How Long Does a Single-Storey Extension Take?

Single-storey extension timelines are often quoted as just the build time — but the real journey from deciding to extend to actually using the new space includes planning, design, and often a wait for materials, all before the visible construction work even starts.

A realistic timeline, start to finish

• Design and planning application (if needed): design typically takes a few weeks; a planning decision can take up to 8 weeks once submitted. Permitted Development projects skip this stage — see our guide on Permitted Development rights for extensions to check whether yours qualifies.
• Building control and final drawings: a few weeks, often running in parallel with securing quotes.
• Groundwork and foundations: 1–3 weeks, weather-dependent.
• Structural walls and roof: 3–5 weeks.
• Windows, doors and weatherproofing (getting the extension watertight): 1–2 weeks.
• First-fix plumbing and electrical: 1–2 weeks.
• Plastering and drying time: plastered walls typically need 1–2 weeks to dry properly before decoration.
• Second-fix, flooring and finishing: 2–4 weeks.

Put together, the on-site build for a straightforward single-storey extension is typically 10–16 weeks, with the planning and design stages adding several weeks to months before that, depending on whether planning permission is required.

What commonly extends the timeline

• Planning permission delays — a decision can take longer than the standard 8 weeks if the application needs revisions or neighbour consultation issues arise.
• Party wall notice periods — if a Party Wall Act notice is required, your neighbour has up to 14 days to respond, and the notice itself must typically be served one to two months before work starts on or near a shared wall. See our Party Wall Agreements guide — leaving this until the last minute is one of the more common, avoidable causes of a delayed start date.
• Weather — groundwork and roofing are both weather-dependent stages; a wet spell can push the programme back regardless of labour availability. Starting groundwork in late autumn or winter carries genuinely more schedule risk than starting in spring or summer.
• Ground conditions found once digging starts — unexpected drainage, tree roots, or poor soil can require design changes mid-build.
• Material lead times — bespoke windows, doors, or roofing materials can have lead times of several weeks that need to be factored in early.
• Living in the property during the build — coordinating access around a household's routine can add time compared with an empty property, particularly at the connection stage between old and new structure.

Preparing before the start date

A few practical steps reduce delay once the crew is on site: clear the garden or area the extension will occupy, agree parking and material-storage arrangements with the contractor in advance, confirm services (gas, electric, water) are correctly marked if groundwork is near existing runs, and — if living in the property throughout — agree which rooms will be affected and when, particularly around the connection stage between old and new structure.

Questions worth asking about the programme

• Is the quoted timeline the on-site build only, or does it include design and planning stages?
• What happens to the programme if the planning decision or party wall notice period runs long?
• How is weather-related delay handled — is it built into the estimate, or treated as an unpredictable extra?
• What's the plan for keeping the property weatherproof and secure between stages, particularly once the roof is off the connecting wall?

Budgeting around the timeline

See our full single-storey extension cost guide and labour costs guide for the budgeting side of the same project, and our common extension mistakes guide for how poor planning at this stage often causes both cost and time overruns together.