Velux to Dormer Conversions
Already have rooflights? Convert the slope to a dormer and gain standing height. Turns a sloped room you stoop in into one you stand up in.
Velux to Dormer Conversions — indicative geometry
What a velux to dormer conversion actually is
Plenty of lofts were converted in the 1990s and 2000s with rooflights only, leaving a room with sloping ceilings you cannot put a wardrobe against. Cutting a dormer into an already-converted loft is a shorter job than starting from scratch — the floor, stair and services are already there — but it does need the existing structure checking before anything is opened up.
Specification at a glance
| Typical gain | 3m² – 8m² of full-height floor |
|---|---|
| Prerequisite | Existing floor structure verified |
| Disruption | Room out of use 3–5 weeks |
| Roof finish | GRP, EPDM or tile to suit |
| Building control | Full plans or building notice required |
| Typical build | 3–5 weeks on site |
How we build it
We would rather tell you at survey that your ridge height will not take what you want than take a deposit and find out in week three. Every velux to dormer conversions job runs the same seven stages — survey and feasibility, design and consent, scaffold and protection, structure, weatherproofing, glazing, then finish and sign-off.
Rear dormers on houses often sit inside permitted development, but the volume allowance is consumed by any previous loft or rear extension — so we check the planning history for the address before we say yes.
What is included in the price
- Measured survey, structural calculations and full working drawings
- Planning application or lawful development certificate, and the building regulations application
- Scaffold design, erection, weekly inspection, hire and dismantling
- All structure, weatherproofing, flashings, gutters and cladding
- Glazing with escape-compliant opening lights where required
- Building control inspections and your completion certificate
- Waste removal and making good
Where this goes wrong elsewhere
The two failures we are called to fix most often on velux to dormer conversions are undersized or incorrectly lapped flashings, and a vapour control layer that stops at the first awkward junction. Neither shows up for a winter or two. Both are cheap to do properly at build stage and expensive to correct afterwards.
The quickest route is a photograph of the rear elevation over WhatsApp — we can usually tell you what is feasible before anyone visits.
Related work
Velux to Dormer Conversions by county
Where we do this work
Specialist teams across 74 counties. Pick your area for local detail, or browse every town.
How a dormer gets built
Seven stages, in this order, every time
The sequence matters. Most dormer problems come from doing one of these out of order — glazing before the flashings, or opening the roof before the materials land.
Survey and feasibility
A specialist measures the roof, checks ridge height, span and structure, and confirms whether you are inside permitted development.
Design and consent
Measured drawings, elevations and sections. We submit the planning application or lawful development certificate and the building regulations application.
Scaffold and protection
Scaffold designed, erected and inspected. Temporary weather protection over the work area before anything is opened.
Structure
Ridge beam and floor steels craned or manhandled in, new joists laid, the dormer frame built and sheathed.
Weatherproofing
Deck, insulation, membrane or lead, cheek cladding, flashings and gutters — the dormer is weathertight from this point.
Glazing
Windows fitted square, sealed and drained, with escape-compliant opening lights where required.
Finish and sign-off
Plaster, electrics, decoration and making good, final building control inspection and your completion certificate.
FAQs
Velux to Dormer Conversions — your questions
How much does a dormer window cost?
A single flat roof dormer typically runs from around £9,000 to £20,000 supplied and fitted, depending on width, cladding finish and access. A full-width rear dormer on a semi is usually £28,000 to £45,000, and a complete dormer loft conversion including stairs, insulation, plaster and building control sign-off is more commonly £40,000 to £70,000. Our online guide-price tool gives you an indicative range in about a minute.
Will my roof be open to the weather?
No. The roof is only opened once the scaffold, temporary weather protection and materials are all on site, and the dormer is made weathertight — structure, deck, membrane and temporary glazing — before the crew leaves at the end of the opening-up phase. We do not open a roof on a Friday afternoon.
Can you build a dormer on a bungalow?
Usually yes, and bungalows often give the best results because the spans are wide and there are no party walls. The deciding factor is ridge height: you generally want at least 2.6m from the top of the ceiling joists to the underside of the ridge. We measure this at survey before committing to anything.
Why is my dormer leaking?
In our experience the window itself is the cause less than a fifth of the time. Far more often it is the apron flashing at the dormer front, the lead soakers where the cheeks meet the roof slope, the back gutter behind a pitched dormer, or a split at an upstand in the flat covering. Water then tracks along a rafter and appears somewhere else entirely, which is why we trace the path before quoting.
What insulation do dormers need?
Current building regulations want around 0.15–0.18 W/m²K for a new dormer roof and 0.18 W/m²K for the cheeks and front. In practice that is a warm-roof build-up using 100–150mm of PIR insulation above or between the structure, with a continuous vapour control layer on the warm side. Getting the VCL continuous matters more than the insulation thickness.
Do you handle the planning application?
Yes. We prepare measured drawings, elevations and sections, submit the application or the lawful development certificate, and deal with the case officer's queries. On conservation and listed work we also put together the heritage justification the application needs.
Is scaffolding included in the price?
Yes. Every quotation we issue includes scaffold design, erection, weekly inspection, hire for the programme and dismantling. Scaffold appearing as an extra afterwards is one of the more common complaints about dormer work, so we price it in from the start.
Find out what your roof will take
A free measured survey tells you what is achievable, what it needs, and what it costs — before you commit to anything.