Timelines are the second thing every London homeowner underestimates — after cost. A single-storey rear extension is often sold as "three to four months on site" and that part is often true, but the four months of design, twelve weeks of planning and eight weeks of party wall administration in front of it are equally real. Here is the honest calendar.
Phase 1 — Brief and feasibility (weeks 1–3)
- Initial architect meeting; brief captured on paper.
- Site measured survey (half a day on site; two weeks for the CAD file).
- Feasibility drawings — two or three options.
- Rough cost sanity-check against per-sqm benchmarks.
London delay to watch: measured survey firms are booked 3–4 weeks out in spring and autumn.
Phase 2 — Design development (weeks 3–8)
- Design frozen. Full planning drawings prepared.
- Pre-application advice submitted (optional but wise for marginal schemes).
- Structural engineer brought in to size steels for the planning submission.
Phase 3 — Planning or LDC (weeks 8–20)
- Lawful Development Certificate: 6–8 weeks to determination.
- Householder planning application: 8 weeks statutory, realistically 10–14 in Camden, Islington, Hackney, K&C.
- Prior approval (larger single-storey): 42 days.
- Conservation area / full planning with objections: 12–20 weeks.
London delay to watch: validation queues at busy councils add 2–3 weeks before the statutory clock even starts.
Phase 4 — Technical design (weeks 16–24, overlapping)
- Building Control drawings.
- Structural engineer produces construction calcs and details.
- Party Wall notices served (allow 2 months minimum before start).
- Tender pack issued to 3–4 vetted builders.
Phase 5 — Tender, appointment, mobilisation (weeks 22–28)
- Tenders returned (3–4 weeks).
- Post-tender interviews and price scrutiny.
- Contract signed (JCT Homeowner or Minor Works).
- Builder mobilises — hoardings, welfare, skip permits.
London delay to watch: parking suspensions require a council permit, typically 5–10 working days.
Phase 6 — On site (weeks 28–46)
Weekly breakdown for a mid-spec 25–30 m² London rear extension:
- Weeks 1–2: Strip-out, protection of existing house, welfare setup.
- Weeks 3–5: Groundworks — excavation, foundations, drainage. Party Wall surveyor's schedule of condition inspection.
- Weeks 6–9: Structural shell — brickwork, blockwork, steel installation.
- Weeks 10–12: Roof, glazing, watertight envelope. First fix electrics and plumbing.
- Weeks 13–15: Plaster, second fix electrics and plumbing, floor screed.
- Weeks 16–18: Kitchen install, joinery, flooring, painting.
London delay to watch: steel lead times are 4–8 weeks — order at contract signature, not week 6.
Phase 7 — Snag, sign-off and handover (weeks 46–50)
- Snag list walk-through with builder.
- Building Control final inspection and completion certificate.
- Party Wall surveyor's post-works inspection.
- Warranties, O&M manuals, retention held (typically 2.5–5% for 12 months).
What compresses the timeline (legitimately)
- Design-and-build under one contract. Removes the tender gap and unifies coordination.
- Choose PD + LDC route where possible. Saves 4–6 weeks vs full planning.
- Serve party wall notices in parallel with planning. The two months run down while you wait for the council anyway.
- Order long-lead items at contract signature. Steel, glazing, bespoke kitchens — not on week 6 of the build.
- Move out during first fix. A builder who is not tiptoeing around a family runs 15–25% faster on site.
What stretches it (avoid where you can)
- Design changes after tender.
- Late kitchen and bathroom selection.
- Under-briefing your neighbours, then negotiating Party Wall dissent mid-build.
- Splitting the contract across too many trades without a lead contractor.