Extension Cost · 2026

How much does an extension cost?

There is no single figure, and the one you find online is usually the shell — the bare build. The number you actually pay is the all-in, once fit-out, fees, contingency and VAT are in. On a single-storey rear extension in the South East, that gap runs to roughly two-thirds again on top of the headline.

Cost your extension See a sample report

The figure you find isn't the figure you pay

Most extension cost guides quote a shell rate — foundations, structure, roof, envelope. It is a real number, and it is not what lands on your account. A single-storey rear in the South East, priced on current day rates and merchant prices, looks like this per square metre.

The headline
you find

£3,575

Shell only — structure, roof, envelope. Per m².

What you
actually pay

£5,882

All-in with a fitted kitchen, fees, contingency and VAT. Per m².

The gap is about £2,307 per m² — roughly 65% on top of the shell. It is not overspend and it is not a builder marking up. It is the fit-out, the professional fees, the contingency and the VAT that the shell figure never included. On a 25–30 m² single-storey rear, that is the difference between a headline near £100,000 and an all-in nearer £150,000 to £175,000.

South East figures, priced on current trade day rates and live merchant pricing (2026). Premium shell specification runs around £4,049 per m². The all-in figure assumes an entry-level bespoke kitchen from local joinery — a Premium-tier fit-out; a higher tier moves it further still, which the anatomy below sets out.

Where you build moves the number

Unlike a kitchen — which is mostly supply, and priced much the same across the country — an extension is mostly build, and build cost follows local labour. The same single-storey rear costs materially more in London than in the north. BuildLens calibrates every figure to the property's location; the pattern, in round terms against the South East, runs like this.

Prime London +25–30%

Greater London +10–20%

South East & Kent baseline

Midlands & South West −8–12%

Northern England −15–18%

Scotland & Wales −15–20%

Indicative regional variation applied by the BuildLens engine, rounded. A single-storey rear that is around £5,882 per m² all-in in the South East lands nearer £4,800 across much of the north and around £7,500 in prime London.

Where the money actually goes

An extension is not one cost, it is a sequence of trades. Itemised, a single-storey rear splits roughly like this — and one slice is the reason the all-in figure is a range, not a fixed number.

Sub
structure

Super
structure

Roof

Envelope
& glazing

Services

Fit-out

Substructure — foundations, slab Superstructure — walls, steel Envelope — cladding, doors, glazing Services — first-fix, heating, electrics Fit-out — kitchen, bathroom, finishes

The fit-out slice is where "all-in" stops being one number

Everything to the left is the build, and it is broadly fixed for a given extension. The fit-out slice is a choice, and it moves. The £5,882 all-in above assumes a Premium kitchen; a self-managed one brings it down, a luxe one pushes it well past — a kitchen-diner is where that choice bites hardest.

See the fit-out ladder — £8,000 to £150,000 for the same room →

Indicative split for a representative single-storey rear extension; proportions shift with roof type, glazing, ground conditions and the fit-out tier chosen.

Why an extension has no single price

An extension varies on three axes at once, which is why a single per-square-metre figure is close to useless on its own. The first is region — local labour, as the strip above shows. The second is specification — the fit-out tier, glazing package and roof type, which is where the fit-out slice climbs. The third is the property itself.

That third axis is the one homeowners rarely price in. An older or period property costs more to build off than a modern one. Restricted access, a boundary that triggers a party wall matter, poor ground conditions, or a conservation setting each add cost before a single course of bricks is laid. None of it is exotic, and all of it is predictable once the property is properly assessed — which is exactly what a desktop analysis of your specific address is for.

How BuildLens gets the number

A BuildLens figure is built from two layers, then calibrated to your property — its region, its age and type, and the scope of what you are building.

Layer 01 — Method

Itemised the way a job is actually priced

Every extension is broken out by trade and by sequence — substructure, superstructure, roof, envelope, services, fit-out — so nothing that gets built sits unpriced between the lines. This is where the shell-versus-all-in gap opens, and where budgets built from a single headline rate come apart.

Layer 02 — Current prices

Live rates, not a published average

The figures come from working trade day rates and weekly merchant pricing, refreshed continuously rather than pulled from an annual index. Location, property age and specification are applied on top, so the number reflects your build now — not a national mean from a guide last updated a year ago.

Common questions about extension cost

How much does a single-storey rear extension cost in 2026?

On current South East pricing, the shell runs around £3,575 per m² for a standard specification and about £4,049 for premium. All-in — with a fitted kitchen, professional fees, contingency and VAT — is nearer £5,882 per m². A typical 25–30 m² single-storey rear therefore lands around £150,000 to £175,000 all-in in the South East, and materially less in lower-cost regions.

Why is the extension cost I found online so much lower than my quotes?

Because most published figures are shell rates — the bare build, without fit-out, fees, contingency or VAT. On a single-storey rear those add roughly two-thirds again on top of the shell. The quote is not high; the headline was incomplete. A useful comparison prices every quote against the same full scope rather than against a shell-only average.

Does where I live change the cost of an extension?

Yes, more than for a kitchen. A kitchen is mostly supply and priced much the same nationally; an extension is mostly build, and build cost follows local labour. The same single-storey rear runs around 15–20% below the South East across much of the north, and 25–30% above in prime London. BuildLens calibrates every figure to the property's location.

What pushes an extension over budget?

Two things, usually. The fit-out — a kitchen or bathroom specified above the budget it was set against, which is where most of the surprise sits. And the property — restricted access, a party wall matter, poor ground conditions or a period structure, each of which adds cost that a generic per-m² figure never accounted for. Both are predictable once the specific property is assessed.

How much does a kitchen-diner extension cost?

A rear extension with a kitchen-diner knock-through carries the build cost above plus the kitchen fit-out, which spans a very wide range depending on tier — from around £8,000 for a self-managed kitchen to £60,000 and beyond for a named bespoke maker. The kitchen extension page sets out the combined figure.