One of the first questions homeowners ask when they start planning a house extension is how long it will take. It’s a reasonable question — and one that deserves a straight answer rather than the vague “it depends” that’s so common in the construction industry. The honest answer is that a typical single storey rear extension takes 6 to 9 months from the first conversation to handover, when you include the design and planning stage. Here’s exactly what happens at each stage and how long each one takes.
The Full Extension Timeline at a Glance
| Stage | Typical Duration |
|---|---|
| Initial consultation and brief | 1–2 weeks |
| Architect design and drawings | 4–8 weeks |
| Planning application (if required) | 8–13 weeks |
| Building regulations drawings | 2–4 weeks |
| Contractor quotes and contract | 2–4 weeks |
| Build — single storey rear | 10–16 weeks |
| Build — double storey or wraparound | 16–24 weeks |
| Snagging and sign-off | 1–2 weeks |
For a permitted development single-storey rear extension, you can skip the planning application stage — cutting 2–3 months from the total programme. For projects needing full planning in a South London conservation area, the planning stage alone can take 3–4 months.
Stage 1 — Initial Consultation (1–2 weeks)
The process starts with a site visit. A good builder or architect will come to your property, assess the space, discuss what you want to achieve, and give you an honest read on what’s feasible — including the planning position for your specific address.
This stage is fast and, with most reputable builders, free. The output is a clear brief: what you want to build, what planning route applies, a ballpark budget, and a realistic sense of the programme.
Don’t skip this step or rush it. The brief you agree at this stage shapes everything that follows. Changing your mind halfway through the design process costs time and money.
Stage 2 — Design and Drawings (4–8 weeks)
Once the brief is agreed, your architect produces the design drawings. This stage takes 4 to 8 weeks depending on the complexity of the project and how quickly you’re able to review and approve the designs.
For a straightforward permitted development rear extension, the drawings don’t need to be elaborate — building control drawings that show the structure and specification are sufficient. For a full planning application, you’ll need planning drawings, elevations, and often a design and access statement.
Be responsive during this stage. Architects typically work on multiple projects simultaneously, and slow client feedback is one of the most common causes of programme slippage in the early stages.
Stage 3 — Planning Application (8–13 weeks, if required)
If your extension requires full planning permission — double storey, conservation area, or larger than permitted development limits — this is the longest single stage of the process.
The council has 8 weeks from the date of validation to determine a householder planning application. Validation typically takes 1–2 weeks from submission. Some applications are determined faster; others — particularly those involving conservation areas, objections from neighbours, or requests for additional information — take longer.
In South London:
- Bromley Council typically processes household applications within 8 weeks of validation
- Royal Borough of Greenwich may take longer for applications near the World Heritage Site or in conservation areas
- London Borough of Lewisham processes most applications within 8 weeks
Factor in 10–13 weeks from submission to decision as a realistic planning programme. Build this into your programme from the start — treating it as a surprise delay is unnecessary and avoidable.
If your extension is permitted development, you skip this stage entirely. Some homeowners also apply for a Lawful Development Certificate even for PD projects — this takes around 8 weeks but provides documented proof of lawfulness for future sale.
Stage 4 — Building Regulations and Structural Engineer (2–4 weeks)
Building regulations approval is separate from planning and required for all extensions regardless of whether planning permission was needed. Your architect or structural engineer produces detailed drawings showing the structure, insulation, drainage, and means of escape.
You submit these to building control before work starts. Building control inspectors visit the site at key stages during the build — foundations, structural frame, insulation, and before plastering. The final inspection triggers the completion certificate.
This stage typically runs in parallel with the planning application or the contractor procurement stage — it doesn’t usually add time to the overall programme if managed properly.
Stage 5 — Contractor Quotes and Contract (2–4 weeks)
Once planning is approved and drawings are finalised, you get detailed quotes from builders and appoint a contractor. A reputable builder will want to properly price the drawings before committing to a figure — allow 2 to 4 weeks for this stage.
Read the contract carefully before signing. A good building contract sets out the scope of work, the programme, the payment schedule, and — critically — how variations are handled. Any change to the agreed scope during the build should be priced in writing before work proceeds. Verbal agreements on variations are a common source of disputes.
Stage 6 — The Build
This is the stage most people think of when they imagine an extension — and it’s often the fastest part of the overall programme.
Single Storey Rear Extension Build Time
A typical single storey rear extension takes 10 to 16 weeks from start on site to practical completion. The programme typically runs:
- Weeks 1–3: Groundworks, foundations, drainage
- Weeks 3–6: Superstructure (walls, steel work, structural frame)
- Weeks 6–8: Roof — flat or pitched, weathertight
- Weeks 8–10: First fix (electrics, plumbing, windows and doors)
- Weeks 10–13: Plastering and screeding
- Weeks 13–16: Second fix, decoration, flooring, final fit-out
Double Storey Extension Build Time
A double storey extension or wraparound takes 16 to 24 weeks from start on site. The additional time reflects the increased structural complexity, more extensive scaffolding requirements, and greater volume of work on each trade.
What Can Slow the Build Down?
The most common causes of delay during the build are:
- Weather — groundworks and roofing are weather-dependent
- Material lead times — specialist glazing, steelwork, or particular brick types can have long lead times
- Subcontractor availability — gaps between trades are a common source of programme slippage on poorly managed projects
- Unforeseen ground conditions — buried rubble, unexpected soil types, or services in the ground can require remedial work
A well-managed builder anticipates these risks and builds programme contingency in from the start. Ask any contractor how they handle delays — the answer tells you a lot about how they work.
Stage 7 — Snagging and Sign-Off (1–2 weeks)
Practical completion is when the extension is finished and you can use it — but it’s not quite the end of the process. A snagging walkthrough identifies any minor defects or incomplete items that need attention before final sign-off.
Building control sign-off (the completion certificate) is issued after the final building control inspection. Keep this document — it’s essential evidence at sale that the work was done properly and signed off.
Most reputable builders include a defects liability period after practical completion — typically 6–12 months during which they’ll return to address any issues that emerge. Make sure this is written into your contract before you sign.
Can You Stay in Your Home During the Extension?
In most cases, yes — and most of our clients do. The build is disruptive, particularly during the breakthrough from the existing house into the new extension. We manage this carefully: sealing off dust, protecting flooring and furnishings, and always leaving the site clean and secure at the end of the day.
For larger projects — full wraparounds or double storey extensions — there may be a period of a few days where temporary relocation makes sense. We discuss this honestly at the planning stage so it’s never a surprise.
Get a Realistic Programme for Your South London Extension
Every project is different — your timeline depends on whether you need planning permission, the size and complexity of the build, and when on site your chosen builder can start.
We give every client a realistic programme in their quote and stick to it. We cover South London including Bromley, Greenwich, Lewisham, Eltham, and Beckenham. Get in touch for a free site consultation. Planning your budget too? See our guide to house extension costs in South London.