How long does a loft conversion take?
Ten to fourteen weeks on site for a rear dormer, or fourteen to twenty for a mansard. Before that, allow eight to twelve weeks for design, structural calculations and either a planning decision or a lawful development certificate, plus two months for party wall notices.
A realistic on-site sequence for a rear dormer is two weeks of scaffolding, strip-out and structural steels, three weeks forming the dormer and making the roof watertight, two weeks on the staircase and first fix, three weeks of plastering and drying, then three weeks of second fix, decoration and the en-suite. Twelve weeks is typical; anyone promising six has left something out.
The pre-construction period is usually longer than clients expect and it is where the programme is really decided. Design and structural calculations take four to six weeks. A lawful development certificate takes around eight weeks; a full planning application takes eight to thirteen. Party wall notices need two months. Run in parallel and started early, these overlap; run sequentially, they add six months.
The good news is that a loft conversion is the least disruptive major project to live through. Most of the work happens above your ceiling, accessed by external scaffolding, and the house stays weathertight throughout. The disruptive period is short — mainly when the staircase is formed and the ceiling below is opened up, usually about a week.
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