Sash Dormer Windows
Vertical sliding sashes sized and detailed for a dormer opening. Period-correct proportions on a Victorian or Georgian roof.
Sash Dormer Windows — indicative geometry
What a sash dormer window actually is
Dormers on Victorian and Georgian houses were sashes, and a casement in the same hole reads wrong from the pavement immediately. Spiral-balance or traditional cord-and-weight sashes are both available at dormer sizes, in timber or in slim heritage uPVC with run-through horns and putty-line profiles.
Specification at a glance
| Operation | Spiral balance or cord and weight |
|---|---|
| Materials | Timber or heritage uPVC |
| Detailing | Run-through horns, putty-line, astragal bars |
| Glazing | Slim heritage units, 12–14mm |
| Best for | Victorian, Georgian and Edwardian dormers |
| Guarantee | 10 years |
How we build it
Everything is priced from a measured survey, not a phone call — which is why our quotations tend to be the ones that still hold at the end of the job. Every sash dormer windows job runs the same seven stages — survey and feasibility, design and consent, scaffold and protection, structure, weatherproofing, glazing, then finish and sign-off.
If the property is in a conservation area or the dormer faces the road, a full planning application is needed. We prepare the drawings and handle the submission either way.
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 sash dormer windows 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.
Send us the address and roughly what you are hoping to gain, and we will come back with a realistic answer rather than an optimistic one.
Related work
Sash Dormer Windows 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
Sash Dormer Windows — your questions
How disruptive is a dormer build?
Less than people expect, because most of the work happens above your ceilings and is reached from the scaffold. The noisy phases are opening up and installing steels, typically two to four days. Once the staircase is cut in there is dust and traffic through the house, and we sheet and protect the route for that period.
Can a dormer include an en-suite?
Very often, yes — a full-width or L-shaped dormer usually has enough floor area for a bedroom plus a shower room. The practical constraints are getting soil and waste down to an existing stack, and whether the head height over the shower tray works. Both are settled at survey.
Can you build a dormer in winter?
Yes, with adjustments. Temporary weather protection over the scaffold lets structural work continue in most conditions. GRP roofing needs dry conditions above about 5°C to cure properly, so in a cold spell we either sequence it differently or specify EPDM, which is unaffected.
Do I need planning permission for a dormer window?
Rear dormers on a house often fall under permitted development, provided the dormer is set at least 200mm back from the eaves, sits below the ridge line, uses materials similar in appearance to the existing house, and stays within the volume allowance — 40m³ for a terrace, 50m³ for a semi or detached. Front-facing dormers, flats and maisonettes, listed buildings, conservation areas and any property with an Article 4 direction all need a full planning application. We check the constraints on your specific address before we quote.
How long does a dormer take to build?
A single box or flat roof dormer is typically three to five weeks on site. A full-width rear dormer runs five to eight weeks, and an L-shaped dormer on a terrace with an outrigger is closer to seven to ten. A complete dormer loft conversion including the staircase, services and finishes is usually eight to twelve weeks from scaffold up to sign-off.
Does a dormer need building regulations approval?
Yes, always — permitted development covers planning, not building control. A dormer needs approval for structure, thermal performance under Part L, fire escape and protected route under Part B, and stairs under Part K if a staircase is involved. We handle the application, the inspections and the completion certificate.
What is the best roof covering for a flat dormer?
GRP fibreglass and EPDM rubber are the two we specify most. GRP cures into a single seamless laminate with bonded trims and suits small tops with several upstands. EPDM is a one-piece membrane that copes better with movement on larger spans. Both carry twenty-year guarantees; mineral felt is cheaper but has a materially shorter life.
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.