How Long Does a House Extension Take in 2026? (UK)
A typical UK house extension in 2026 takes 14–24 weeks on site from groundworks to handover. Add 4–6 months of pre-construction design, planning and supplier lead time and the realistic homeowner-to-handover horizon is 8–12 months. Single-storey rear extensions land at the fast end (10–14 weeks build); two-storey side+rear or kitchen wraparounds push to 18–24 weeks. This guide breaks the timeline down phase by phase, names the 6 most common delay risks, and shows when to start if you want the build finished before next Christmas.
The 6 Phases of a UK House Extension — and How Long Each Takes
1. Design & planning (10–18 weeks)
Architect drawings 3–6 weeks, planning or LDC application 8 weeks statutory (often 10–14), structural calcs 2–3 weeks. Run in parallel where possible.
2. Tender & contractor selection (3–6 weeks)
Issue full tender pack to 3 builders, allow 2–3 weeks for pricing, 1 week negotiation, 1–2 weeks contract review. Don't skip JCT.
3. Groundworks & foundations (2–4 weeks)
Excavation, footing pours, oversite slab. Add 1 week per metre of foundation depth if you hit clay or soil bearing issues.
4. Shell & roof (4–8 weeks)
Brickwork, steels, joists, roof structure, slate or tile, weatherproofing. Single-storey: 4–5 weeks. Two-storey: 7–9 weeks.
5. First & second fix (4–7 weeks)
First fix: electrics, plumbing, plasterboard. Second fix: kitchen, sanitaryware, flooring, decorating, snagging. The fitted kitchen is usually the long-lead item.
6. Building control sign-off (1–2 weeks)
Final inspection, completion certificate, snag list close-out. Don't pay the final retention until the certificate is in your inbox.
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