Adding an extension is one of the best ways to gain space and add value to your existing home.
But how long does it take to build an extension?
This guide walks you through the extension timeline in detail, outlining what to expect week by week from design to finish.
Whether it’s a single-storey extension, a kitchen extension, or a full two-storey extension, we’ll show you what happens and when.
Weeks 1–3: Design and Planning
Engage an Architect
Start by hiring a professional architect to develop your extension plans.
They’ll help you shape the design process, provide layout options, and prepare for permissions.
A measured survey of your existing house is usually the first step.
Prepping for Planning Permission
Most extensions require approval.
Your architect or structural engineer will guide you through submitting a planning application.
You may also need a party wall agreement if you’re in a conservation area or building on a boundary.
Expect this stage to take 2–4 weeks minimum.
Weeks 4–8: Permissions & Regulations
Submit the Planning Application
Allow at least 8 weeks for your local council to respond.
Building Regulations
You’ll also need to obtain approval for the building regulations.
A building control officer will check that your extension meets safety, energy, and construction standards.
You may need to revise drawings or include extra details.
Weeks 9–10: Tendering & Finding a Builder
Once approvals are in, it’s time to:
- Finalise specs
- Send to contractors
- Get competitive quotes
Take time to find the right builder. A good one is often busy, so allow for lead-in time.
Use this period to finalise your building contract and schedule the start of your extension project.
Weeks 11–12: Site Setup
- Erect safety barriers
- Order materials
- Set delivery schedules
Your builder will liaise with building control and coordinate the trades.
Site setup is usually done in a week or two.
Weeks 13–20: Groundworks & Foundations
This is where building work begins:
- Dig footings
- Pour a concrete base
- Build the slab
This part can take 2–4 weeks, depending on weather, soil, and the extension type.
Weeks 21–28: Build the Shell
- Construct walls and a structural frame
- Install windows and roof
- Make your extension watertight
At this point, you’ll see your extension take shape.
If you’re adding a two-storey extension, expect framing to take longer.
Weeks 29–36: Internal Works Begin
Now your house extension moves inside:
- First, fix the electrics and plumbing
- Install fixtures and fittings
- Apply insulation
- Plumb new bathrooms or kitchen utilities
Every extension requires these core services spanning 4–6 weeks.
Week 37–42: Finishes and Decoration
- Plastering and painting
- Flooring and tiling
- Final carpentry
Your extension timeline here depends on complexity and finish quality.
A single-storey extension will usually be faster than multi-level builds.
Weeks 43–44: Snagging & Final Checks
You’re almost there.
This stage includes:
- Final touches
- Addressing snag issues
- Building inspector sign-off
Once complete, you’ll receive final approval and handover.
Total Timeline: 6–10 Months
Depending on the size and complexity of the build, most house extension projects take 4–6 months from start to finish.
However, some can take 8–16 weeks longer due to:
- Weather delays
- Changes in design
- Planning or contractor hold-ups
Be realistic. A complex extension or a two-storey extension may take longer.
Key Tips to Keep Your Extension On Time
- Hire a good builder early
- Follow your step-by-step guide and timeline
- Account for delays and weather
- Make decisions quickly on materials and finishes
The more organised your extension planner, the smoother your extension from start to finish.
Common Delays to Avoid
- Late planning responses
- Waiting on party wall agreements
- Changes after work starts
- Late deliveries
These can all significantly impact the timeline.
Having a project manager or an experienced builder keeps things on track.
