Quick Answer
A loft conversion typically takes 4 to 14 weeks depending on the type. A simple Velux conversion takes around 4–6 weeks, a dormer 6–8 weeks, while a full Mansard conversion can take 10–14 weeks. These timescales are for the building works only — add 6–12 weeks beforehand for design, planning and Building Regulations approval.
A loft conversion is one of the most valuable home improvements you can make in 2026, adding an extra bedroom, office or living space without extending your footprint. But how long does it actually take from start to finish? The answer depends heavily on the type of conversion you choose. This guide breaks down realistic timescales at every stage — including what causes delays and how to avoid them.
In this guide
Duration by Loft Conversion Type
Velux (Rooflight) Conversion — 4 to 6 Weeks
A Velux or rooflight conversion is the simplest type. The existing roof structure is retained and reinforced, and windows are inserted into the existing roof slope. Because there's minimal structural alteration, a skilled crew can complete the work in 4–6 weeks. This is the fastest and least disruptive option — an ideal choice if your loft has sufficient headroom already.
Dormer Conversion — 6 to 8 Weeks
A rear dormer is the most popular loft conversion type in the UK. It involves cutting back part of the roof and adding a box-shaped extension with vertical walls and a flat or pitched roof. The extra structural work and weatherproofing take longer — typically 6–8 weeks on site. Most rear dormers fall under Permitted Development, which speeds up the pre-build phase.
Hip-to-Gable Conversion — 8 to 10 Weeks
Common on end-of-terrace and detached homes with a hipped roof, this conversion involves rebuilding the hipped roof end as a vertical gable wall, dramatically increasing headroom and floor space. The more extensive structural work involved typically adds 2–4 weeks compared to a dormer, bringing the total on-site time to 8–10 weeks. This type often requires Planning Permission.
Mansard Conversion — 10 to 14 Weeks
A Mansard conversion involves replacing one or both sides of the roof with near-vertical walls and a shallow-pitched top. This maximises internal space but is the most complex and expensive type, requiring the most structural alteration. Expect 10–14 weeks on site, and note that Mansard conversions almost always require Planning Permission due to the significant change to the roofline.
Loft Conversion Timeline Comparison
| Conversion Type | On-Site Build Time | Planning Needed? | Total Project (incl. design) |
|---|---|---|---|
| Velux / Rooflight | 4–6 weeks | Usually no (PD) | 10–18 weeks |
| Rear Dormer | 6–8 weeks | Usually no (PD) | 12–20 weeks |
| Hip-to-Gable | 8–10 weeks | Often yes | 16–28 weeks |
| Mansard | 10–14 weeks | Yes | 20–30 weeks |
| L-shaped Dormer | 8–12 weeks | Usually no (PD) | 14–24 weeks |
Stages of a Loft Conversion (On-Site)
Understanding what happens week-by-week helps you manage expectations and plan around disruption:
Week 1–2: Structural Work
The most disruptive phase. Steel beams are lifted in, existing timbers are reinforced or replaced, and any structural changes to the roof are made. Scaffolding goes up. This stage is noisy and you may notice cold draughts through the house until the roof is weathertight.
Week 2–4: Roofing and Weatherproofing
New roof covering, felt and battens are installed. Windows (Velux or dormer) are fitted and made weathertight. For dormers, the box structure is built and clad. This stage is critical — the building inspector will visit to check the structural work before it's closed in.
Week 3–5: Insulation and First Fix
Insulation is installed between and over the rafters. First-fix electrics and any plumbing runs (for an en-suite) are routed. The building inspector will check insulation specification before it's boarded.
Week 4–6: Boarding and Staircase
Plasterboard goes up on walls and ceiling, and the staircase is installed. This is often the point where the space starts to look like a real room. The staircase fitting itself takes 1–2 days but the opening in the floor below needs careful preparation.
Week 5–8: Second Fix and Finishes
Plastering, second-fix electrics and plumbing (sockets, lights, radiators, sanitaryware), flooring, decoration and joinery. The pace often slows at this stage as trades need to follow sequence — plastering must dry before decoration, for example.
What Causes Delays?
Even with a well-organised contractor, loft conversions can run longer than planned. Common causes:
- Building inspector delays: Local authority inspectors can be busy, and you must wait for sign-off at key stages before proceeding. Private Approved Inspectors are often faster.
- Unexpected structural issues: Rotten timbers, substandard previous repairs or unexpected beam configurations can add days or weeks.
- Materials lead times: Structural steel, specialist windows and staircases often have 2–4 week lead times. A good contractor orders these before work starts.
- Wet weather: Roofing work cannot safely proceed in heavy rain or high winds. Winter projects typically run 10–15% longer.
- Trade availability: Plasterers, electricians and plumbers are in high demand. Gaps in the trade schedule are one of the most common causes of overruns.
- Design changes: Changing the spec mid-project — adding an en-suite, changing the window size — adds time and cost.
Planning and Permissions: Add 6–16 Weeks Before Build
Before work starts on site, you'll need:
- Architect or designer drawings — typically 2–4 weeks to produce
- Building Regulations application — see our Building Regulations guide for timescales (usually 5–8 weeks for Full Plans)
- Planning Permission (if required) — target 8 weeks, often takes 10–12 weeks
- Structural engineer calculations — usually 1–2 weeks
For most Velux and rear dormer conversions, you can use the Building Notice route for Building Regulations, which starts as soon as you serve notice — cutting the pre-build timeline significantly. For a full overview of PD rights and when planning is needed, see Permitted Development rules in the UK.
Once you're ready to find a builder, our guide on how to hire a builder in the UK covers everything from getting quotes to checking credentials.
For cost information, our loft conversion cost guide has up-to-date 2026 prices by type.
Frequently Asked Questions
Can I live in my house during a loft conversion?+
Yes, in most cases. The main disruptive periods are the structural work in week 1–2 and the staircase installation. During the roofing phase there may be some cold or noise, but the house remains habitable. If you have young children or work from home, you may want to arrange to be away for the noisiest days.
How long does a loft conversion take from start to finish including design?+
For a straightforward Velux or dormer conversion, plan for around 3–5 months total from appointing a designer to completion — including design, Building Regulations approval, finding a contractor and the build itself. For a Mansard requiring planning permission, 6–9 months is more realistic.
Do I need to move out during a loft conversion?+
Most homeowners stay in the property throughout. The main impacts are noise during structural work, dust (particularly when plasterboarding) and loss of access to one bedroom when the staircase opening is made. Talk to your contractor about the most disruptive days so you can plan accordingly.
How long does Building Regulations approval take for a loft conversion?+
If you use the Building Notice route, you can start work almost immediately after serving notice. If you use the Full Plans route, approval typically takes 5–8 weeks. The advantage of Full Plans is that the inspector checks your drawings in advance, reducing the risk of remedial work on site.
What is the fastest type of loft conversion?+
A Velux (rooflight) conversion is the fastest — typically 4–6 weeks on site. It involves no structural changes to the roofline, just reinforcing existing timbers and inserting windows. It works well where the loft already has sufficient headroom (at least 2.2m at the ridge is recommended).
Can a loft conversion be done in winter?+
Yes, but it typically takes longer. Roofing and external work is weather-dependent, and wet or cold weather can slow progress. Some contractors prefer to start loft conversions in spring. If you start in autumn, make sure your contractor has a clear plan for managing weather delays.
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