The Short Answer
A typical loft conversion takes 6–12 weeks on site once work begins. A simple Velux conversion can be done in around 4–6 weeks; a dormer in 6–8 weeks; a hip-to-gable or mansard at the longer end of the range.
That on-site figure is the part most people mean when they ask how long a loft conversion takes. But it is only one stage of the project. The full journey — from your first enquiry through design, approvals and the build — usually spans 3 to 6 months depending on whether planning permission is required and whether your neighbours' party walls are involved.
The good news is that much of that pre-build work can run in parallel. With a contractor who plans the sequence properly, a permitted-development dormer with no party wall complications can move from first idea to finished room in around three to four months.
“On Site” Is Not the Whole Story
When a builder quotes a duration, they usually mean the time their team is physically on site building. That covers:
- Structural works — installing steel beams, posts and the new floor structure
- First fix — carcassing, plumbing and electrical runs, insulation and the dormer or roof structure
- Second fix — plastering, staircase, fitting out, en-suite, decoration and final finishes
- Building Control inspections at each notifiable stage of the build
What that figure does not include is everything that has to happen before the first day on site:
- Architectural design and measured drawings
- Structural engineer’s calculations for the new beams and floor
- Planning permission, where the conversion does not fall under permitted development
- Party Wall notices and agreements, where neighbours’ walls are affected
- Building Control application and contractor scheduling
Understanding the difference matters. A “six-week” conversion can still take four months overall if planning and party wall have not been started early — which is why a clear programme from the outset is so important.
The Loft Conversion Timeline, Phase by Phase
Here is how a typical loft conversion breaks down, stage by stage. Some phases overlap — party wall notices, for example, can run while drawings are being finalised — so the durations below do not simply add up to the total.
Design & drawings — 2–4 weeks
Your architect or designer produces measured survey drawings and the proposed layout, and a structural engineer prepares calculations for the new steel beams and floor. This is the foundation of the whole project — getting it right here prevents delays later.
Planning permission (if needed) — ~8 weeks for a decision
Many loft conversions are permitted development and skip this stage entirely. Where planning is required — mansards, conservation areas, flats, or designs over the permitted limits — a local authority decision typically takes around 8 weeks from validation.
Party Wall notices (if applicable) — up to 2 months notice period
If the work affects a shared or neighbouring wall, you must serve party wall notices. Neighbours have up to two months to respond. Crucially, this period can run in parallel with your design stage, so it need not delay the start if it is handled early.
Structural works — steels & floor — 2–3 weeks
On site, the build begins with the heavy structural work: installing the steel beams and posts that carry the new loads, and forming the new floor structure that the room will sit on. Scaffolding goes up and the roof may be opened to install the dormer structure.
First fix — 2–3 weeks
First fix is everything that happens before the walls are closed up: timber carcassing and stud walls, plumbing and electrical cable runs, soil pipes for any en-suite, insulation to meet Building Regulations, and the dormer or roof structure being made watertight. Building Control inspects key stages here.
Second fix — 2–3 weeks
Second fix is the finishing stage: plastering and skimming, fitting the new staircase, hanging doors, installing the en-suite sanitaryware and electrics, fitting sockets and switches, and decoration. This is where the space turns from a building site into a finished room.
Building Control final inspection & completion certificate
A final inspection confirms the conversion complies with Building Regulations. Once passed, the completion certificate is issued — usually within a few weeks. Keep this safe: you will need it when you sell or remortgage the property.
How Long by Type of Conversion
The type of loft conversion has the biggest single effect on the on-site duration. The more the roof shape changes, the more structural work is involved, and the longer the programme.
Velux (rooflight) conversion
The quickest type. No change to the roof shape — only roof windows are added — so there is far less structural work. Often completed in around 4–6 weeks on site, and almost always permitted development.
Dormer conversion
The most common type in the UK. A box dormer extends the roof to create headroom and floor space, so it involves more structural and roofing work. Typically 6–8 weeks on site, sometimes longer for larger or twin dormers.
Hip-to-gable conversion
The sloping hip end of the roof is rebuilt into a vertical gable wall, creating significantly more internal volume. More substantial structural work means a longer programme — usually 8–10 weeks, and often combined with a rear dormer.
Mansard conversion
The most extensive type, effectively rebuilding one or both roof slopes to near-vertical. It delivers the most space but takes the longest — commonly 10–12 weeks or more, and almost always requires planning permission.
What Can Delay a Loft Conversion
Most overruns come from a handful of predictable causes. A good contractor anticipates and manages these rather than discovering them mid-build:
The two most avoidable delays are planning and party wall. Both can be started early and run alongside the design stage — so if your contractor leaves them until the last minute, that is a warning sign about how the rest of the project will be run. Learn more in our guides on the party wall agreement and Building Control.
Living in Your Home During the Works
A loft conversion has a real practical advantage over an extension: most of the work happens above your existing living space, accessed through the roof and scaffolding rather than through the heart of the house. That means the great majority of homeowners stay in their property throughout the build.
There will be noisy and dusty days — the structural-steel stage and the roof-open stage are the most disruptive — and you should expect scaffolding around your home for several weeks. The single biggest moment of internal disruption is when the new staircase is fitted, because this connects the loft to the floor below and temporarily opens up a landing or hallway.
A well-organised contractor will tell you in advance which days will be the worst, keep dust contained with screening, and clean the site down regularly. If you work from home or have young children, talk to your builder early so the most disruptive days can be planned around.
RCB's Approach to the Programme
At RCB Design & Build, every loft conversion starts with a clear written programme: a confirmed start date and a set of milestones you can hold us to. We do not give a vague “a few weeks” estimate — we set out exactly when structural works, first fix, second fix and completion are scheduled.
Because we handle the design, approvals and build under one roof, we start planning and party wall early so they run in parallel with the design rather than holding up your start date. We order long-lead items — steels, roof windows, staircases — ahead of time, and we engage Building Control from the outset so inspections are booked, not chased.
The result is a conversion that is kept to schedule, with no surprises about when your team will be on site or when your room will be finished.
Questions to Ask About the Timeline
Before you commit to a contractor, make sure their programme answers all of these:
- A written programme showing the start date and key milestones
- Confirmation of whether planning permission is needed before any start date is given
- Party Wall notices served early so the notice period runs in parallel with design
- Material and steel lead times ordered ahead of the build, not during it
- A clear stage payment schedule linked to defined points of progress
- Building Control engaged from the outset so inspections are booked, not chased
Why Homeowners Trust RCB
RCB Design & Build delivers loft conversions across London, Essex and Kent with a fixed programme, transparent pricing and full Building Control sign-off on every project. We are fully insured, we serve party wall notices early, and we keep you informed at every milestone — so you always know exactly where your project stands.
Explore our loft conversion serviceWondering about budget as well as time? Read our companion guide on how much a loft conversion costs to plan both the timeline and the spend together.