How Long Does a Home Extension Take?
Realistic timelines for every stage - design, planning, Building Regs, construction, and handover. Plus what causes delays and how to avoid them.
"How long will it take?" is the question every homeowner asks - and the answer is almost always longer than they expect. Builders quote the construction phase, but the total timeline includes design, approvals, procurement, and construction. Understanding each stage helps you plan realistically and avoid the frustration of missed deadlines.
The Full Timeline: Stage by Stage
Stage 1: Design (4–10 Weeks)
Your architect or technologist designs the extension:
| Task | Duration |
|---|---|
| Initial consultation and site survey | 1 week |
| Concept design options (2–3 schemes) | 2–3 weeks |
| Detailed design development | 1–2 weeks |
| Planning drawings preparation | 1–2 weeks |
Overlap opportunity: Brief a structural engineer as soon as the design is agreed - their calculations can run in parallel with the planning application.
Stage 2: Approvals (6–16 Weeks)
| Approval | Duration | Overlap Possible? |
|---|---|---|
| Planning permission | 8–12 weeks | - |
| Lawful Development Certificate (if PD) | 6–8 weeks | Instead of planning |
| Building Regulations | 2–5 weeks | Yes - after planning submission |
| Party Wall notices | 2–10 weeks | Yes - as soon as design is agreed |
| Structural engineer calculations | 1–3 weeks | Yes - during planning |
Key insight: Party Wall is the hidden bottleneck. Most homeowners leave it until last, but it can delay the start by 2–3 months. Serve notices as soon as your design is finalised - you don't need to wait for planning approval.
Stage 3: Procurement (2–6 Weeks)
Ordering materials and getting builder quotes:
| Task | Duration | Lead Time |
|---|---|---|
| Get 3 builder quotes | 2–4 weeks | - |
| Order structural steel | 2–4 weeks | Can be ordered during approvals |
| Order windows/doors | 4–12 weeks | Crittall takes 8–14 weeks |
| Order kitchen | 4–8 weeks | If kitchen extension |
| Scaffolding booking | 2–4 weeks | Book early in summer |
Critical path items: Steel and windows are the two items that most often cause construction delays. Order them as soon as the structural engineer and architect have finalised specifications - don't wait for the builder to start.
Stage 4: Construction
| Project Type | Construction Duration |
|---|---|
| Garage conversion | 2–4 weeks |
| Loft conversion (dormer) | 6–10 weeks |
| Single-storey rear extension | 10–16 weeks |
| Kitchen extension (with kitchen fit) | 12–18 weeks |
| Side return / wrap-around | 12–16 weeks |
| Double-storey extension | 14–22 weeks |
| Basement conversion | 16–30 weeks |
Stage 5: Snagging and Handover (1–3 Weeks)
After the builder finishes, walk through the project and create a snagging list - minor defects, incomplete items, and quality issues to be rectified. Common snags:
- Paint touch-ups and unfinished decoration
- Doors not closing properly
- Sealant gaps around windows and sanitaryware
- Scratched glass or damaged surfaces
- Missing or loose fixtures
- Building Control completion certificate not yet issued
Your builder should return to fix snags within 2–4 weeks. Hold back the final 5–10% of payment until snags are resolved. Find vetted builders through the Federation of Master Builders who follow formal snagging procedures.
Total Timeline by Project
| Project | Design + Approvals | Construction | Total |
|---|---|---|---|
| Garage conversion | 4–8 weeks | 2–4 weeks | 2–3 months |
| Loft conversion | 8–16 weeks | 6–10 weeks | 4–6 months |
| Single-storey extension (PD) | 6–12 weeks | 10–16 weeks | 4–7 months |
| Single-storey extension (planning) | 12–20 weeks | 10–16 weeks | 6–9 months |
| Kitchen extension | 12–20 weeks | 12–18 weeks | 6–10 months |
| Double-storey extension | 14–22 weeks | 14–22 weeks | 7–11 months |
| Basement conversion | 16–24 weeks | 16–30 weeks | 8–14 months |
Construction Phase Breakdown
Here's what happens week by week on a typical 14-week single-storey kitchen extension:
| Week | Phase | What Happens |
|---|---|---|
| 1 | Groundworks | Excavate foundations, pour concrete, drainage |
| 2 | Foundations | Foundation walls, DPC, ground-floor slab preparation |
| 3 | Slab | Insulation, underfloor heating pipes, concrete floor slab |
| 4–5 | Walls | Blockwork walls, cavity insulation, lintels |
| 6 | Roof | Roof structure, membrane, flat roof or tile finish |
| 7 | Weathertight | Windows and doors fitted, building sealed |
| 8 | First fix | Electrics, plumbing, underfloor heating connected |
| 9 | Plastering | Walls and ceilings plastered, screed floor |
| 10 | Drying | Plaster and screed drying (heat and ventilate) |
| 11 | Second fix | Sockets, switches, lighting, radiators, sanitaryware |
| 12 | Kitchen | Kitchen units, worktops, appliances fitted |
| 13 | Decoration | Painting, tiling, flooring |
| 14 | Snagging | Final fixes, cleaning, Building Control final inspection |
What Causes Delays
| Delay | Time Added | How to Avoid |
|---|---|---|
| Late Party Wall notices | 4–10 weeks | Serve notices during design, not after planning |
| Planning refusal + resubmission | 12–20 weeks | Get pre-application advice first |
| Bad weather (foundations, roof) | 1–4 weeks | Start in spring, not late autumn |
| Steel delivery delay | 2–4 weeks | Order as soon as engineer specifies |
| Window/door lead time | 2–8 weeks | Order before construction starts |
| Design changes during build | 2–6 weeks | Finalise design before breaking ground |
| Unexpected ground conditions | 1–3 weeks | Get a ground investigation survey (£500–£1,500) |
| Builder capacity / sub-contractor delays | 1–4 weeks | Use a builder with in-house trades |
Tips for Staying on Schedule
- Start Party Wall notices immediately after design is agreed - don't wait for planning
- Order steel and windows early - the two longest lead-time items
- Use permitted development if possible - skip the 8–12 week planning wait
- Agree the design before starting - changes during construction cause delays and cost overruns
- Book your builder 4–8 weeks ahead - good builders are in demand, especially spring–summer
- Consider timber frame or SIPs - 2–4 weeks faster to weathertight than masonry
- Keep a decision log - record every agreed change to avoid disputes later
The Financial Cost of Delays
Delays aren't just frustrating - they cost money:
| Delay Impact | Cost |
|---|---|
| Extended scaffolding hire (per week) | £50–£150 |
| Temporary kitchen / eating out (per week) | £100–£300 |
| Builder standing time (per day) | £200–£500 |
| Extended accommodation if uninhabitable (per week) | £500–£1,500 |
| Material price increases (per month of delay) | 0.3–0.5% of build cost |
A 4-week delay on a £60,000 extension can add £1,000–£3,000 in direct costs. Plan realistically and you'll avoid most of it.
Next Steps
- Map your timeline - use the stage-by-stage guide above
- Identify your critical path - planning, Party Wall, and material lead times
- Brief your architect early - the design phase sets the pace for everything
- Get 3 builder quotes with start dates and completion timelines
- Budget for the timeline - see our hidden costs guide
- Get a cost estimate - use our free calculator for your project
- Check the Planning Portal for current application processing times in your area
Frequently Asked Questions
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