Dormer Window Replacement
Swap tired, draughty or rotten dormer windows without touching the structure. Warmer room, lower bills, no scaffolding-heavy rebuild.
Dormer Window Replacement — indicative geometry
What a dormer window replacement actually is
Most dormer windows fail long before the dormer does. Rotten timber sills, blown double-glazed units, seized hinges and perished seals are all replaceable in a day or two without disturbing the cheeks or roof. We survey the surrounding timber first — there is no point putting a new frame into a rotten sub-sill, and that is where a lot of cheap replacements go wrong.
Specification at a glance
| Typical duration | 1–2 days per dormer |
|---|---|
| Scaffolding | Tower or scaffold depending on height |
| Sub-sill check | Timber probed and replaced if soft |
| Glazing upgrade | 1.2 W/m²K or better |
| Certification | FENSA or building control notice |
| Guarantee | 10 years on frames and units |
How we build it
We only do dormers. That sounds like a limitation until you have watched a general builder improvise a back gutter on a wet Thursday. Every dormer window replacement job runs the same seven stages — survey and feasibility, design and consent, scaffold and protection, structure, weatherproofing, glazing, then finish and sign-off.
We run the permitted development test on your specific property: set-back from the eaves, height against the ridge, volume allowance and whether an Article 4 direction applies locally.
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 dormer window replacement 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
Dormer Window Replacement 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
Dormer Window Replacement — your questions
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.
Do you work on listed buildings and in conservation areas?
Yes. That work is detail-led: matching sightlines, glazing bar widths, cill projections and cheek finishes to what survives, and specifying timber, lead and natural slate rather than modern equivalents. We prepare the drawings and heritage statement for listed building consent alongside the planning application.
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.