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

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

The cleanest way to budget is per square metre of new floor area, then add fit-out (kitchen, bathroom, flooring, glazing). In London, a single-storey rear extension typically costs £2,200–£3,000 per m² for the shell and standard finish; kitchens, roof lanterns and bifold doors are extra. A 20m² extension therefore sits around £44,000–£60,000 before you fit the kitchen. Basements are in a different bracket entirely because of excavation, underpinning and waterproofing.

Two things explain why the per-m² rate is a band and not a fixed number. The first is finish level: the same shell can carry a builder-standard kitchen or a bespoke one costing three times as much, and structural glazing, roof lanterns and bifold doors add quickly. The second is groundworks — a shallow, straightforward foundation on firm ground is cheap, but London's clay soils, nearby trees, and drainage runs that have to be diverted or built over can all deepen the foundations and lift the price before a single wall goes up. Budget the shell from the table, then add fit-out as a separate, honestly-priced line.

Why is a double-storey extension cheaper per square metre?

It sounds counter-intuitive, but adding a second storey to an extension is usually the best value per m² you can get. The most expensive elements — foundations, drainage diversions and the roof — are shared across two floors instead of one, so the marginal cost of the upper floor is largely just walls, a floor slab and internal finishes. That is why a double-storey extension often lands around £2,000–£2,600 per m² against £2,200–£3,000 for single-storey.

The trade-off is planning, not cost. A double-storey extension is far more visible to neighbours and from the street, so it almost always needs a full householder application rather than permitted development, and councils apply rules on how far it can project and how close it can sit to the boundary. Design it from the outset as two storeys — rather than building single and hoping to add later — and the structure, foundations and stair are all sized once for the finished building, which is both cheaper and far less disruptive than returning to a completed extension to build on top of it.

• One set of foundations and one roof serve both floors
• Scaffolding, site setup and professional fees are spread across more area
• The upper floor adds a bedroom or bathroom — usually the highest-value space per pound
• Structural design is only marginally more complex if planned from the outset

What does a side-return or wraparound extension cost?

Side-return and wraparound extensions are the classic London terrace move — infilling the narrow alley beside the kitchen, sometimes wrapping around into the rear. Because they involve more party-wall interfaces, awkward roof junctions and often a structural beam where the old external wall is removed, they tend to price at the upper end of the single-storey range. Budget the build itself in the £45,000–£70,000 region for a typical terrace, plus a Party Wall surveyor where a neighbour does not consent in writing.

The reason a side-return costs more per square metre than a plain rear extension, despite adding less floor area, is that almost all of it is expensive interface. You are removing the full-height side wall of the house and holding the structure above on a new steel, matching brick and roof lines into a Victorian original, and often re-roofing the join. A wraparound adds a full planning application to that, because turning the corner past the original rear building line usually falls outside permitted development. It buys the transformational open-plan kitchen most people are really after, which is why it remains the most popular — and most carefully-designed — London extension.

What do extension drawings, planning and building regs cost?

Separate from the build, three professional items are needed: measured survey and design drawings, the planning or permitted-development route, and building-regulations drawings with structural calculations. As a fixed fee these typically total £1,800–£3,500 for a London house extension, depending on whether the project is permitted development or needs full planning, and how much steelwork the structural engineer has to design. It is a small slice of the overall spend, but it is the slice that determines approval and gives your builder something firm to price.

It helps to see these as two consents, not one. Planning permission (or a Lawful Development Certificate) decides whether you are allowed to build the extension at all; the building-regulations drawings decide that what you build is structurally sound, properly insulated and correctly drained, and they are what building control inspects on site. You need both, and they are usually cheaper bought together because the survey is done once and the building-regs set develops directly from the approved planning drawings. On top of the fixed fee sit the statutory charges — the council's planning fee and the building-control fee — which are paid to the authorities, not to the designer.

• Measured survey + proposed design drawings
• Planning application or Lawful Development Certificate
• Building-regulations drawings + structural calculations (steel beams, foundations)
• Where needed: Party Wall notices, drainage/build-over agreement with the water authority

How can I keep a London extension on budget?

Most overspend is designed in before a single brick is laid. The reliable savings are: check permitted-development eligibility so you may avoid the planning route and its risk; keep the structure simple (a rectangular plan with a standard beam layout is far cheaper than cranked walls and structural glazing); and commission complete, accurate building-regulations drawings so every builder prices the same specification and there are no mid-build variations. A cheap set of drawings that a builder has to interpret is where budgets quietly blow.

Which extra costs do people forget when budgeting an extension?

The build cost per square metre is the number everyone quotes, but it is rarely the number people actually spend. The gap is made up of professional fees, statutory charges and neighbour-related processes that are easy to leave off a first estimate and awkward to find money for later. Knowing them in advance is the difference between a budget that holds and one that creeps. The two categories that catch London homeowners most often are the party-wall process and the below-ground surprises, so it is worth understanding both before you commit.

None of these are optional extras a good adviser can make disappear — they are simply part of building next to other people on London's constrained plots. Naming them at the start lets you carry a realistic contingency rather than a hopeful one.