Homeowners often ask 'how long will my extension take?' expecting a number like 'about 12 weeks'. The honest answer is more nuanced: site work is 10-22 weeks depending on type, but the full project from first call to keys is usually 6-9 months including planning, design, and Party Wall agreements. Here's the realistic timeline.
The five phases of a London extension
- Design and planning
- Party Wall agreements (if applicable)
- Pre-start (contractor procurement, drawings, finalising spec)
- Site work (the actual build)
- Snagging and handover
Site work — the bit people picture when they think 'extension build' — is usually only 40-50% of the total project duration. Front-end design and approvals often take just as long.
Phase 1: Design and planning (6-14 weeks)
Permitted development route (faster)
If your extension falls under permitted development rights — typical for single-storey rear extensions up to 6m deep on terraces — you don't need full planning permission. You may need prior approval for the larger PD route (extensions over 4m deep on terraces), which takes 6-8 weeks. Architectural drawings take 2-4 weeks.
Full planning permission route
Double-storey extensions, side extensions, and most extensions in conservation areas need full planning permission. Architectural drawings: 4-6 weeks. Planning application processing: 8-12 weeks (8 weeks is the statutory determination period). Allow more time if your borough is slow or your case is contentious. In total, plan for 12-18 weeks from instruction to consent.
Phase 2: Party Wall agreements (4-8 weeks)
If your extension affects shared walls with neighbours (very common in London terraces and semis), you must serve notice under the Party Wall Act 1996. The process:
- Serve formal notice on each adjoining owner (2 months' notice required before work starts on most issues)
- Neighbour responds within 14 days — can consent, dissent, or do nothing (which counts as dissent)
- If dissented, each party appoints a Party Wall Surveyor (you can share one 'Agreed Surveyor')
- Surveyor(s) produce a Party Wall Award documenting the work, condition of neighbour's property, and dispute mechanism
Typical timeline from first notice to a signed Award: 6-10 weeks. Skip this and the neighbour can obtain an injunction stopping the work mid-build.
Phase 3: Pre-start (2-4 weeks)
Even after planning and Party Wall are sorted, you need:
- Building Regulations submission — Building Notice or Full Plans (£600-1,200 fee, processing 5-15 working days)
- Structural engineer's calculations — for steels, foundations, openings
- Finalised specification — exact kitchen, doors, finishes, fittings
- Contractor procurement — signed contract, deposit paid, materials ordered (some lead times like bi-fold doors are 6-10 weeks)
Phase 4: Site work — the build (10-22 weeks)
Typical London extension site timelines:
| Extension type | Site time |
|---|---|
| Single-storey rear (3m × 5m) | 10-14 weeks |
| Side-return only | 10-14 weeks |
| Wrap-around (rear + side combined) | 14-18 weeks |
| Double-storey rear | 16-22 weeks |
| Glazed box / structural glass extension | 16-24 weeks |
Inside the site work timeline (single-storey example)
| Weeks | Activity |
|---|---|
| Week 1-2 | Site setup, scaffolding, excavation |
| Week 3-4 | Foundations, drainage, sub-floor |
| Week 5-7 | Walls up, structural steels installed |
| Week 8-9 | Roof, glazing, weather-tight envelope |
| Week 10-11 | First fix — electrical, plumbing, plastering |
| Week 12 | Second fix — kitchen install, finishes, doors |
| Week 13-14 | Snagging, final clean, handover |
Phase 5: Snagging and handover (1-3 weeks)
After the contractor declares 'practical completion', there's a snagging period — typically 2-4 weeks — where you note any defects (gaps in finishes, doors not closing properly, paint touch-ups needed). The contractor returns to fix snags. Final retention (typically 5% of contract value held back from final payment) is released once snags are signed off.
Total timeline — realistic
For a typical London single-storey rear extension going through full planning:
- Design + planning: 12-16 weeks
- Party Wall: 6-10 weeks (often overlaps with end of planning)
- Pre-start: 2-4 weeks
- Site work: 10-14 weeks
- Snagging: 2 weeks
- Total: roughly 6-8 months from instruction to keys
Permitted development route can shave 6-10 weeks off the front end. Larger projects (double-storey, wrap-around with planning) typically run 8-12 months end to end.
What can extend the timeline
- Bad weather — heavy rain or snow halts excavation, roofing, exterior brickwork. Allow 1-2 weeks of weather contingency in any UK build.
- Material lead times — bi-fold doors, custom kitchens, structural glass: 8-14 weeks on some items. Order early.
- Planning amendments — if neighbours object during planning, expect 4-8 extra weeks for design revisions or appeals.
- Surprises during groundworks — discovering old wells, drains in wrong locations, asbestos, contamination. Add 2-6 weeks.
- Borough planning team delays — some London boroughs (notably Camden, Westminster, Hackney) are notably slow during summer months.
Frequently asked questions
Can I do anything to speed up the timeline?
Yes. Start design early (before you've sold your current house if you're buying-to-renovate). Serve Party Wall notices as soon as you have permission. Order long-lead materials (bi-folds, custom kitchen) as early as the spec is locked. Choose a builder with capacity rather than the cheapest one that's 6 months booked-up.
Is there a 'fastest' London borough for planning?
Wandsworth and Bromley tend to be relatively quick (8-10 weeks for straightforward applications). Camden, RBKC, Westminster and Hackney often run 12-16+ weeks. Conservation area applications add 4-6 weeks anywhere.
What time of year is best to start an extension?
Spring (Mar-May) is ideal — site work runs through summer with good weather, and you're typically in by autumn. Avoid starting major external work in November-January if you can — wet weather slows everything.
Can I stay in the house during a 14-week extension?
Yes for most single-storey extensions. The original rear wall stays in place until 'break-through' late in the build (typically week 8-10), so the existing kitchen/living area remains usable. The break-through phase (1-2 weeks) is the most disruptive — dust, no kitchen, no rear access — and many clients move out briefly for it.
What happens if the builder runs over time?
Most contracts have a target completion date with extension provisions for legitimate delays (weather, agreed variations). Liquidated damages clauses (typically £200-500/week) penalise the builder for late completion. A good contract protects both sides — make sure yours has clear extension-of-time provisions, not just damages.
Related services
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We deliver every type of extension across London — single-storey through to double-storey wrap-arounds.
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Learn more →Glass Extension Cost Guide
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