London Extension Timeline 2025: Complete Process Guide
From initial idea to moving in: the complete timeline for your London extension. Realistic timescales for each stage, what causes delays, and how to keep your project on track.
Key Takeaways
- Total timeline: 6-12 months from initial idea to completion (typical single-storey extension)
- Planning permission alone takes 8-12 weeks (plus 4-8 weeks design beforehand)
- Party wall process runs in parallel but can add 2-3 months if disputed
- Get instant timeline & cost estimate for your extension (Free)
The Complete Timeline Overview
Building an extension in London is a significant project spanning 6-12 months for a typical single-storey rear extension. Two-storey extensions or complex projects can take 12-18 months. Understanding each stage helps you plan around work, school terms, and family life.
The timeline below is based on real London projects completed in 2024-2025, accounting for typical borough processing times and construction realities.
Typical Timeline at a Glance
Stage 1: Planning & Design (4-8 weeks)
What Happens
This is where your vision becomes a concrete plan. An architect or designer will:
- Survey your property (measurements, existing structures, services)
- Check planning constraints (conservation areas, Article 4 directions, tree preservation orders)
- Create initial concept designs (2-3 options)
- Develop detailed planning drawings (floor plans, elevations, sections)
- Prepare planning statements and supporting documents
Timeline Factors
Faster (4-5 weeks): Simple rear extension, quick decision-making, experienced designer
Slower (6-8 weeks): Complex design, multiple revisions, conservation area requiring heritage statement, structural surveys needed
Pro tip: Start this stage in autumn/winter if you want to build in summer. Many projects have permission granted just as good building weather begins.
Stage 2: Planning Permission / Lawful Development (8-12 weeks)
Planning Permission Route
Most London extensions require planning permission due to Article 4 directions or conservation area restrictions.
Lawful Development Certificate (Faster Alternative)
If your extension qualifies under Permitted Development rights, you can apply for a Certificate of Lawful Development instead. This typically takes 6-8 weeks and has a higher approval rate since the council only confirms you meet PD criteria.
Important: If planning permission is rejected, you'll need to redesign and resubmit, adding another 8-12 weeks. This is why experienced designers familiar with your borough's preferences are worth the investment.
Borough-Specific Timings
Fast Boroughs (8-9 weeks)
- Wandsworth
- Hammersmith & Fulham
- Bromley
Slower Boroughs (10-13 weeks)
- Westminster
- Camden
- Islington (conservation areas)
Stage 3: Building Regulations (2-4 weeks)
Building regulations ensure your extension is structurally safe, thermally efficient, and accessible. You can apply via:
Full Plans Application
- Submit detailed drawings upfront
- Council reviews before work starts
- Takes 5-8 weeks for approval
- Recommended for complex projects
Building Notice Route
- Submit notice to start
- Details provided during construction
- Can start in 2 days after notice
- Faster for simple extensions
Pro tip: Use Building Notice if your contractor starts quickly after planning approval. Use Full Plans if there's a gap or you're coordinating complex structural work.
Most projects now also use an approved inspector (private building control) instead of council building control. This can be faster and more flexible, with inspections scheduled around your contractor's programme.
Stage 4: Party Wall Process (2-3 months)
If your extension affects a shared wall or is within 3-6m of a neighbour's structure, you must serve Party Wall notices under the Party Wall Act 1996.
The Process
- 1Serve notice to neighbours (minimum 2 months before work starts for new structures, 1 month for other work)
- 214-day response period for neighbours to consent or dissent
- 3If they consent: you can proceed after the notice period (2 months)
- 4If they dissent or don't respond: both parties appoint surveyors who agree a Party Wall Award
- 5Award issued: typically takes 4-8 weeks from surveyor appointment
Timeline Scenarios
Best case (2-2.5 months): Neighbours consent immediately, no surveyor needed
Typical case (2.5-3 months): Surveyors appointed, straightforward award
Worst case (3-6 months): Disputes, multiple site visits, complex awards, or appeals
Pro tip: Start the party wall process immediately after planning approval. This runs in parallel with building regulations and tendering, so it won't delay your overall timeline if managed well.
Costs
You (the building owner) pay for all surveyors' fees:
- Your surveyor: £800-1,500
- Neighbour's surveyor: £800-1,500 per neighbour (if they appoint one)
- Total for terraced house (2 neighbours): £2,400-4,500 typical
Stage 5: Tender & Contractor Selection (2-4 weeks)
Once you have planning approval and building regulations underway, you need to select your contractor. This stage is often overlooked but is critical to project success.
The Process
- Week 1: Request quotes from 3-5 contractors (provide drawings, specifications, provisional sums)
- Weeks 2-3: Site visits, detailed quotes returned
- Week 3-4: Review quotes, check references, verify insurance, negotiate terms
- Week 4: Sign contract, agree start date, pay deposit (typically 10-15%)
Important: Don't select purely on price. Check previous work, references, insurance (public liability £5m+), and whether they're realistic about timescales. A cheap quote often becomes expensive through delays and poor quality.
What to Check
Essential Checks
- Public liability insurance (£5m minimum)
- Employers' liability if they have staff
- References from last 3 projects
- Visit a current or recent project
Red Flags
- Wants 30%+ upfront payment
- No written contract offered
- Can't provide insurance documents
- Reluctant to provide references
Stage 6: Construction (8-16 weeks)
The main event. Construction timescales vary significantly by project size and complexity.
Typical Construction Timescales
Construction Phases
Phase 1: Groundworks (1-2 weeks)
Site setup, foundations excavation, concrete pour, drainage connections. This is the messiest phase.
Phase 2: Structure (2-3 weeks)
Blockwork walls, structural steel installation, first floor joists (if two-storey), roof structure and covering.
Phase 3: Weathertight & First Fix (2-3 weeks)
Windows and doors installed, first fix electrics and plumbing, internal walls framed, insulation fitted.
Phase 4: Plastering & Second Fix (2-3 weeks)
Plastering (needs 2 weeks to dry properly), second fix electrics and plumbing, kitchen and bathroom installation begins.
Phase 5: Finishes (2-3 weeks)
Flooring, tiling, decoration, appliances fitted, external works completed (patio, landscaping).
Building Control Inspections
Your building control inspector will visit at key stages. Typical inspection points:
- Foundation trenches (before concrete)
- Concrete foundations poured
- Drainage before covering
- Damp proof course installed
- Structural steels installed
- Insulation and ventilation installed
- Final inspection (completion certificate)
Stage 7: Finishing & Snagging (2-3 weeks)
The contractor should create a snagging list of minor defects before handover. This typically includes:
- Paint touch-ups and final decoration
- Door and window adjustments
- Grouting and sealant work
- Electrical faceplate alignment
- Final clean and rubbish removal
You should also conduct your own inspection and create your own snagging list. Withhold 5-10% of final payment until all snagging items are resolved.
Pro tip: Don't expect perfection, but do expect professional standards. Small paint chips or minor scratches are normal; crooked tiles or gaps around windows are not.
Handover Documents
Before final payment, ensure you receive:
- Building control completion certificate
- Electrical installation certificate (Part P)
- Gas Safe certificate (if applicable)
- FENSA certificates for windows/doors
- Product warranties and manuals
- Structural engineer's certificates
What Causes Delays
Planning Stage Delays (add 2-6 months)
- Planning rejection: Need to redesign and resubmit (adds 2-4 months)
- Planning appeal: If you appeal a refusal (adds 3-6 months)
- Missing documents: Incomplete applications or requested amendments (adds 2-4 weeks each time)
Party Wall Delays (add 1-3 months)
- Neighbour disputes: Extended negotiations and award amendments (adds 1-2 months)
- Late notice service: Starting party wall too late (avoidable with good planning)
Construction Delays (add 2-8 weeks)
- Weather: Prolonged rain delays foundations and external work (adds 1-3 weeks in winter)
- Undiscovered issues: Hidden drainage, asbestos, structural problems (adds 2-4 weeks)
- Material delays: Supply chain issues, custom items on long lead times (adds 2-6 weeks)
- Client changes: Design changes mid-construction (adds 1-4 weeks per change)
- Failed inspections: Building control issues requiring rework (adds 1-2 weeks per failure)
How to minimize delays:
- Hire an experienced architect who knows your borough's preferences
- Start party wall process as soon as planning is submitted
- Order long-lead items (bifold doors, bespoke windows) as soon as planning is approved
- Build in spring/summer to avoid weather delays
- Have a contingency budget (10-15%) for undiscovered issues
Living Through Construction
Most London families stay in their homes during extension projects. Here's what to expect and how to make it bearable.
The Reality
Difficult Phases
- Weeks 1-2 (Demolition): Very dusty and noisy
- Weeks 2-3 (Foundations): Heavy machinery, early starts
- Weeks 4-6 (Opening up): House exposed to elements, plastic sheeting everywhere
Better Phases
- Weeks 6-7 (Weathertight): Windows in, much calmer
- Weeks 8+ (Second fix): Cleaner trades, progress visible daily
Practical Tips
- Set up a temporary kitchen: Microwave, kettle, and toaster in another room
- Seal off work area: Plastic sheeting and zippered doors reduce dust spread
- Agree working hours: Typical 8am-5pm Monday-Friday, 8am-1pm Saturday
- Toilet and washing facilities: Arrange site toilet or dedicated bathroom access
- Daily cleanup: Insist on end-of-day sweeping and waste management
- Plan escapes: Book weekend trips or stay with family during the worst phases
- Protect valuables: Move furniture, art, and electronics away from work areas
Reality check: It will be worse than you expect. Budget for eating out more, increased heating bills (from openings), and family stress. Most families say it was worth it, but few would want to repeat the experience immediately.
When to Move Out
Consider temporary accommodation if:
- You have young children (under 3) or vulnerable family members
- You're doing a full house renovation or reconfiguration
- Kitchen or bathroom are out of action for more than 2 weeks
- Major structural work (underpinning, full rear wall removal) where safety is a concern
Get Your Extension Timeline & Cost
Our free estimator provides a realistic timeline and cost breakdown for your specific extension project, based on thousands of completed London extensions.
- Stage-by-stage timeline for your extension type
- Cost breakdown by phase
- Planning permission likelihood check
- Instant results, no signup required
Frequently Asked Questions
How long does a single-storey extension take from start to finish?
6-9 months typically. This includes 4-8 weeks design, 8-12 weeks planning permission, 2-3 months party wall (running in parallel), and 8-12 weeks construction. If you qualify for Permitted Development and avoid planning permission, you can reduce this to 4-6 months.
How long does a two-storey extension take?
9-15 months typically. The planning and party wall process is similar to single-storey, but construction takes 12-16 weeks instead of 8-12 weeks due to additional structural work and first floor construction.
Can I speed up the planning permission process?
Not really. The statutory determination period is 8 weeks (13 weeks for major applications). Some boroughs offer pre-application advice services (£200-800) where you can discuss your proposal before submitting. This doesn't speed up the process, but increases your chances of first-time approval.
What's the best time of year to start construction?
April-May or September. Starting in spring means the structure is weathertight before winter. Starting in September means foundations are complete before heavy winter rain. Avoid starting groundworks in December-February due to weather delays.
How much of the process can run in parallel?
Quite a bit if planned well: Start party wall process as soon as planning is submitted. Submit building regulations immediately after planning approval. Begin tendering while waiting for building control approval. Order long-lead items (windows, doors, kitchen) as soon as planning is approved. Good project management can save 4-8 weeks on the overall timeline.
What if I want to pause the project mid-construction?
Very difficult and expensive. Contractors have scheduled work and materials. You'll likely need to pay for any materials ordered and may face termination fees. The site must be made secure and weathertight. If you need to pause, do it between clear phases (e.g., after the structure is weathertight) rather than mid-phase.
How often should I expect building control inspections?
5-8 inspections typical for a standard extension: foundations trenches, concrete pour, drainage, DPC level, structural steels, insulation/ventilation, and final inspection. Your contractor should notify the inspector 24-48 hours before each inspection point. Failed inspections require rework and re-inspection (adding 1-2 weeks).
Can I move in before the snagging is complete?
Yes, but withhold payment until snagging is complete. Once you've moved in and paid in full, contractors are notoriously difficult to get back for minor fixes. Hold 5-10% until all snagging items are resolved and you have all certificates.
Summary
A London extension takes 6-12 months from initial idea to completion for a typical single-storey project. The timeline breaks down into clear stages: design (4-8 weeks), planning permission (8-12 weeks), building regulations (2-4 weeks), party wall (2-3 months, often overlapping), tender (2-4 weeks), construction (8-16 weeks), and finishing (2-3 weeks).
Delays are common and typically come from planning rejections, party wall disputes, weather, and undiscovered site issues. Building in 10-15% contingency time and budget helps manage expectations.
Living through construction is challenging but manageable with good planning. Most families stay in their homes, especially for single-storey rear extensions. The first 4-6 weeks are the hardest; it gets significantly better once the structure is weathertight.