How long does a loft conversion take? (2026)

A rear dormer loft conversion takes about 6 to 8 weeks on site. A simpler rooflight loft takes 4 to 6 weeks, a hip-to-gable with a rear dormer 8 to 10 weeks, and a mansard 10 to 12 weeks. Those are our guide build times, for the work on site only.

From the first site visit to a finished room, plan for roughly four to six months for a typical rear dormer. The extra time is drawings, approvals and notices, and most of it happens before anyone climbs a scaffold. Here's how it breaks down.

How long each type of loft takes to build

These are the guide build times on our loft conversions page.

• Rooflight (Velux): 4 to 6 weeks on site. The roof keeps its shape, so there's no dormer to build.
• Rear dormer: 6 to 8 weeks on site. The most common loft on a north London terrace.
• Hip-to-gable with rear dormer: 8 to 10 weeks on site. The extra time is the new gable wall, built up from the side of the house.
• Mansard: 10 to 12 weeks on site. The whole back slope is rebuilt, so it's the closest thing to adding a storey.

Not sure which your roof suits? Our guide on rear dormer or hip-to-gable explains the choice.

What happens before the build

The build is the shorter part of the job. These steps come first, and some of them run side by side.

Survey. We visit, measure, check the structure and the headroom, and tell you what's realistic for your house.

Drawings and structural calculations. An architect or designer draws the loft, and a structural engineer designs the floor and the steel beams. This usually takes a few weeks.

Planning, or proof you don't need it. Most rear dormers are permitted development. It's still worth getting a lawful development certificate to prove it. Most councils should decide either that or a householder planning application within eight weeks. A mansard nearly always needs full planning permission. Our guide to loft conversions and permitted development in Barnet and Haringey explains which you're likely to need.

Building Control. Every loft needs Building Regulations approval. A full plans application has to be decided within five weeks, or two months if you agree to extend it. We book and attend every inspection during the build.

Party wall notice. On a terrace or a semi, the new floor beams usually bear into the party wall. That needs two months' notice to your neighbour before the work starts. It's the one that catches people out, so serve it as soon as the drawings show where the beams go. Our guide to party wall notices explains how.

What happens on site

On a rear dormer, the build usually goes in this order. Our Bounds Green case study has photos of every stage.

• Scaffold and a temporary roof, so the house stays dry while the roof is open.
• The new floor. New joists go in across the loft to the engineer's design, with the steel beams where needed.
• The dormer and the roof. The back slope comes off, the dormer is framed and the new roof goes on. After this the house is watertight again.
• Stud walls, pipes and cables, which turn the space into rooms.
• Plasterboard, windows and plastering.
• The staircase. This is when we open into the house below.
• Bathroom, electrics, doors and decorating, then the final Building Control inspection and sign-off.

Our site hours are Monday to Friday, 8am to 5pm, with a team of four to five on site. You get a live build schedule as a Google Sheet, updated daily, so you can see where the job is.

Can you live at home during the build?

In most cases, yes. Most of the work happens from the scaffold, so the house below carries on as normal for most of the job. It gets dustier and noisier when we open up for the staircase towards the end, and that's the part to plan around.

What can make it take longer

• A party wall notice served late. The two months start when it's served, not when you decide to build.
• Waiting on planning. A mansard, or a house in a conservation area, adds a planning decision before anything else can happen.
• Changes during the build. Moving the bathroom or changing the windows after work starts costs time as well as money. Fix the spec first.
• What's found when the roof opens. Old timbers, chimneys and wiring can need work nobody could see at the survey. We stop, show you, price it and agree it before carrying on.

Paying for it as it goes

We don't take a deposit. You pay weekly in arrears, with an invoice every Friday for the work done that week. So you pay as the loft goes up, and never for work that hasn't happened yet.

If you're working out the budget as well as the timeline, read our guides to loft conversion costs in London and how much value a loft conversion adds.

Sources and dates

• Promaester, guide build times on our loft conversions page, checked 5 October 2026.
• Planning Portal, Lawful development certificates and Loft conversions, checked 5 October 2026.
• gov.uk, Party walls and building work, checked 5 October 2026.

Every loft is different, so these are guide times, not a promise. For a timeline for yours, book a free site visit. We'll look at the roof and the stairs, and tell you what needs approving before we can start.