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

The short answer

A single-storey rear extension in London costs between £3,000 and £5,000 per square metre for construction alone, depending on specification. Add professional fees, statutory costs, contingency and VAT, and the figure a homeowner actually pays is roughly 45 to 55 per cent above the construction cost.

A 30 square metre rear extension at mid specification therefore sits at around £108,000 to £129,000 of construction cost, and closer to £170,000 to £200,000 as a total project cost with everything included.

What the per square metre rate covers

The rate is only meaningful if you know what sits inside it. A construction rate should include demolition and strip-out, foundations, structure and steels, the external envelope and roof, windows and doors, first and second fix mechanical and electrical work, plastering and standard finishes, and the contractor's preliminaries and overheads.

It does not include the kitchen, the sanitaryware, bespoke joinery, landscaping, or the professional and statutory costs of getting the project consented. Comparing a rate that includes prelims against one that does not is the single commonest reason two quotations appear thousands apart when they are in fact describing the same building.

The three specification tiers

Standard, at £3,000 to £3,600 per square metre, means a sound, well-built extension using contract-grade components: standard rooflights, a good but off-the-shelf glazed door set, painted plasterboard, engineered timber flooring.

Mid, at £3,600 to £4,300, is where most London projects land. Better glazing, a more considered roof build-up, improved insulation values, some fitted joinery, and finishes chosen rather than defaulted to.

High, at £4,300 to £5,000 and upwards, covers structural glazing, minimal-frame sliding doors, exposed structure, bespoke joinery throughout and the detailing that makes those things work.

Two-storey extensions are usually slightly cheaper per square metre than single-storey, because the foundation and roof costs spread across two floors of new space.

Refurbishment is priced differently

Work to existing parts of the house is not new construction and should not be priced as if it were. Refurbishing an existing room, meaning finishes, partitions, doors, rewiring, replumbing and decoration, typically runs at roughly 40 to 50 per cent of the new build rate.

This distinction is worth insisting on. A project that extends by 25 square metres and refurbishes 80 square metres of the existing house has a very different cost profile from one that only extends, and an estimate that applies a single blended rate across the whole floor area will be wrong in one direction or the other.

The costs that sit outside the build

Professional fees run at around 15 per cent of construction cost for a full service covering design, planning, building regulations and the contract on site. Where a structural engineer, party wall surveyor or specialist consultant is required, those are additional and usually appointed directly.

Statutory costs and surveys account for roughly 3 per cent, covering measured survey, planning and building control fees, and any reports the application triggers.

Contingency should be 10 per cent as a minimum on an extension to an existing house, and more on a Victorian or Edwardian property where the ground conditions and existing structure are unknown until opened up.

VAT applies at 20 per cent to most householder work. Certain conversions and some energy efficiency measures attract a reduced rate, and genuinely new dwellings are zero-rated, but the standard rate is the sensible planning assumption.

What moves the number most

Ground conditions and trees. A mature tree near the extension can require piled or deepened foundations, and that is a five-figure variable that no drawing reveals in advance.

Structural openings. Removing a rear wall and a chimney breast to make an open plan space is a different structural exercise from adding a box behind the house, and steels, padstones and temporary works add up quickly.

Access. A terraced house with no side return means every skip, every delivery and every load of spoil travels through the house. Contractors price that, and correctly.

Glazing. The step from a good bi-fold set to a minimal-frame sliding system is frequently the largest single line item after the structure itself.

Getting to a real number

A per square metre rate is a planning tool, not a quotation. It tells you whether a project is broadly affordable before you commit to design fees, and it is the right instrument at that stage.

What it cannot do is account for your ground, your structure, your access or your specification. That requires a site visit, a measured survey and a scheme drawn to the point where a contractor can price it rather than guess at it.

The sequence that avoids waste is to test affordability with a rate, then establish feasibility, then design to the budget rather than discovering the budget after the design is finished.