House Extension Cost Calculator

Single-storey extensions in South London run £1,800 to £2,500 per square metre depending on specification, so a typical 20 square metre rear extension lands between £45,000 and £60,000 including structural work, glazing, and professional fees. Wraparound extensions sit at the top of that range because of the extra steelwork. Double-storey extensions cost more in total but noticeably less per square metre — roughly 20% less — because the foundations and roof are shared across two floors.

Foundations and groundwork, structural steelwork sized by our engineer, walls, roof, standard glazing, first and second fix electrics and plumbing, plastering, and decoration. The calculator adds professional fees — structural calculations, Building Control, FENSA registration — as a separate line because they're fixed-ish costs that don't scale with size. What it deliberately excludes is kitchen and bathroom fit-out: a kitchen can be £8,000 or £25,000 depending on your taste, so we price it separately. Our kitchen extension calculator includes it if that's the room you're building.

Often not. Most single-storey rear extensions fall under permitted development: up to 3 metres deep on a terraced house (4 on detached), maximum 4 metres high. Double-storey extensions have tighter limits and need planning more often. Conservation areas — common across Dulwich, Blackheath, and Crystal Palace — change the rules, as does any previous extension that used up your permitted development allowance. We check all of this at the free site visit and handle applications where they're needed, including lawful development certificates.

Most single-storey extensions take 10 to 14 weeks from breaking ground: two weeks of groundwork, two to three for structure and steel, one to two for the roof and weatherproofing, then first fix, plaster, second fix, and decoration. Double-storey adds 3 to 4 weeks. Weather matters most during groundwork, which is why we steer projects towards a March-to-October start where the diary allows.

Per square metre, up. A double-storey extension reuses one set of foundations and one roof across two floors, which is where the money goes — so the rate drops to roughly £1,500 to £2,000 per square metre against £1,800 to £2,500 for single-storey. The trade-offs: planning is harder to get for two storeys, and the first floor usually needs to be bedrooms or bathrooms rather than the open-plan kitchen most people are actually after. Budget permitting, the question is usually what the house needs, not which is cheaper.