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.
Quick Answer
A London extension project typically takes 10-12 months from initial planning to completion: 4 weeks initial planning, 8 weeks design, 12 weeks approvals, 8 weeks procurement, and 16+ weeks construction. Early planning permission and party wall checks are critical.
4-12 weeks
Planning Phase
8-12 weeks
Approvals
12-20 weeks
Construction
Check your specific property constraints
Free Property CheckPhase 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.
Start your project with confidence
Our AI assistant can check your property constraints, estimate costs, and tell you if you need planning permission - all in 2 minutes.
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.
Before submitting your planning application, make sure you understand your property constraints.
Free property check →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.
Free AI Assistant
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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.
Ready to start?
Ready to start your extension project?
Our AI assistant can check your property, estimate costs, and guide you through the planning process. Free, instant, no signup required.
Or email hello@mayfairstudio.co.uk · 07405 920944
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.