4 September 2026
What a rear extension involves, week by week
From the first dig to the day the doors go in, most single-storey extensions follow the same rhythm. Here is what to expect and when.
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A rear extension looks like one big job. On site it is a run of smaller ones, each of which has to be finished and checked before the next can start.
Weeks one and two: groundwork
We mark out, dig the foundations and wait for the building inspector to approve them before any concrete goes in. This is the noisiest and muddiest part, and the garden will look worse before it looks better.
- Foundations dug, inspected and poured.
- Drains moved or extended where the extension sits on them.
- Floor slab and damp-proof membrane laid.
Weeks three to six: walls and roof
Blockwork goes up, steel beams go in over the new openings, and the roof follows. Once the roof is on, the shell is weathertight and work can continue whatever the weather.
Weeks seven to ten: inside
Windows and doors, first-fix wiring and plumbing, plastering, then second fix and decorating. Only at the very end do we open up into the existing house, so you live with the dust for days, not months.
What slows it down
Rain during groundwork, a late inspection and changes made after the order for windows has gone in. We tell you on day one which decisions have deadlines.