Extension Cost Per m² in Surrey (2026): What the Rate Includes — and What It Doesn't
£2,500–£3,500/m² is the honest 2026 range for Surrey extensions — but the per-metre rate is only useful if you know what sits inside it. What moves you up or down the band, and the costs that never appear in it.
Every extension conversation starts with the same question: what's the cost per square metre? It's a fair question — and the honest Surrey answer for 2026 is £2,500–£3,500/m² including VAT for single-storey extensions, the same banding we use in our guide to builder estimates. But the rate is only useful if you know what's inside it.
What sits inside the rate
At £2,500–£3,000/m² you're buying a solid standard specification: masonry to match the house, standard bi-fold or French doors, plastered and decorated, standard electrics, and a contribution-level allowance for the kitchen rather than a full high-end fit-out.
At £3,000–£3,500/m² you're adding the things that quietly eat budgets: structural glazing, longer steel spans for open-plan layouts, underfloor heating, and premium floor and wall finishes.
A 21m² rear extension (3m × 7m) therefore lands between £52,000 and £74,000 — a range wide enough to prove the point: the per-m² number is a budgeting envelope, not a quote.
What moves you up or down the band
- Structure. Removing the back wall of the house for open-plan means steels, and steels mean structural engineering, cranes or manpower, and building control scrutiny.
- Glazing ratio. Glass is the most expensive wall you can buy. A gable-height glazed rear elevation can move the whole project up a band on its own.
- Ground conditions. Sloped plots — everywhere along the North Downs — mean stepped foundations and drainage complexity that flat-site rates ignore.
- Kitchen scope. "Extension with a new kitchen" is two projects sharing a roof. Be explicit about which parts of the kitchen budget sit inside the build cost.
- Two-storey economics. One roof and one set of foundations serving two floors usually drops the per-m² rate by 10–20% — the total is bigger, but each square metre works harder.
What never sits inside the rate
Per-m² figures quoted in isolation exclude, at minimum:
- Professional fees — drawings, structural calculations, approvals: typically 10–12% of build cost
- Party wall costs — most attached houses in Surrey need party wall notices, and if a neighbour dissents, surveyor fees follow
- Building control — application and inspection fees
- Contingency — hold 10%; roofs and grounds hide surprises
- VAT — always confirm whether a builder's rate includes it
A per-m² figure that looks dramatically better than the market is nearly always excluding several of these — which is how projects "overspend" without a single thing going wrong on site.
From envelope to real number
The per-m² rate gets you a budget envelope. What turns it into a real number is a design and a specification of works that every builder prices identically — the discipline we walk through in how to plan a home extension. Local context matters too: see our Godstone & Tandridge cost guide for how the green belt shapes what you can build in the first place.
We design and build home extensions across Surrey and South London. Book a free consultation and we'll give you a realistic budget for your actual house — not a brochure rate.