How Long Does an Extension Take? The Honest Timeline, Stage by Stage

"How long will it take?" is usually the second question we're asked, right after "what will it cost?" — and it deserves the same honest treatment. The short answer for a typical single-storey extension: around 10 to 12 weeks on site — but six months or more door to door, once design, planning and approvals are counted.

That gap between "weeks on site" and "months door to door" is where most timeline disappointment lives. So here's the whole journey, stage by stage, with the numbers we actually plan around.

Stage one: design — 2 to 3 weeks (plus thinking time)

Measured survey, layouts, elevations: allow 2–3 weeks to prepare drawings, plus revision rounds while you decide what you actually want. Don't rush this part to "save time" — every decision deferred now resurfaces later as a variation, and variations cost more than decisions. This is also when the approvals strategy gets set: permitted development or planning application, and what building regulations will need.

Stage two: approvals — 6 to 12 weeks, mostly waiting

The statutory clocks tick at their own speed, and no builder can hurry them:

• Householder planning application: 8 weeks statutory target from validation — allow 10–12 weeks end to end including drawings and validation, longer when the department is busy. (Everyone wants to build in summer; planning departments know this.)
• Permitted development, larger-home scheme: a 42-day prior approval neighbour consultation.
• Building regulations, Full Plans route: approval in roughly 4–5 weeks — and worth doing properly, because it surfaces the technical risks before anyone prices or builds them.
• Party wall notices (attached houses): allow one or two months, depending on whether the work needs a boundary/excavation notice or a party-structure notice — these periods run happily in parallel with the above if you serve notices early. Our party wall guide covers the process.

Overlap what you can, but the rule of thumb stands: if you haven't allowed at least three months between "let's do it" and "start on site", you're under pressure from day one. And pressure means fast-tracking, which always costs — in money or quality.

Stage three: pricing and mobilisation — don't forget the queue

With drawings and specification complete, tendering takes a few weeks — builders need time to price properly, and a rushed quote is a wrong quote. Then the queue: good builders are booked ahead, and lead times on glazing, steels and kitchens need ordering into the programme. This stage is invisible in most people's mental timeline, and it's why the door-to-door number surprises them.

Stage four: the build — 10 to 12 weeks for a typical extension

Once on site, a typical single-storey extension means builders in your home for around 10 to 12 weeks. Larger and more complex projects — two-storey additions, wrap-arounds, structural reworking of the existing house — run 16 weeks or more. The broad rhythm:

1. Groundworks and foundations — weather-exposed and surprise-prone; slopes and poor ground add time
2. Shell — walls, roof, weathertight
3. First fix — structure opened to the house, services routed
4. Second fix and finishes — plastering, kitchen or joinery, decorating: the stage that looks nearly done for weeks, because finishing well is slow

Timeline by project type

The same stages apply to every project; what changes is how long each one runs. Broad-brush expectations, assuming a well-planned job:

• Single-storey rear or side-return extension. The benchmark case above: 2–3 weeks of drawings, the approvals window (often shorter if permitted development applies), then 10–12 weeks on site. Door to door, roughly 6–9 months.
• Two-storey extension. More structure, more scaffold, and almost always a full planning application rather than permitted development — so a longer approvals stage and a longer build. Think in terms of the 16-weeks-plus end of the on-site range, and add the extra weatherproofing care of working at roof level.
• Wrap-around and open-plan remodels. The build isn't just the new footprint — it's the structural surgery where new meets old: steels, temporary works, and the existing ground floor out of action for part of the programme. Budget time accordingly and expect the second half of the build to feel slower than the first; finishing a big connected space well is unhurried work.
• Loft conversions. A different rhythm: much of the early work happens in the roof and off the scaffold, so the household disruption is more back-loaded and the overall on-site period is often shorter than a ground-floor extension of similar value. Approvals are frequently quicker too, since many lofts fall within permitted development — see our loft planning guide.
• Extension plus whole-house renovation. The programme is no longer "an extension with extras" — it's a refurbishment project that includes an extension, and it should be planned (and priced) as one, with the trades sequenced across the whole house rather than colliding in it.

The pattern worth noticing: the pre-site stages don't scale with project size nearly as much as the build does. Drawings and approvals for a big project take only somewhat longer than for a small one — which means on larger schemes, proportionally more of the total timeline is on site, and the payback from thorough pre-site planning gets bigger, not smaller.

What actually causes overruns

After decades around building projects, the honest list — and note that weather is not at the top:

• Delayed design. Finalising the kitchen, the glazing, the bathroom "as we go" is the classic. Every late decision generates variations, and as we set out in how to plan a home extension, unplanned projects routinely overspend by 25% — the time overrun tracks the money.
• Scope creep. The new work looks so good you add the cloakroom mid-build. A build programme is a fine-tuned machine of trades and materials in sequence; mid-build changes throw a spanner into it every time.
• Unrealistic start dates. Starting before approvals and design are complete doesn't save the time — it borrows it at interest. Better to start two weeks late than finish three months late: it's not about the start; it's about the completion.
• Genuine surprises. Ground conditions, hidden structure in the existing house. You can't eliminate them — you can hold a contingency of time and money — around 10%, and up to 15% on complex or older properties — so they're absorbed rather than fatal.

Living with it — and the 20% question

Can you shorten the build? One lever genuinely moves the needle: an empty house lets builders work roughly 20% quicker, because nothing they do has to be sequenced around family life. Renting nearby for the messiest weeks costs money but buys speed and sanity — worth the sums on bigger projects. If you stay put, a temporary kitchen, separate site access and realistic expectations do most of the work; our guide to avoiding disruption during an extension has the practical checklist.

What you shouldn't do is choose speed for its own sake. A project has three elements — speed, quality and cost — and you only get to prioritise two. Choose speed and you will sacrifice one of the others.

The realistic overall answer

For a typical single-storey extension, door to door: roughly 6 to 9 months — a month or so of design and decisions, two to three months of approvals and pricing running their statutory and practical courses, then 10–12 weeks on site. Larger projects, longer. Could parts compress in a perfect world? Sometimes. Should you plan on the compressed version? Never — a programme with slack in it is a programme that finishes on time.

As a surveyor-led design and build firm, we run design, approvals, specification and construction as one connected process — which is exactly what keeps the stages overlapping efficiently instead of queuing behind each other. Book a free consultation and we'll map a realistic timeline for your actual project, statutory clocks and all — so the only surprises left are the pleasant ones.