Costs & Guides
How Long Does a Loft Conversion Take?
Six to ten weeks on site for a typical conversion. The part people underestimate is everything that happens before anyone arrives.
The Whole Timeline, End to End
Ask a loft conversion company how long the job takes and you'll usually be told six to eight weeks. That's honest, and it's also only half the answer, because it's measuring from the day scaffolding goes up.
The full timeline — from deciding you want a loft conversion to having a completion certificate in a drawer — is more often four to seven months. On a conversion needing planning permission it can be longer. Almost all of that extra time sits before the build, and almost none of it is the builder's doing.
| Stage | Typical duration |
|---|---|
| Getting and comparing quotes | 2 – 4 weeks |
| Design and structural drawings | 2 – 4 weeks |
| Planning application (only if needed) | 8 – 13 weeks |
| Lawful Development Certificate (if used instead) | 6 – 8 weeks |
| Building control application | Runs alongside the above |
| Party wall notices and agreement | 8+ weeks minimum |
| Build on site | 6 – 10 weeks |
| Snagging and completion certificate | 1 – 3 weeks |
Several of these overlap. Party wall notices can be served while drawings are finalised; building control runs in parallel with everything. But the party wall notice period sets a floor that nothing can compress: at least two months' notice before work starts, by law.
The Part that Takes the Longest
Quotes — 2 to 4 weeks. Three companies, three site visits, three quotes back. Some are quick; some take a fortnight. This stage runs longer than people expect largely because of diary availability.
Design and structural drawings — 2 to 4 weeks. Layout, roof design, and calculations from a structural engineer for the steels and new floor.
Planning permission — 8 to 13 weeks, if you need it. A householder application has a statutory determination period of eight weeks, but the clock starts when the application is validated, not when it's submitted, and validation can add time if anything's missing. Build in three months and be pleasantly surprised.
Lawful Development Certificate — 6 to 8 weeks. If your conversion is permitted development, this is the optional-but-sensible route.
Party wall — at least eight weeks, and this one is immovable. A party structure notice must be served at least two months before work begins. Your neighbour has fourteen days to respond; if they don't reply at all, they are deemed to have dissented and a dispute is deemed to have arisen, which means surveyors get appointed and an award drawn up. Serving notices early is the single most effective thing you can do to protect your timeline. Note also that a notice lapses if work hasn't begun within twelve months.
Week by Week on Site
A typical dormer or hip-to-gable conversion, six to ten weeks:
Week 1 — Scaffolding and Opening Up
Scaffold goes up, the roof is opened or stripped where the new structure goes, temporary weather protection is installed. Noisy, and the week that feels most alarming from inside the house.
Weeks 1–2 — Structure
Steel beams craned or carried in and set, new floor joists laid. On a terrace with no side access this stage is slower, because everything comes through the hallway.
Weeks 2–4 — the Shell
Dormer framed and built, or the gable wall raised and the roof extended over it. Roof covering, flashing and guttering. By the end of this stage the house is weathertight again and the shape of the room is visible.
Weeks 3–5 — Stairs and First Fix
The staircase is formed — usually the most disruptive interior stage, because it opens up the landing below. Electrics, plumbing and any bathroom pipework are run in.
Weeks 4–6 — Insulation and Plasterboard
Roof and walls insulated to current standards, then boarded out. Building control inspects at key points.
Weeks 5–8 — Plastering and Second Fix
Plaster skimmed and left to dry, then sockets, switches, radiators, doors and the bathroom fitted out.
Weeks 7–10 — Decoration and Finish
Painting, flooring, handover. Scaffolding comes down.
Then — Completion Certificate
Final building control inspection and sign-off. This can take a week or three after the builders leave, and it is the document that matters most. Chase it.
By type: a rooflight conversion is quicker, often four to six weeks, because there's no external structure. A mansard is longer — commonly three to four months on site, because the rear roof is removed and rebuilt.
The Four Things that Add Weeks
1. Party Wall Notices Served Late
Easily the most common cause of delay, and completely avoidable. Two months is the statutory minimum, and a neighbour who simply doesn't reply triggers the surveyor process. Serve early.
2. Discovering the Roof Isn't What Everyone Assumed
Modern trussed rafters, an undersized existing structure, or previous work done badly. A design-stage problem when it's found early and a mid-build problem when it isn't — which is an argument for a proper survey rather than a quote from photographs.
3. Planning Conditions and Revalidation
An application that gets validated late, or comes back with conditions requiring revised details, can add a month without anyone doing anything wrong.
4. Changing Your Mind Mid-Build
Moving a window, adding a bathroom, relocating the stairs. All reasonable, all costly in time, and all much cheaper to decide on paper.
Can You Live in the House During the Work?
Most people do, and it's manageable rather than pleasant.
Weeks one to three are the worst — noise, dust, and a period where the roof is open under temporary cover. Once the shell is closed in, it improves markedly.
Practical things that make it easier:
- Agree working hours in writing before the start, and check what your council's rules are for construction noise.
- Empty the loft completely beforehand. All of it. It takes longer than you think.
- Expect the landing to be out of action while the staircase is formed, and plan for dust reaching further into the house than anyone promises.
- Talk to your neighbours before the scaffolding arrives rather than after. It costs nothing and it prevents most party wall problems from becoming adversarial.
A mansard is the type where moving out for part of the work is genuinely worth costing, because the rear roof comes off entirely.
Timeline Questions
How long does a loft conversion take from start to finish?
Six to ten weeks on site for a typical dormer or hip to gable. Four to seven months in total once quotes, design, party wall notices and any planning application are included.
How long does a loft conversion take if I need planning permission?
Add roughly three months. A householder application has an eight-week determination period, but the clock starts at validation and conditions can extend it.
What's the quickest type of loft conversion?
A rooflight conversion — often four to six weeks on site, because the roof structure isn't altered.
Do I need to move out?
Usually not, though the first two or three weeks are the hardest. A mansard is the exception worth planning around.