Dormer windows in Thornbury
From the first measured survey to the completion certificate, we run dormer windows and dormer loft conversions projects in Thornbury as a single, managed piece of work.
What we build in Thornbury
Because Thornbury is largely Victorian and Edwardian terraces, interwar semi-detached houses and post-war bungalows, the roof pitches and span widths we meet are fairly predictable, and that makes feasibility a quick conversation rather than a guess.
Whether that is a single flat roof dormer to make a back bedroom usable, or a full-width rear dormer with a bedroom and en-suite, the process is the same: a free measured survey first, a fixed written price second, and nothing else until you have both in front of you.
Planning permission in Thornbury
Whether you need planning permission or a lawful development certificate depends on the elevation, the volume already used and the local authority's Article 4 position — all of which we confirm before design work starts.
Thornbury sits in Gloucestershire, and local authority interpretation of the volume allowance does vary — which is why we check the planning history at your specific address rather than assuming the neighbours' dormer sets a precedent.
How the job runs
Scaffold and protection
Scaffold designed, erected and inspected. Temporary weather protection over the work area before anything is opened.
Weatherproofing
Deck, insulation, membrane or lead, cheek cladding, flashings and gutters — the dormer is weathertight from this point.
Finish and sign-off
Plaster, electrics, decoration and making good, final building control inspection and your completion certificate.
Design and consent
Measured drawings, elevations and sections. We submit the planning application or lawful development certificate and the building regulations application.
We only do dormers. That sounds like a limitation until you have watched a general builder improvise a back gutter on a wet Thursday. The quickest route is a photograph of the rear elevation over WhatsApp — we can usually tell you what is feasible before anyone visits.
Popular in Thornbury
Dormer work we are asked for most
Full-Width Rear Dormers
Wall-to-wall across the back of the roof — the biggest gain in usable floor area.
Learn moreCat Slide Dormers
The main roof continues down past the dormer — a soft, cottage-style profile.
Learn moreDormer Window Installation
Supply and fit of the window itself into new or existing dormer openings.
Learn moreAluminium Dormer Windows
Thermally broken aluminium — the slimmest frames and the largest possible glass area.
Learn moreGRP Fibreglass Dormer Roofing
Seamless glass-reinforced polyester on flat dormer tops — no joints, no seams.
Learn morePitched Roof Dormers
A miniature pitched roof over the dormer — the traditional, period-correct option.
Learn moreFAQs
Dormers in Thornbury — FAQs
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.
Can you convert an existing Velux loft to a dormer?
Yes, and it is usually a shorter job than starting from scratch because the floor, staircase and services already exist. We verify the existing floor structure and any previous building control approval first, then cut and form the dormer into the finished room. Expect the room to be out of use for three to five weeks.
Find out what your roof will take
A free measured survey in Thornbury tells you what is achievable, what it needs, and what it costs — before you commit to anything.