How long does an extension really take?
The build itself: 10 to 14 weeks for a typical single-storey rear extension. The honest total from first drawing to finished room is closer to six to nine months once design, planning or prior approval, building regs, party wall and builder lead times join the queue.
The honest timeline
- Design and drawings: 4 to 8 weeks including revisions.
- Planning permission or the prior approval route: about 8 weeks when needed; permitted development skips this queue.
- Building regs drawings, structural engineer, party wall: parallel, weeks not days.
- Builder lead time: good ones book 2 to 4 months ahead.
- The build: 10 to 14 weeks for single-storey rear.
Know your budget before you enter the queue: the rear extension cost guide and the calculator give you the range in minutes, and the budget planner shows when each payment falls due along this timeline, and the snagging checklist guards the final payment at the end.
Sources: national 2026 timeline guides (BestBuilders, Beams and comparable), checked August 2026; the build-week bands match the phase data behind the budget planner.
Frequently asked questions
What takes longer, the build or the paperwork?
Usually the paperwork and the queue: design takes weeks, a planning application eight weeks plus, and good builders book months ahead. The 10 to 14 build weeks are often the most predictable part of the journey.
Can I live in the house during the build?
For a rear extension usually yes, with dust, noise and weeks without a kitchen at the knock-through stage. Many families plan the knock-through around a holiday and set up a temporary kitchen; it is survivable when you see it coming. Think about pets too: dust, noise and open doors are stressful for cats and dogs, so give them a quiet closed-off room or board them for the loudest weeks.
What makes extensions overrun?
Late decisions and surprises in the ground or the existing structure. Fix your choices (glazing, kitchen, finishes) before the build starts and budget 10 to 15 percent contingency; weather delays are real but usually smaller than decision delays.