- The Full Timeline at a Glance
- Stage 1: Design and Planning Preparation (4 to 8 Weeks)
- Stage 2: Planning Permission (8 to 13 Weeks)
- Stage 3: Structural Engineering and Building Regulations (4 to 6 Weeks)
- Stage 4: Procurement and Mobilisation (2 to 4 Weeks)
- Stage 5: The Build Phase (8 to 20 Weeks)
- What Can Delay a London House Extension?
- How to Keep Your Project on Track
- What About Cost?
- FAQs
Planning a house extension is exciting. But it doesn't take long before a very practical question surfaces: how long is this actually going to take?
The honest answer is that it depends — on the type of extension, the complexity of your design, and how smoothly planning and build stages run. What you can do is understand the realistic timeline from the start, so you can plan around it rather than be caught off guard.
This guide walks through every stage from initial design to final handover, with realistic timeframes for London projects in 2026.
The Full Timeline at a Glance
A typical London house extension takes anywhere from 4 to 12 months from appointing a contractor to walking into your finished space. That range is wide because a single-storey rear extension on a straightforward semi-detached property moves very differently from a double-storey side extension on a period terrace in a conservation area.
Here's how the stages typically break down:
| Stage | Typical Duration |
|---|---|
| Design and planning preparation | 4 to 8 weeks |
| Planning permission (if required) | 8 to 13 weeks |
| Structural engineering and building regs | 4 to 6 weeks |
| Procurement and contractor mobilisation | 2 to 4 weeks |
| Build phase | 8 to 20 weeks |
| Snagging and final sign-off | 1 to 2 weeks |
These stages often overlap. A good design and build team will run structural engineering work in parallel with the planning application, for example, rather than treating each stage as a sequential queue.
Stage 1: Design and Planning Preparation (4 to 8 Weeks)
Before any application is submitted, your design needs to be drawn up, refined, and agreed. This covers architectural drawings, structural considerations, and the specification of materials and finishes.
In London, this stage often takes longer than homeowners expect. Properties here tend to be period terraces, Victorian semis, or flats with complex party wall situations. Getting the design right before you submit saves time — and headaches — later.
When your contractor handles design alongside build under one roof, this stage moves faster. You're not waiting for a separate architect to coordinate with a separate builder. The design is developed with buildability in mind from day one.
Stage 2: Planning Permission (8 to 13 Weeks)
Not every extension requires full planning permission. Many single-storey rear extensions fall under Permitted Development rights, meaning no formal application is needed — though a Lawful Development Certificate is still worth obtaining for peace of mind when you come to sell.
Where planning permission is required, the statutory determination period is 8 weeks for straightforward applications and 13 weeks for more complex ones. London boroughs vary in how efficiently they process applications. Some are reliably within the statutory window; others run close to or slightly beyond it.
Conservation areas, listed buildings, and Article 4 directions add another layer of complexity. If your property sits within a conservation area, expect the design stage to take longer too, as drawings need to reflect local character guidelines carefully.
One thing worth knowing: the planning clock starts from the date your application is validated, not the date you submit. Incomplete applications get returned, which costs you weeks. Getting the submission right first time matters.
Stage 3: Structural Engineering and Building Regulations (4 to 6 Weeks)
Building regulations approval runs separately from planning permission and covers structural safety, fire safety, insulation, drainage, and more. A Full Plans application submitted before work starts is the approach most contractors recommend for extensions of any real complexity.
Your structural engineer will produce calculations and drawings to accompany the building regs submission. Run this work in parallel with the planning application and you save several weeks on the overall programme.
Building regulations approval typically takes 4 to 6 weeks, though this can extend if the inspector requests additional information.
Stage 4: Procurement and Mobilisation (2 to 4 Weeks)
Once permissions are in place, materials need to be ordered and the site set up. For a well-organised contractor, this is a 2 to 4 week stage. For one juggling multiple projects without a clear programme, it can drift.
This is where having a single team managing your project from concept to completion makes a real difference. There's no handover between a design firm and a separate builder. The same team that designed your extension knows exactly what's been specified and can move quickly.
Stage 5: The Build Phase (8 to 20 Weeks)
This is the stage most people picture when they think about how long an extension takes. The range is wide because the build itself varies enormously by size and complexity.
Single-Storey Rear Extension
A straightforward single-storey rear extension typically takes 8 to 12 weeks on site. This covers foundations, structural frame, roof, external envelope, windows and doors, first and second fix, plastering, and finishes.
Double-Storey Extension
A double-storey extension adds structural complexity, more materials, and more trades to coordinate. Expect 12 to 16 weeks on site as a realistic baseline, with more complex projects running to 20 weeks.
Kitchen Extension
A kitchen extension combines the structural build with a full kitchen fit-out — cabinetry, worktops, appliances, and often bespoke joinery. The build phase typically runs 10 to 14 weeks, depending on the level of finish.
Side Return Extension
Side return extensions are common in London terraces and involve working in a narrow, often awkward space. They tend to take 10 to 14 weeks on site. Party wall matters can add complexity where a neighbour's wall is involved.
What Can Delay a London House Extension?
Understanding the risks is just as useful as knowing the typical timeline. These are the most common causes of delay on London extension projects.
Planning complications. Conservation area requirements, neighbour objections, or an incomplete application can add weeks or months to the planning stage.
Party wall matters. If your extension affects a shared wall, you'll need a Party Wall Agreement with your neighbour. If they appoint their own surveyor and the process becomes contested, this can take 2 to 3 months. Starting early is essential.
Structural surprises. Older London properties sometimes reveal unexpected conditions once work begins — poor existing foundations, hidden drainage runs, or substandard previous work. A thorough pre-build survey reduces this risk, though it can't eliminate it entirely.
Material lead times. Bespoke windows, structural steel, and specialist roofing materials can carry lead times of 6 to 12 weeks. Ordering during the planning or building regs stage — rather than waiting for the build to start — keeps the programme on track.
Contractor availability. An overcommitted contractor won't give your project the attention it needs. Monthly progress reports with budget tracking and on-site photography are one way to stay informed and hold your contractor accountable throughout the build.
How to Keep Your Project on Track
There are practical steps you can take as a homeowner to protect your timeline.
Start the Party Wall process as soon as you have a design. Don't wait for planning approval.
Choose a contractor who manages design, planning, and build under one roof. Coordination between separate firms is one of the biggest sources of delay and miscommunication on extension projects.
Get a clear programme at the start. A good contractor will give you a week-by-week schedule and update it as the project progresses.
Ask how you'll be kept informed. Monthly reports with budget tracking and on-site photography are a reasonable expectation on any project of this size. If a contractor can't tell you how they communicate progress, that's worth noting before you sign anything.
What About Cost?
Timeline and budget are closely linked. A project that runs over time almost always runs over budget too — extended site time means extended preliminaries, more supervision, and sometimes abortive work.
If you want a ballpark figure before committing to a consultation, the instant cost estimator at MVV gives you a starting point in minutes, with no obligation. It's a useful way to sense-check your budget before you start talking to contractors.
FAQs
How long does a single-storey house extension take in London?
From appointing a contractor to completion, a single-storey rear extension typically takes 4 to 6 months, covering design, planning (if required), building regulations, and the build itself. The on-site build phase alone is usually 8 to 12 weeks.
Do I need planning permission for a house extension in London?
Not always. Many single-storey rear extensions fall under Permitted Development rights and don't require a formal application. However, properties in conservation areas, listed buildings, or those subject to Article 4 directions may need full planning permission. A Lawful Development Certificate is worth obtaining even for Permitted Development projects.
What is the longest part of a house extension project?
The planning permission stage is often the longest single stage, taking 8 to 13 weeks once an application is validated. For extensions that don't require planning permission, the build phase itself is typically the longest element.
Can design and build overlap with planning permission?
Yes — and it should. A well-run project will progress structural engineering and building regulations work in parallel with the planning application, saving several weeks on the overall programme.
What causes the most delays on London extension projects?
The most common causes are planning complications, Party Wall Agreement disputes, structural surprises uncovered during the build, and long lead times on bespoke materials. Starting the Party Wall process early and ordering long-lead materials during the planning stage both help significantly.
How do I know if my contractor is keeping to the programme?
Ask for a written project programme at the start and request monthly progress updates with budget tracking and on-site photography. These give you a clear picture of where the project stands and flag any drift before it becomes a serious problem.
Does a double-storey extension take significantly longer than a single-storey one?
Yes. A double-storey extension typically adds 4 to 8 weeks to the on-site build phase compared with a single-storey extension, due to the additional structural work, materials, and trades involved. The planning and building regulations stages are broadly similar in duration.
A well-planned extension in London is entirely achievable within a clear, predictable timeline. The key is choosing a team that manages the whole process, keeps you informed at every stage, and treats your budget with the same care as your design.
If you're at the early stages of planning your extension, get a free instant estimate at themvv.co.uk to see what your project might cost before your first conversation with a contractor.