How Long Does a House Extension or Loft Conversion Actually Take?

Ask a builder how long your extension will take and you'll usually get an answer for the build alone. Ask how long the whole project will take, from the first sketch to actually cooking dinner in the new kitchen, and the honest answer is almost always longer than homeowners expect.

Design and planning: 2–4 months

Concept design, developing it into planning drawings, and submitting typically takes four to eight weeks before anything even reaches the local authority. A householder planning application then has an eight-week statutory determination period (thirteen weeks for anything requiring committee input), though it's common for a first submission to come back with a request for amendments, adding several more weeks. Permitted development still needs a lawful development certificate in most cases if you want mortgage-lender or future-buyer confidence, which follows a similar timeline.

Party wall and building control: overlaps, but adds time

Party wall notices need a minimum two-month notice period before work on a shared wall can start (one month for excavation-only work), and this can run in parallel with the tail end of planning rather than after it — but only if notices go out as soon as the design is fixed. Building regulations approval (full plans or building notice) is a separate process again, usually handled alongside the party wall stage.

Tendering and appointing a builder: 4–8 weeks

Getting three comparable quotes, having them properly reviewed, negotiating scope and signing a contract realistically takes a month to two months if it's done properly rather than rushed. This is also where a two-week gap in a builder's availability can shift your whole start date by months if their diary fills up while you're still deciding.

The build itself

• Single-storey rear extension (15–30m²): 16–18 weeks
• Wraparound extension: 20–26 weeks
• Rooflight/Velux loft conversion: 4–6 weeks
• Dormer loft conversion: 10–12 weeks
• Hip-to-gable or mansard loft conversion: 8–12 weeks

These are build-only windows for straightforward projects with no major undiscovered conditions — add contingency time for anything found once walls or roofs are opened up, which is common in period property.

These are deliberately more honest than the twelve-to-thirteen weeks contractors often quote for a kitchen extension. I have set out exactly where the difference goes in why a realistic kitchen extension build is 17–18 weeks, not 12.

Snagging and completion: 2–6 weeks

The last 5% of a job — final finishes, fixing minor defects, commissioning services — routinely takes longer than either side expects, and is the stage most likely to drift without someone tracking it deliberately.

The realistic total

Put together, a straightforward rear extension is commonly 10–15 months from first design conversation to finished, snagged completion — even though the build itself might only be four months of that. A loft conversion under permitted development can be considerably quicker, sometimes 5–7 months end to end, since it skips the longer planning route.

Where projects actually lose time

The recurring pattern isn't dramatic delay — it's small gaps adding up: a decision on kitchen layout left two weeks too late, a party wall notice sent after the design changed and needing to be reissued, or a builder's start date slipping because tendering ran on longer than planned. Building a realistic programme at the outset, and someone keeping an eye on which stage is actually on the critical path, is usually the difference between a project landing close to its original timeline and one that drifts by months without anyone quite noticing why.