How Long Does a Loft Conversion Take? A Week-by-Week Timeline
Quick Answer
A dormer loft conversion takes 8–12 weeks on site. A Velux conversion can be done in 6–8 weeks. A mansard conversion takes 12–16 weeks. Add 4–8 weeks for pre-construction (structural drawings, Building Regulations, Party Wall notices) and the total programme from instruction to completion is typically 16–24 weeks.
A dormer loft conversion typically takes 8–12 weeks from start to finish. But that figure hides a lot of variation. The type of conversion, the structural condition of your roof, and whether you need planning permission all affect the programme significantly. What follows is a realistic week-by-week breakdown of what happens and when.
In 2026, a Velux conversion in Hertfordshire costs between £18,000 and £28,000. A rear dormer runs £38,000–£60,000. A Mansard conversion typically costs £55,000–£85,000. For a full breakdown by type and finish tier, see our Loft Conversion Cost Guide. This article focuses on programme — what happens, in what order, and how long each stage takes.
Duration by Conversion Type
| Conversion Type | Typical Range | TCM Median (n=63) | Notes |
|---|---|---|---|
| Velux / Roof Light | 6–8 weeks | 7 weeks | No structural changes to roof shape. Fastest option. Rarely needs planning permission. |
| Dormer (rear) | 8–12 weeks | 10 weeks | Most common in Hertfordshire terraces. Requires structural steelwork. |
| Hip-to-Gable | 10–14 weeks | 12 weeks | Requires rebuilding one or both hip ends. More complex groundwork. |
| L-Shaped Dormer | 10–14 weeks | 12 weeks | Two dormers joined at a corner. Common on Victorian terraces. |
| Mansard | 12–16 weeks | 14 weeks | Significant structural alteration. Almost always requires planning permission. |
Week-by-Week: A Typical Dormer Loft Conversion
What Slows a Loft Conversion Down
Party Wall disputes are the single most common cause of programme delays. If a neighbour dissents, the appointment of a Party Wall surveyor and agreement of an Award adds 4–8 weeks before work can start. Serve notices as early as possible — ideally the day you receive your Building Regulations approval.
Structural surprises are the second most common cause. Older London properties sometimes have undersized or damaged ceiling joists that require additional steel before the loft floor can be formed. A pre-contract structural survey identifies these issues before they become programme problems.
Building Control inspection delays can also add time if the inspector is not booked in advance. Your builder should be booking inspections at least a week ahead of each stage. If they are not, ask why.
From Our Work
What Our 63 Loft Projects Actually Took
Across 63 loft conversions completed in Hertfordshire and North London since 2020, our median on-site programme was 10 weeks for a rear dormer and 7 weeks for a Velux conversion. The fastest dormer we completed was 8 weeks (a semi-detached in Borehamwood with no party wall requirement and a straightforward roof structure). The longest was 18 weeks — a Mansard on a Victorian mid-terrace in Barnet where the party wall process with two neighbours added six weeks to the pre-construction phase.
The single most consistent finding from our project data: homeowners who serve party wall notices on the day they receive Building Regulations approval save an average of 3.5 weeks compared with those who wait until a week before the planned start date. On a 10-week programme, that is a 35% reduction in total elapsed time from instruction to completion.
The programme is largely within your control. The decisions you make in the first two weeks determine whether your loft conversion takes 14 weeks or 22 weeks.
Ready to Start Your Loft Conversion?
TCM manages the entire process — from structural engineer to Building Control sign-off. Get a fixed-price quote today.