How Long Does a House Extension Take? A Realistic Timeline (2026)
The short answer
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From first conversation to finished room, a house extension in Cheshire or North Wales usually takes 6–12 months. The build itself is the shorter part - around 10–16 weeks on site for a single-storey extension, or 20–26 weeks for a double-storey. Most of the timeline is the before: design, planning permission and technical drawings, which together take 3–6 months before a single brick is laid.
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Almost everyone underestimates the front half and overestimates the build. Here's the honest breakdown, stage by stage.
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The full timeline, stage by stage
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1. Design and brief - 4 to 8 weeks Site visit, measured survey, working out what you actually want, and developing the design. The pace here is mostly set by how quickly decisions get made — yours as much as the designer's.
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2. Planning permission - 8 to 12 weeks If you need it. The council has a statutory 8 weeks to decide a householder application, though it can run longer. If your project is permitted development, you can skip most of this — which is why PD projects start on site months sooner.
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3. Technical drawings and Building Regs - 4 to 6 weeks The detailed, buildable drawings, structural engineer's calculations, and Building Regulations package. This runs partly alongside planning to save time.
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4. Finding and pricing a builder - 4 to 8 weeks Getting quotes, comparing them properly, and — the part people forget — waiting for a good builder to be free. The best ones are often booked 2–3 months ahead.
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5. The build - 10 to 26 weeks
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Single-storey rear extension: 10–16 weeks
Double-storey extension: 20–26 weeks
Add time for kitchens, complex glazing, or bad weather
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Typical totals by project type
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Small single-storey extension (permitted development): 4–7 months total
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Standard single-storey rear (needs planning): 6–9 months total
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Double-storey side or rear extension: 9–14 months total
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The single biggest variable is whether you need planning permission. A permitted-development project can be on site in 6–10 weeks from instruction; one needing full planning is more like 4–6 months before the build even starts.
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What slows a project down (and what you can control)
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Adds time:
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Indecision. The most common delay by far. Every decision left open - tiles, doors, kitchen layout - stalls the whole chain. Decisions made calmly on paper are quick; the same decisions made mid-build are slow and expensive.
Planning conditions or objections. A neighbour objection or an awkward condition can add weeks.
Long-lead items. Bespoke windows, bi-folds and roof lanterns can take 8–12 weeks to manufacture. Order them too late and the build waits.
Waiting for the right builder. Good ones are worth waiting for, but factor the wait in.
Unknowns below ground. Drains, old foundations and poor soil can surface once you dig.
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Saves time:
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Decide everything before you start on site. The single most powerful thing you can do.
Run stages in parallel. Technical drawings can progress while planning is being decided.
Order long-lead items early, as soon as the design is fixed.
A complete, detailed drawing set - so the builder isn't stopping to ask questions.
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Why "starting sooner" often finishes later
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There's a tempting shortcut: skip the detailed design, get a builder in fast, and "figure it out as we go".
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It almost always finishes later. Every unresolved detail becomes a pause on site while someone decides, sources a material, or reworks something already built. The project that starts a month later with a complete set of drawings routinely overtakes the one that rushed on site with half a plan - and it's usually cheaper and less stressful too.
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Time spent getting the design right isn't a delay. It's what makes the build fast.
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A realistic plan: working back from when you want it done
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Want the new kitchen ready for next Christmas? Work backwards:
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Christmas - finished
3–4 months earlier - build starts (single-storey)
1–2 months before that - builder appointed, drawings complete
2–3 months before that - planning submitted (if needed)
1–2 months before that - design begins
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That puts the first conversation around 8–10 months ahead for a project needing planning. Start the conversation early - it's free, and it's the cheapest month in the whole project.
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Frequently asked questions
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How long does a single-storey extension take to build?
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The on-site build for a single-storey rear extension is typically 10–16 weeks. The whole project, including design, planning and technical drawings, usually runs 6–9 months from first conversation to finished room.
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How long does a double-storey extension take?
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Around 20–26 weeks on site, and 9–14 months in total once design, planning and drawings are included. The larger structure, roof and scaffolding all add build time.
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Can I speed up a house extension?
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Yes - mostly by deciding everything before you start on site, running technical drawings alongside planning, and ordering long-lead items like glazing early. Most delays come from decisions left open, not from the building work itself.
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How long does planning permission take?
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A householder planning application in England has a statutory 8-week decision period, though it can take longer. If your extension is permitted development, you can skip this stage and start on site months sooner.
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When should I start planning if I want it finished by a certain date?
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For a project needing planning permission, allow 8–10 months from first conversation to completion. Working backwards from your target date is the best way to set a realistic start.
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Thinking about an extension in Cheshire or North Wales?
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Studio 9 Architecture is an architectural design studio based in Chester, working across Cheshire, North Wales and Lancashire. The earlier we talk, the more of the timeline works in your favour - we'll map out a realistic schedule for your project before you commit to anything.
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