Timelines are the second thing every London homeowner underestimates, after cost. A single-storey rear extension is 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)
- 1
Write the brief before you meet an architect
Week 1One page: how you cook, eat and work at home; must-haves; the budget you will actually spend; the date you need to be finished by.
- 2
Book the measured survey
Week 1Half a day on site, two weeks for the CAD file. London survey firms are booked 3–4 weeks out in spring and autumn.
- 3
Two or three feasibility options
Weeks 2–3Not one preferred scheme. Options force the trade-offs into the open while changing them is still free.
- 4
Sanity-check against per-m² benchmarks
Week 3If the favoured option is 40% over budget at benchmark rates, fix it now — not after planning.
Phase 2 — Design development (weeks 3–8)
- Design frozen and signed off by both ownersYou
Every later change costs money and weeks. Sign the plan.
- Planning drawings completeArchitect
All elevations, sections, dimensioned depth and eaves height, materials schedule.
- Structural engineer engagedEngineer
Indicative steel and foundation strategy for the submission, so the scheme is buildable as drawn.
- Pre-application advice, if marginalOptional
£250–£800 and 4–6 weeks. Worth it for ambitious or precedent-free schemes.
- Neighbours briefed informallyYou
Before the council's consultation letter arrives, not after.
Phase 3 — Planning or LDC (weeks 8–20)
| Route | Statutory | London reality |
|---|---|---|
| Lawful Development Certificate | 8 weeks | 6–8 weeks |
| Householder planning | 8 weeks | 10–14 weeks in Camden, Islington, Hackney, K&C |
| Prior approval (larger single-storey) | 42 days | 6–7 weeks |
| Conservation area / objections | 8 weeks | 12–20 weeks |
Phase 4 — Technical design (weeks 16–24, overlapping)
- Building Control application — council or Approved Inspector.
- Structural engineer issues construction calculations and details.
- Party Wall notices served — allow two months minimum before start.
- Drain build-over agreement applied for, where a shared drain is crossed.
- Tender pack issued to three or four vetted builders, with a full schedule of works.
Phase 5 — Tender, appointment, mobilisation (weeks 22–28)
- 1
Issue an identical pack to every builder
Week 22Same drawings, same schedule of works, same provisional sums. Different packs make the returns incomparable.
- 2
Tenders returned
Weeks 25–26Three to four weeks is realistic. Anyone who prices a London extension in 48 hours has not read the drawings.
- 3
Post-tender interviews
Week 26Meet the site manager who will actually run your job, not only the salesperson. Ask to visit a live site.
- 4
Contract signed
Week 27JCT Homeowner or Minor Works, with a stage payment schedule tied to completed work and 2.5–5% retention.
- 5
Mobilisation
Week 28Hoardings, welfare, skip and parking permits, and the long-lead order for steel and glazing on day one.
Phase 6 — On site (weeks 28–46)
Phase 7 — Snag, sign-off and handover (weeks 46–50)
- Snag list walked and signed offBoth
Walk it in daylight, with the site manager, room by room.
- Building Control completion certificateBuilder
The document your buyer's solicitor will ask for. No certificate, no final payment.
- Party wall post-works inspectionSurveyor
Surveyor confirms no damage against the schedule of condition.
- Electrical installation certificate and gas safety recordBuilder
Part P certification and any Gas Safe work notified.
- Glazing, roof and appliance warrantiesBuilder
Registered in your name, with serial numbers.
- As-built service drawingsYou
Photographs of first fix before plaster are worth more than any drawing later.
- Retention agreed in writingBoth
2.5–5% held for 6–12 months for defects.
Serve your party wall notices while the council still has your planning application. Two months of statutory waiting can either run in parallel or bolt onto the end — that choice is yours, and it is free.
— The single most useful piece of timeline advice
What legitimately compresses the programme
- Serve party wall notices in parallel with planning
- Take the PD + LDC route where the design allows — saves 4–6 weeks
- Order steel, glazing and bespoke kitchens at contract signature
- Use design-and-build under one contract to remove the tender gap
- Move out during first fix — a builder not tiptoeing around a family runs 15–25% faster
- Change the design after tender
- Leave kitchen and bathroom selection until the shell is up
- Under-brief neighbours and negotiate party wall dissent mid-build
- Split the contract across many trades with no lead contractor
- Book a holiday for the week of practical completion
+Can a London extension really be done in six months?
Yes, if the works are permitted development, you use design-and-build, party wall notices are served early and the design never changes. Twelve months is more common with full planning.
+How long is the build itself?
Sixteen to eighteen weeks on site for a mid-spec 25–30 m² rear extension. A side return alone can be twelve to fifteen; a double-storey rear runs twenty-two to thirty.
+Should we live in the house during the build?
Most families stay through groundworks and shell, then move out for first fix through plastering. That window is the dustiest and the noisiest, and moving out speeds the site up.
+What is the best time of year to start on site?
Late winter or early spring. Groundworks in deep winter risk frost delays, and you want the watertight envelope closed before the following autumn.
+Who owns the programme?
The contractor produces it; you approve it and hold them to it at weekly site meetings. Ask for a one-page updated programme at every meeting — the drift shows up there first.