How Much Does a Rear Extension Cost in London? (2026)

How much does a rear extension cost in London in 2026?

In 2026, most single-storey rear extensions in London cost between £2,200 and £3,200 per square metre to build, with a standard-finish job around £2,400–£2,700 and a high-specification one with structural glazing and a fitted kitchen pushing past £3,000. On a typical 20 square metre extension that is roughly £44,000 to £64,000 for the build shell and standard finishes, before the kitchen, the professional fees and VAT are counted. The reason it is a band rather than a single figure is that finish level, glazing, ground conditions and access all move the number, and no two London plots behave the same way.

Layered on top of the build are the professional and statutory costs that many homeowners forget until they arrive. A complete drawing package — planning drawings, building-regulations drawings and structural calculations — typically runs £2,200–£3,850 as a fixed fee. The council's householder planning application fee and the Building Control charge are separate again and paid to those bodies directly. Getting accurate, approvable drawings first is what stops the build number drifting, because a design that sails through planning and Building Control avoids the abortive work and mid-project redesigns that quietly inflate a builder's final account.

What is included in the build cost per square metre?

The per-square-metre rate is a useful shorthand, but it hides a lot of variation. A standard rate covers the structural shell — foundations, walls, a flat or pitched roof, a rooflight or two, plastering, basic electrics and plumbing, and standard doors — but it rarely includes the kitchen, high-end flooring, underfloor heating or a full run of structural glazing. When you compare two builders' rates, the honest question is what sits inside the rate and what has been quietly left out as a provisional sum or a client-supplied item.

It is also worth separating the shell from the fit-out in your own head. The shell is relatively predictable; the fit-out is where personal choices multiply cost. A modest kitchen and porcelain tiles keep you near the bottom of the band, while a bespoke kitchen, bifold or slim-framed sliding doors and a polished-concrete floor lift a 20 square metre room by £20,000 or more on their own.

What swings the price of a London rear extension?

Beyond finish level, four site-specific factors move the total most, and they are largely outside your control. Ground conditions come first: London is riddled with shrinkable clay, mature trees with protected root zones, old drainage runs and the occasional made-up ground, any of which forces deeper, reinforced or piled foundations that a simple trench-fill quote never anticipated. A site investigation before you commit is cheap insurance against a five-figure foundation surprise.

Access is the second lever — a house with no side gate, where every barrow of muck and every pallet of brick must be carried through the hallway, costs more in labour than one with rear vehicle access. The third is the design complexity that the glazing and roof choices above create. The fourth is the borough and the planning route: a conservation area, an Article 4 direction removing permitted-development rights, or a listed building each adds design constraint and evidence, and therefore cost, before a spade goes in the ground.

• Ground conditions — clay, trees, drains and made-up ground can force deep or piled foundations
• Access — no side return or rear access means slower, costlier material handling
• Glazing and roof — structural glass, bifolds and lanterns lift both steel design and price
• Kitchen and fit-out specification — the largest discretionary line in most rear extensions
• Planning context — conservation areas, Article 4 directions and listed status add design and evidence work

How is a rear extension different from a side-return or wraparound?

These three terms describe different footprints, and the price per square metre differs because of it. A rear extension projects straight out from the back wall into the garden and is the simplest of the three. A side-return fills the narrow alley of dead space that runs alongside the back room of a typical Victorian or Edwardian terrace — it adds less floor area but transforms the light and width of the existing kitchen, and because it is a fiddly, small-footprint job it often carries a higher rate per square metre. A wraparound combines both, extending to the rear and filling the side return in one L-shaped project, which usually gives the best value per square metre because the setup, roof and structural work are shared across a larger area.

Knowing which you actually want matters for both budget and planning. Many rear and side-return extensions fall within permitted development, avoiding a full planning application, while a large wraparound more often needs full householder planning permission. Getting the drawings and the planning route right at the design stage is what keeps a wraparound from being priced and built as two separate, more expensive jobs.

Does the price include planning, drawings and VAT?

Usually not — and this is where budgets slip. A builder's quote covers construction; it rarely covers the drawings, the structural calculations, the planning application or the statutory Building Control charge. Planning permission and building-regulations approval are two separate consents: planning decides whether you may build the extension, while building regulations govern how it is constructed and are checked by Building Control during the works. You need drawings for both, and a complete package covers planning drawings, building-regs drawings and structural design as one coordinated set that a build can actually be priced and constructed from.

VAT is the final line people underestimate. Almost all domestic extension work is standard-rated at 20%, charged by a VAT-registered builder on labour and materials, and it is not usually recoverable by a homeowner. When you read a market range, check whether it is quoted inclusive or exclusive of VAT, because a figure quoted plus VAT is one-fifth larger than it first appears. A Party Wall Act 1996 award, where the extension sits against or near a neighbour's boundary, is a further cost — the surveyors' fees, usually paid by the building owner, are separate again.

How do I keep a rear extension on budget?

The most reliable saving is made before anyone lifts a tool, in the quality of the drawings and the clarity of the specification. A complete, accurate set of planning and building-regulations drawings, backed by real structural calculations, lets several builders price the exact same job — which turns quotes into true like-for-like comparisons instead of guesses, and removes the variations and provisional sums that inflate a final account. Vague drawings, by contrast, invite every builder to price defensively and then bill the gaps as extras once you are committed.

Fix the specification early, hold a sensible contingency of around 10–15% for the unknowns that live underground, and resist changing your mind once the build starts, because mid-project changes are the most expensive kind. It also pays to settle the planning route before you commission a builder: confirming whether the project is permitted development or needs full householder permission avoids abortive design and lets the drawings be produced once, correctly. Approvable drawings are not a cost to minimise but the tool that keeps the far larger build cost under control.