Extension Project Checklist: Step-by-Step Guide
Your complete checklist for planning and building an extension in London. Every step from initial ideas through to moving into your new space.
TL;DR - The Quick Answer
A London extension project runs across six phases and typically takes 10-12 months from start to finish. Initial planning (weeks 1-4) covers budget-setting and permitted development checks, followed by design (weeks 5-12), approvals including an 8-week planning determination period (weeks 13-24), procurement with 3-5 builder tenders (weeks 20-28), and 16+ weeks of construction. Completion requires a building control sign-off, trade certificates, and a 2.5-5% retention period.
Phase 1: Initial Planning (Weeks 1-4)
What do you need? Extra bedroom, larger kitchen, home office?
Include 15-20% contingency. Consider professional fees, not just build costs.
Use our permitted development checker to see what you can build without planning.
Conservation area? Listed building? Tree preservation orders?
Early communication prevents problems later. Good neighbours make projects easier.
| Phase | Key Tasks | Typical Duration |
|---|---|---|
| 1. Initial planning | Budget, PD check, constraints research, neighbour consultation | Weeks 1–4 |
| 2. Design | Appoint architect, survey, concept + developed design, D&A statement | Weeks 5–12 |
| 3. Approvals | Planning application (8-week determination), party wall, building regs | Weeks 13–24 |
| 4. Procurement | Technical drawings, tender to 3–5 builders, sign contract | Weeks 20–28 |
| 5. Construction | Groundworks, superstructure, first fix, plastering, second fix, snagging | Weeks 28–44+ |
| 6. Completion | Building control sign-off, certificates, retention, insurance update | After construction |
Source: Mayfair Studio project data • Data as of 2025
Phase 2: Design (Weeks 5-12)
Get recommendations, review portfolios, compare quotes (3 minimum).
Measured survey of existing building. May need topographical survey too.
Sketch options showing different approaches. Agree on direction before detail.
Floor plans, elevations, sections. Enough detail for planning application.
Written justification for your design. Required for conservation areas.
Phase 3: Planning & Approvals (Weeks 13-24)
£258 fee for householder applications. 8 week determination period.
£129 fee. Confirms your development is permitted. Good for resale.
If building on or near boundary. 2 months notice before work starts.
For beam calculations, foundation design, structural drawings.
Full plans or building notice. Full plans recommended for extensions.
Phase 4: Procurement (Weeks 20-28)
Detailed construction drawings for builders to quote from.
Written specification of materials, finishes, workmanship standards.
Send identical information. Allow 3-4 weeks for quotes.
Visit completed projects. Verify public liability and employer's liability.
JCT Minor Works or similar. Include scope, price, timeline, payment terms.
Phase 5: Construction (Weeks 28-44+)
Agree programme, access, working hours, site setup, communication.
Building control inspection at foundation stage before concrete pour.
Walls, steel, roof construction. Multiple building control inspections.
Pipes and cables before plastering. Last chance to change positions.
Walls finished, sockets and switches installed, kitchen fitted.
Final finishes. Walk through with builder, create snagging list.
Phase 6: Completion
Inspector signs off work. Completion certificate issued.
Electrical Part P certificate, gas certificate, FENSA for windows.
Hold 2.5-5% retention for defects liability period (typically 6-12 months).
Operation manuals for appliances, warranties for materials, guarantee certificates.
Notify insurer of increased floor area and rebuild value.
Keep planning approvals, building control certificate, warranties for resale.
A London extension project runs across six phases and typically takes 10–12 months from start to finish. Allow 4 weeks for initial planning, 8 weeks for design, up to 12 weeks for planning approval, 8 weeks for procurement, and 16+ weeks for construction. Retain 2.5–5% of the build cost until the defects liability period expires.
Frequently Asked Questions
Frequently Asked Questions
How long does an extension project take from start to finish?
A typical London extension takes 10-12 months total: 4 weeks initial planning, 8 weeks design, 12 weeks approvals, 8 weeks procurement, and 16+ weeks construction.
When should I start looking for a builder?
Start your builder search during the planning and approvals phase, typically 4-6 months before you want to start construction. Good builders are booked 2-6 months ahead.
Do I need planning permission for my extension?
Many single-storey rear extensions fall under permitted development and don't need planning permission. Check your PD rights first-if you exceed limits or have constraints (conservation area, listed building), you'll need to apply.