How long does a house extension take?
Fourteen to twenty weeks on site for a single-storey rear or side-return extension. Add eight to twelve weeks beforehand for design, structural calculations and planning, plus two months for party wall notices — so nine to twelve months from first conversation to finished room.
On site, a typical sequence runs three to four weeks for groundwork and foundations, two weeks for the structural opening and steels, four to five weeks for brickwork and roof, three weeks for glazing and making watertight, then four to six weeks of services, plastering and finishing. Weather affects the groundwork stage more than any other.
The pre-construction phase is where projects actually slip. Design and structural calculations take four to six weeks. A planning decision takes eight to thirteen weeks; a lawful development certificate around eight. Party wall notices need two months to run. Started in parallel at the outset these overlap almost entirely; started sequentially they add half a year.
The disruption is more concentrated than a whole-house renovation but more severe while it lasts. The rear of the house is open to the elements for part of the build, and if the extension contains the kitchen you will be without one for most of the programme. Most clients stay in the house and set up a temporary kitchen elsewhere; we build that into the programme when asked at quoting stage.
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