What will your dormer cost?
Four questions and you'll have a realistic range. It is a guide, not a quotation — the firm figure comes from a free measured survey, because ridge height and existing structure move the number more than anything you can tell us over the phone.
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What moves a dormer price, and by how much
Three things account for most of the spread between two quotations on the same roof.
1. Width, and what carries it
Price scales roughly with the width of the dormer front — but not smoothly. Once you go past about four metres you generally need a steel ridge beam rather than timber, and that beam has to get into the loft, which sometimes means a crane and a road closure. That step is where a price jumps rather than climbs.
2. The finish on the cheeks and the top
GRP fibreglass is the baseline. Tile-hung cheeks add roughly a fifth; lead and standing-seam zinc add a third or more, because both are slow, skilled work priced by the day. On a rear elevation nobody sees, GRP is usually the sensible spend. On a front elevation in a conservation area, it may not be an option at all.
3. Where the scope stops
A dormer quotation and a loft conversion quotation are different animals. Ours separate them explicitly: the structure and weathertight shell, then internal finishing, then extras like an en-suite or a new staircase. The most common reason a cheap quote turns expensive is that the internal finish was never in it.
What is always included in our price
- Measured survey, structural calculations and working drawings
- Planning or lawful development certificate application, and building regulations submission
- Scaffold design, erection, weekly inspection, hire for the programme and dismantling
- Structure, insulation, weatherproofing, flashings, gutters and cladding
- Glazing, including escape-compliant opening lights where required
- Building control inspections and your completion certificate
- Skips, waste removal and making good
What is not
- VAT, shown separately on every quotation
- Anything found once the roof is open that could not reasonably have been seen — rotten purlins, undersized existing joists, asbestos. We price these as a variation with your written approval first, never as a surprise on the invoice.
- Party wall awards where a surveyor is appointed, and local authority planning fees, both of which are paid direct
FAQs
Cost 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.
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.