Rear extension cost London 2026 — realistic £/m² ranges and what drives them

Every cost guide promises 'from £2,000/m²'. Our project ledger says otherwise: in inner London in 2026, a properly built, properly specified single-storey rear extension lands between £3,000 and £4,600 per square metre of new floor area — before VAT, professional fees and the kitchen. Here's where in that range your project sits, and why.

The headline numbers

Straightforward site, standard spec	£3,000–£3,400 /m²
Typical project (good spec, party walls both sides)	£3,500–£4,000 /m²
Conservation spec, restricted access	£4,200–£4,600+ /m²
Typical 15m² extension, all-in build	£50,000–£70,000
Typical 18m² with structural opening + glazing	£68,000–£95,000

Single-storey rear extension, inner London, 2026 — build cost only

What actually moves the number

Access: if materials travel through the house or over a neighbour's land, add 10–15%. Skip lorries that can't park add more.
Conservation specification: matching stock brick, lime mortar and bespoke glazing bars are real money — often the whole gap between mid and top of range.
Structural openings: removing the original rear wall full-width means steel, temporary works and a structural engineer; the glass you put in the opening can cost as much as the steel.
Party walls: awards on both flanks typically cost £2,000–£5,000 in surveyor fees before any build cost.
Drainage: building over or near a shared sewer needs a build-over agreement and sometimes a diversion — discovered late, this is a five-figure surprise.
Groundworks: clay, trees and Victorian made-ground all show up in the foundation price, not the brochure.

The budget lines people forget

Architect / designer (drawings to tender)	£4,000–£12,000
Structural engineer	£1,200–£3,000
Planning application or LDC fee	£100–£300
Party wall surveyors (two awards)	£2,000–£5,000
Building control	£800–£1,500
VAT on the lot	20%
Kitchen (if the point was a kitchen)	£15,000–£60,000+

On top of the build cost

Does the spend come back?

On family-house streets, converting a rear galley kitchen into a full-width kitchen-diner is one of the most reliably priced improvements in London sold data — but the margin between cost and uplift varies sharply by postcode. That's a checkable fact, not a hunch: the Siteline dossier puts real Land Registry sold prices for your street next to the build-cost range for your project type.

Get the cost range and constraint picture for your specific address — the area report is free and cites every source.

Check an address

How much is a 6m rear extension in London?

A 6m-deep extension across a typical 5m-wide terrace is ~30m² of new space: realistically £95,000–£135,000 build cost in 2026, plus fees, VAT and kitchen. Note that 6m depth needs prior approval (outside conservation areas) or full permission.

Is it cheaper per m² to build bigger?

Somewhat — preliminaries, scaffold and the structural opening are semi-fixed, so a 25m² scheme is usually cheaper per metre than a 12m² one. But the saving is modest in inner London; don't let it talk you past what the planning context supports.

Can I reclaim VAT on an extension?

Not on a standard extension to your home — domestic extension work is standard-rated at 20%. Exceptions exist (listed-building works lost theirs in 2012; some conversions and long-empty properties qualify for reduced rates) but assume 20% unless a VAT adviser says otherwise.

When is the cheapest time of year to build?

Good London builders are booked 3–6 months out year-round; chasing seasonal discounts mostly selects for the builders nobody else booked. Programme around your own life (and the weather for the open-roof phase) instead.

How much does a rear extension cost in London? 2026 £/m² ranges