A straightforward single-storey rear extension typically runs 10 to 16 weeks from breaking ground to final snagging, though the exact figure depends on size, weather, and how much of the existing house is affected.
Groundworks and foundations usually take one to two weeks, followed by two to three weeks for the structural shell — walls, steel beams and roof. First-fix electrics and plumbing, insulation and plastering typically take another three to four weeks, with the remainder spent on flooring, kitchen fit-out, decoration and snagging.
Planning permission or Permitted Development approval, if needed, sits outside this timeline entirely and is worth starting as early as possible so it doesn’t delay the build once you’re ready to go.
