The short answer
A simple Velux conversion typically takes 4–6 weeks on site; a dormer takes 7–12 weeks; a mansard or complex project can run to 14 weeks or more. Before site work starts, design, structural engineering and building regulations typically add 6–12 weeks. See loft conversion costs to understand how the type and timeline affect budget.
Homeowners often focus on the on-site construction time and are surprised by the weeks of design, structural engineering, building regulations approval and planning (where required) that happen before anyone arrives with tools. A realistic total project timeline, from the first survey to the completion certificate, is more often four to six months than four to six weeks. This guide breaks down the phases so you can plan effectively and ask the right questions when comparing contractors.
Loft conversion timeline at a glance
- Velux / rooflight 4–6 weeks on site
- Rear dormer 7–12 weeks on site
- Hip-to-gable 8–12 weeks on site
- Mansard 10–14 weeks on site
- Pre-site phases 6–12 weeks for design, structs, regs
- Total project (typical) 4–6 months from survey to cert
Phase one: survey and design (2–6 weeks)
Before any structural or planning work begins, your loft needs a proper measured survey and a design. A loft conversion company or architect will measure the space, assess the roof structure, confirm head height, identify the staircase position and establish which conversion type is feasible. A simple Velux project may go straight to building regulations drawings; a dormer or more complex project needs architectural drawings first, which takes two to four weeks depending on the practice’s workload. If planning permission is required (for mansard, hip-to-gable or front dormers), allow eight to ten weeks for the planning application to be determined after submission, with another two to four weeks to prepare the application drawings.
Phase two: structural engineering and building regulations (4–8 weeks)
The structural engineer designs the new joist layout, any steels required and, where a trussed roof is involved, the replacement structural frame. This typically takes two to four weeks. Building regulations submissions (a full plans application) take around five weeks for the local authority to check and approve. Some contractors use a “building notice” approach, which allows work to start within 48 hours of submission — but this carries more risk if the inspector requires changes to work already completed. In parallel, any party wall notices must be served, and the two-month notice period runs from service, so doing this early saves considerable time. See party wall agreements for how to manage this process.
| Phase | Typical duration | Notes |
|---|---|---|
| Survey and design | 2–6 weeks | Plus 8–10 weeks if planning is needed |
| Structural engineering + building regs | 4–8 weeks | Full plans application: ~5 weeks LABC check |
| Party wall notice period | 2 months (statutory) | Serve early to avoid delaying the build |
| On site — Velux | 4–6 weeks | Mainly internal; lower weather risk |
| On site — dormer | 7–12 weeks | External work needed; weather dependent |
| On site — mansard | 10–14 weeks | Major roof rebuild; most complex |
Phase three: the build
On-site work follows a broadly predictable sequence regardless of conversion type: scaffold erection, structural work (steels, new joists, dormer or roof alterations), making the external envelope weather-tight, first-fix electrics and plumbing, insulation, plasterboard, second-fix and finishing. The building regulations inspector visits at key stages — typically when structural steels are in place and when first-fix wiring and insulation are complete. Failing an inspection stage means corrective work before proceeding, which can extend the programme by a week or more.
What causes delays
The most common causes of programme overrun in loft conversion projects are: planning decision delays (where an application is submitted and the authority takes longer than the eight-week target); building regulations plan check delays; party wall disputes that delay the two-month notice period; structural engineer availability; material supply issues; unexpected structural discoveries during the build (particularly in older properties with irregular rafters or decayed timbers); and bad weather during the roof-level phase. Budget at least four to six weeks of contingency in your total plan and discuss the programme milestones with your contractor before signing a contract.
Can I stay in the house during the conversion?
In almost all cases, yes — most homeowners remain in the property throughout. The disruptive phase is mainly the first two to three weeks of structural work on a dormer project, when scaffold is up, the roof is opened and dust and noise increase. Once the external envelope is weathertight and the scaffold comes down, the work becomes more internal and less invasive. The top-floor landing and staircase may be inaccessible at certain stages; discuss access with your contractor before work starts. This page is general information, not professional building or planning advice. Timescales vary by project — your contractor will provide a specific programme for your job.
Get a realistic timeline for your project
A specialist can survey your loft, confirm which type of conversion is feasible and give you a programme that accounts for planning, building regulations and the build itself.
Frequently asked questions
How long does a dormer loft conversion take?
A dormer conversion typically takes 7–12 weeks on site, from scaffold to completion. Including design, structural engineering and building regulations, the total project from first survey to completion certificate is more often 4–5 months.
Can a loft conversion be done in 4 weeks?
A very simple Velux conversion in a property with all permissions in place and a straightforward roof structure can complete in 4–6 weeks on site. Most projects take longer because of pre-site design, planning and building regulations phases.
Does a party wall agreement delay a loft conversion?
It can. The Party Wall etc. Act 1996 requires a minimum two-month notice period if your neighbour does not consent immediately. Serve the notice as early as possible — ideally before finalising the design — to avoid it delaying the build start.
When is the building regulations inspection?
Inspectors visit at key stages: typically when structural steels are installed, when first-fix electrics and insulation are in place, and at final completion. Your building regulations inspector will advise on the specific stages they need to visit for your project.
Sources & further reading
- LABC (Local Authority Building Control) — building regulations inspection stages and completion certification
- Planning Portal — planning application processing timescales for householder applications
- GOV.UK — Party Wall etc. Act 1996 — notice periods and dispute resolution
- GOV.UK — Building regulations Approved Document A (structure) and B (fire safety)
This is general information about loft conversions in the UK, not professional planning, structural, building or legal advice. Costs are typical illustrations, not quotes; timescales and outcomes vary with your property, location and chosen specialist. Always consult a qualified specialist and your local planning authority before starting work.