How Long Does an Extension Take? Real Timeline Breakdown 2026
Every builder says "three months." Your neighbour says theirs took six. Your architect mentions eight weeks but adds "depending on planning."
So how long actually?
The truth is: most extensions take 5-7 months from the day you decide to build until the day you move furniture in. But the build itself? Usually 3-4 months. The confusion comes from not understanding what happens before the first brick is laid.
Here's the realistic timeline, broken down phase by phase, with the delays nobody mentions until they happen.
The Complete Timeline (Typical Single-Storey Rear Extension)
Quick Answer: Planning permission (if needed): 8-13 weeks. Design & quotes: 4-6 weeks. Actual build: 12-16 weeks. Total: 6-9 months from start to finish.
Phase 1: Design & Planning (8-20 weeks)
Weeks 1-2: Initial Design
- Meet architect or architectural technician
- Site survey and measurements
- First draft drawings
- Discuss materials, windows, roof style
Typical cost at this stage: £800-£1,500
Weeks 3-4: Revisions & Structural Calculations
- Refine design based on your feedback
- Structural engineer calculates beam sizes, foundations
- Finalise drawings for planning/building control
Typical cost: £600-£1,200 for structural engineer
Weeks 5-16: Planning Permission (if required)
- Week 5: Submit planning application to council
- Weeks 5-8: Validation period (council checks application is complete)
- Week 9: Neighbour consultation period starts (21 days)
- Weeks 10-12: Council assesses application
- Week 13-16: Decision issued
Official timeline: 8 weeks. Realistic timeline: 10-13 weeks.
Common Delay: Council asks for amendments (revised drawings, additional survey). Add 3-4 weeks.
Week 17-18: Get Builder Quotes
- Send drawings to 3-5 builders
- Wait for quotes (1-3 weeks typical)
- Review and compare
- Check references
Weeks 19-20: Building Control Approval
- Submit to building control (council or private inspector)
- Approval typically 2-3 weeks
- Can happen in parallel with getting quotes
Phase 2: Pre-Construction (2-4 weeks)
Weeks 21-22: Prep Work
- Builder orders materials (windows, beams, bricks)
- Custom items have 4-8 week lead times
- Party wall agreement (if needed - add 2-6 weeks)
- Builder books in their schedule
Why the wait? Good builders are booked 4-8 weeks ahead. Bi-fold doors, structural beams, and custom windows aren't sitting in a warehouse.
Phase 3: The Build (12-16 weeks)
This is what people mean when they say "three months." But here's what actually happens week by week:
Week 1: Groundworks
- Mark out extension footprint
- Excavate foundations (typically 1m deep)
- Pour concrete foundations
- Building control inspection #1: Foundation inspection before concrete
Weather delay risk: High. Rain stops concrete pours.
Weeks 2-3: Foundations & Floor
- Build up blockwork to ground level
- Lay drains and water pipes
- Install damp proof membrane
- Pour concrete floor slab
- Install underfloor heating (if planned)
Building control inspection #2: Drains before covering
Weeks 4-6: Walls Go Up
- Build brick/block walls to full height
- Install lintels above window openings
- Cut opening from house into extension (the big moment!)
- Install steel beam across opening
Delay risk: Wrong beam size delivered, bricks on backorder
Weeks 7-8: Roof Structure
- Install roof joists/trusses
- Lay roof boarding
- Install insulation
- Weatherproof with felt and battens
- Tile or install flat roof system
Building control inspection #3: Before roof covering
Weeks 9-10: Windows, Plastering Prep
- Install windows and doors (bi-folds, sliding doors)
- Weatherproof building
- First fix electrics (cables in walls)
- First fix plumbing (pipes for radiators)
- Install insulation in walls and ceiling
Week 11: Plastering
- Plasterboard walls and ceiling
- Skim coat plaster
- Drying time (minimum 2-3 weeks before painting)
Common mistake: Rushing to paint before plaster fully dries = cracking and peeling
Weeks 12-14: Second Fix
- Install kitchen units
- Fit sockets, switches, lights
- Install radiators
- Fit skirting boards and architraves
- Hang internal doors
Weeks 15-16: Finishing
- Paint walls and ceilings
- Lay flooring
- Install worktops
- Fit bathroom/WC (if included)
- External works (patio, step, drainage)
Building control inspection #4: Final completion certificate
Why Projects Overrun: The Real Delays
1. Weather (Add 1-3 weeks)
You can't pour concrete foundations in heavy rain. You can't lay bricks in frost. November-February builds take longer.
2. Material Delays (Add 2-6 weeks)
Your bi-fold doors were supposed to arrive week 8. The supplier has a 4-week backlog. Everything stops. This is the most common delay in 2026.
3. Builder Availability (Add 1-4 weeks)
Your builder is finishing another job. It's running behind. Your start date slips from January to February.
4. You Change Your Mind (Add 2-8 weeks)
Week 6: "Actually, can we move that window 500mm to the left?" New window, new lintel, new delivery time. Delay.
5. Hidden Problems (Add 1-4 weeks)
Week 2: "We've hit a main sewer under your patio." Utility companies take weeks to respond. Work stops.
6. Building Control Delays (Add 1-2 weeks)
Inspector can't come for 10 days. Work can't proceed until inspection signed off.
Reality Check: A "12-week build" finishing in 12 weeks is rare. 14-18 weeks is normal when you factor in delays.
Different Extension Types: Timeline Comparison
Single-Storey Rear Extension (Most Common)
- Planning: 8-13 weeks (if required)
- Build: 12-16 weeks
- Total: 5-7 months
Double-Storey Extension
- Planning: 8-13 weeks
- Build: 16-24 weeks (scaffolding, more complex structure)
- Total: 7-10 months
Side Return Extension (London)
- Planning: Not usually required (permitted development)
- Build: 8-12 weeks (smaller footprint)
- Total: 3-4 months
Wrap-Around Extension (Side + Rear)
- Planning: 10-14 weeks
- Build: 16-22 weeks
- Total: 8-11 months
Loft Conversion
- Planning: Often not required
- Build: 8-12 weeks
- Total: 3-4 months
How to Avoid Delays
1. Lock Down Design Before Starting
Every design change mid-build adds 1-3 weeks. Finalize everything before breaking ground.
2. Order Long-Lead Items Early
Windows, doors, structural beams, kitchen units. Order them the day you sign with your builder, even if installation is weeks away.
3. Book Building Control Upfront
Schedule all inspection dates when the project starts. Don't wait until the day before you need them.
4. Have a Contingent Builder
If your builder gets sick or walks off, you're stuck. Know who you'd call next.
5. Start in Spring/Summer
May-September builds run smoother. Less rain, longer days, fewer weather delays.
Track Your Extension Timeline
Stay on top of your build with daily progress tracking, task lists, and milestone management. Ted helps you document delays, track payments, and keep your builder accountable.
What Slows Everything Down: The Truth
Materials. This is the killer in 2026. Bricks, timber, windows—everything has lead times. Your builder can't magic them into existence.
You. Every time you ask "can we just..." mid-build, the clock resets. Decisions made in week 10 that should have been made in week 0.
Other trades. Your builder can't plaster until the electrician finishes first fix. The electrician is finishing another job. You wait.
Final Timeline Reality Check
If someone tells you their extension took exactly the time quoted, one of three things is true:
- They got exceptionally lucky
- They're rounding up (16 weeks becomes "four months")
- They're lying to make themselves look better
A more honest conversation: "The builder said 12 weeks. It took 16. Not their fault—windows were delayed and we had two weeks of rain. Finished in February, started in September."
Expect the realistic timeline. Plan for delays. Don't book your kitchen renovation to finish the week before Christmas.
Track Your Build for Free
Keep a complete record of every payment, task, and conversation with your builder. Ted is free to download on iOS and Android.
