex VAT | Share | |
|---|---|---|---|
| Joinery (incl bespoke)M · Joinery (incl bespoke) | £13,954 | 16.1% | |
| Structural steelworkD · Structural steelwork | £8,514 | 9.8% | |
| Substructure & groundworksC · Substructure & groundworks | £7,884 | 9.1% | |
| External envelope wallsE · External envelope walls | £7,647 | 8.8% | |
| Mechanical servicesJ · Mechanical services | £6,938 | 8.0% | |
| Premium windows/doorsP · Premium windows/doors | £6,386 | 7.4% | |
| Roofs & ceilingsG · Roofs & ceilings | £5,676 | 6.5% | |
| Painting & decoratingN · Painting & decorating | £5,046 | 5.8% | |
| Electrical servicesK · Electrical services | £4,730 | 5.4% | |
| Demolition / strip-outB · Demolition / strip-out | £4,415 | 5.1% | |
| Preliminaries & site setupA · Preliminaries & site setup | £3,627 | 4.2% | |
| Floors & skirtingsF · Floors & skirtings | £3,311 | 3.8% | |
| DrainageL · Drainage | £2,523 | 2.9% | |
| DoorsI · Doors | £2,207 | 2.5% | |
| Windows & rooflightsH · Windows & rooflights | £2,129 | 2.5% | |
| External worksO · External works | £1,813 | 2.1% |
This is the same stage allocation our written estimates are built from, not a set of percentages written for a web page. Every rate above is all-in: site set-up, supervision and the risk allowance sit inside the rate rather than arriving as a separate line at the bottom, and nothing is added after this except VAT.
What is always in, and what depends on your ground.
The rates we price with, at the unit we price them in. The first table is what every extension carries whether a quote itemises it or not; the second is the part your site decides, and foundation depth is the largest unknown on the page.
Always in the job
These are not add-ons. They are in every figure quoted above, and they are listed so you can check whether a cheaper quote has left one out.
| Item | Rate inc VAT | Why |
|---|---|---|
| Structural engineer | £3,360per project | Foundation design and the steel that opens the rear wall. Every extension needs them. |
| Building control | £1,800per project | Notification, staged inspections and the completion certificate. |
| Rear scaffold | £5,400per project | A single drop to the rear elevation. A wrap-around needs more. |
| Party wall surveyor | £1,440per agreement | One per affected neighbour. A wrap-around usually means two. |
| Below-ground drainage | £3,000per project | New runs, connections and the make-good. Priced as a bundle because it is dug as one. |
Depends on your house and your choices
Add the ones that apply. This is the part of a quote that usually arrives as a surprise, so it is published instead.
| Item | Rate inc VAT | Why |
|---|---|---|
| Deeper foundations | £540per m³ | Clay, a mature tree or a nearby sewer can double the dig. This is the single largest unknown before we open the ground. |
| Drainage CCTV survey | £540per project | Finds the run under your garden before the excavator does. |
| Inspection chamber | £432each | Required where a new run changes direction or meets an existing one. |
| Sliding or bifold doors | £1,440per set | Installation. Supply is separate and is where the range really is — see the provisional sums below. |
| French doors | £660per set | The cheaper way to open a rear elevation, and on a narrow extension often the better one. |
| Roof lantern | £4,560each | Daylight into the middle of a deep plan, which is the part a glazed rear wall cannot reach. |
| Underfloor heating | £173per m² | Plus a manifold. In an extension with a glazed rear wall it is close to standard. |
| Kitchen supply and install | £300per linear m | Our own workshop. Install-only against a client-supplied kitchen is roughly half. |
| Kitchen install only | £132per linear m | Where you buy the kitchen and we fit it. You own the supplier relationship and the lead time. |
| Quartz worktop | £456per linear m | Templated after the cabinetry is in, which is why it is the last thing to arrive. |
| Stone worktop | £696per linear m | Marble or granite. Same install, different material and a different maintenance conversation. |
| Boiler relocation | £3,360per project | Almost always needed when the extension takes the wall the boiler is on. |
| External paving | £264per m² | Supplied and laid. Install-only against client-supplied stone is considerably less. |
| Fitted joinery | £2,640per linear m | Media walls, banquette seating, full-height storage. The line that separates two identical shells. |
These are the rates we price with, at the unit we price them in. Add the ones that apply to your house and you will land within a few thousand pounds of our written estimate before you have spoken to anybody.
The same extension, four London build zones.
Access, parking suspensions, permit costs and the labour premium of working centrally. This is a build factor, not a property-value one — values rise far more steeply between these zones than build costs do.
| Zone | Build factor | Same 28 m² job | Where |
|---|---|---|---|
| Outer London | ×1.00 | £104,160 | Bromley, Croydon, Enfield, Kingston, Twickenham |
| Inner London | ×1.10 | £114,576 | Chiswick W4, Hammersmith W6, Clapham SW4, Wandsworth SW18 |
| Prime | ×1.20 | £124,992 | Fulham SW6, Battersea SW11, Notting Hill W11, Islington N1 |
| Prime central | ×1.40 | £145,824 | Chelsea SW3, Belgravia SW1, Kensington W8, Hampstead NW3 |
This is a build-cost factor, not a property-value one — access, parking, ULEZ, permit costs and the labour premium of working in central London. Property values rise far more steeply between these zones than build costs do, which is why a central London project usually shows the better return despite costing more to build.
What is not settled until the drawings are.
An extension has more genuine unknowns than any other job on this site, and two of them are underground. We survey for both before quoting rather than discovering them with an excavator.
Foundation depth
The single largest variable on any extension. A mature tree near the line can add a metre of depth across the whole run.
Existing drainage
Building over or near a public sewer needs a build-over agreement from Thames Water, and that has its own timetable.
Glazing supply
The widest range on the whole job. Off-the-shelf bifolds against a slim-sightline slider is a five-figure difference on the same opening.
Kitchen and joinery
Two extensions with identical shells routinely differ by more than the shell cost, entirely here.
Planning outcome
A conservation area or an Article 4 direction removes PD entirely, which changes what can be built before it changes what it costs.
Party wall awards
One agreement per affected neighbour, and the fees are yours rather than theirs. Budget for two on a mid-terrace.
None of these carries a figure here, and that is deliberate. We work to a contract plus variations, every variation is priced and agreed before the work happens, and a site risk allowance is already inside the rates above. Putting an invented allowance against an unknown would make the total look more settled than it is.
How accurate is a number from a web page?
As accurate as what you have told us. A figure from a size bracket is a starting point rather than a quotation, and the ladder below is what closes the gap.
| Accuracy | What we know | Why it narrows |
|---|---|---|
| ±20%Ballpark | Project type and a size bracket | A starting point. Useful for deciding whether to keep reading. |
| ±14%Good estimate | A measured floor area | Your actual area instead of a bracket — the single biggest improvement available. |
| ±10%Sharp estimate | Postcode and finish level | Your build zone and the specification you actually want. |
| ±7%Survey-grade | Scope answered at a measured survey | Room count, drainage, structure and access seen rather than assumed. |
The rungs are cumulative — a postcode cannot sharpen a guessed floor area, because the area is the number the rate multiplies. ±7% is a statement about that set of answers, not a promise that the figure cannot move. We work to a contract plus variations, and every variation is costed and agreed with you before the work happens rather than after.
Permitted development or planning permission?
It changes your timeline by 8 weeks.
Permitted development (PD)
For single-storey rears up to 3m (terrace/semi) or 4m (detached), within height limits, no balconies, materials matching the existing house. Excluded if you’re in a conservation area or under an Article 4 direction.
Best for: simple kitchen-diner rears outside conservation areas.
Full planning permission
Required for wraparounds, double-storeys, contemporary glazing, anything over PD limits, and most conservation areas. Slower but unlocks much better design — especially for Victorian terraces with side returns.
Best for: wraparounds, double-storeys, anything in SW3/SW6/W11 conservation zones.
Party wall awards
Required by law if you’re excavating within 3m of a shared wall, building on the boundary line, or affecting a party structure. Each neighbour usually appoints their own surveyor. Costs typically £1,000–£3,000 per agreement.
Required for: most rear extensions, all basement digs, almost every London terrace project.
The 2026 London Extension Cost Guide.
Twenty-four pages built from real client projects across SW6, SW3, W11, SW11 and W4. Sent to your inbox the moment you submit.
- ✓Side-return, rear, wraparound & double-storey costs side by side
- ✓Full £200k extension cost breakdown, line by line
- ✓Borough multipliers — Zone 1 vs Zone 2 vs Zone 3
- ✓Planning permission vs PD — the real timeline difference
- ✓Party wall awards & build-over agreements explained
- ✓The 7 hidden costs that catch homeowners out
Send me the guide.
What homeowners ask us before they book.
How much does a house extension cost in London in 2026?
Inclusive of design, build, site set-up, the risk allowance and VAT, at outer-London rates: side-return infill £31,680–£69,600 at 12–20 m² · builder's to balanced finish; single-storey rear extension £51,840–£111,600 at 18–30 m² · builder's to balanced finish; large rear / kitchen-diner £111,600–£237,600 at 30–45 m² · balanced to signature finish; wrap-around (rear + side return) £90,720–£183,600 at 28–45 m² · builder's to balanced finish; double-storey rear £93,600–£198,000 at 30–50 m² · builder's to balanced finish. Your postcode, the finish level and what the ground turns out to be move you within those. Basement dig-outs are priced individually — they are not a £/m² job and we will not publish a band we cannot stand behind. Every figure here comes from the same engine as our written estimates.
What's the cost per m² for a London extension in 2026?
£2,640 to £3,960 per m² including VAT at outer-London rates, depending on the build type and the finish grade. A double-storey works out lower per m² than a single-storey of the same total area, because the foundations and the roof are shared across two floors rather than one. Postcode moves it on top: prime central London is +40% on build cost, which is the zone table on this page.
Do I need planning permission for my extension?
Single-storey rear extensions up to 3m (semi-detached/terrace) or 4m (detached) on the original house often fall under permitted development (PD) rights — no planning needed. Side returns, wraparounds, double-storeys and anything in a conservation area or under Article 4 direction usually require full planning permission. We assess your address before quoting.
How long does a London extension take from first call to handover?
Allow 8–12 weeks for design and planning, then the on-site programme from the size table above — side return 6 weeks, modest rear 9 weeks, standard rear 12 weeks, large rear 16 weeks, wrap-around 19 weeks at a balanced finish. Those are engine figures for the area, not a sales estimate. The completion date goes into the contract as a date rather than a duration, and only a variation you have approved can move it.
What hidden costs catch London homeowners out?
The top seven: party wall awards (£1,000–£3,000 per neighbour), build-over agreements with Thames Water (£300–£1,500), planning fees and conservation consultations, structural engineer fees, drainage surveys, asbestos surveys on pre-2000 properties, and VAT (which is 20% on top of the build cost). All of these are itemised inside our quote — no surprises.
Should I use permitted development or apply for planning permission?
PD is faster (8 weeks for a Lawful Development Certificate vs 8–13 weeks for planning) but has hard size limits and excludes many conservation areas. Full planning unlocks larger and more contextual designs — usually worth the extra time on Victorian terraces in SW or W postcodes where you want a wraparound or contemporary glazing.
How can the price change once we have signed?
The quotation becomes the contract sum on signing, itemised across twelve stages with a quantity, a unit and a rate on every line. It can move afterwards in one way only: a variation you have approved in writing, priced and dated before any work starts on it. Where something genuinely cannot be priced until the ground or the structure is opened, it is marked as a provisional sum rather than buried. The contract carries a completion date, and a 12-month workmanship warranty on second-fix fittings follows handover — with an optional 10-year HomePro insurance-backed guarantee available on request, arranged with HomePro and paid directly to them at 2.5% of the contract value.
Do you cover my borough?
Our core extension territory is Fulham (SW6), Chelsea (SW3), Notting Hill (W11), Chiswick (W4), Battersea (SW11), Hammersmith (W6), Clapham (SW4), Wandsworth (SW18), Wimbledon (SW19), Putney (SW15) and Paddington (W2). We work across Zones 1–3 and selectively beyond.
Extension cost in 60 London areas.
Twenty minutes with Nick Elias RIBA.
On us.
Bring your floorplan, sketches, or a single Pinterest pin. Walk out with a feasibility verdict, planning route, realistic budget and a programme — even if we’re not the right fit.
Studio at 20 Dawes Road SW6, video call, or on site.
A real studio, in Fulham.
One company, one address, one registered number — not a lead form that sells your details on. You can walk into the office on Dawes Road and ask for Vladimir by name.
One number. Sixty seconds. No callback.
Price your extension using the same 2026 rate card we quote real clients with.
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