Dormer Windows Installation and Replacement Cost Calculator

How this calculator works

This calculator estimates a realistic price range rather than a single figure, because real-world quotes genuinely vary that much. It starts from typical UK supplied-and-fitted price ranges drawn from published 2026 industry cost guides, then multiplies them by the number of units you enter and by adjustment factors for each choice you make below. The result is a low-to-high band that a fair quote is likely to fall within, not a promise of the exact price you will pay.

Replacing the window within an existing dormer typically costs around to in 2026, depending on size, material and access. Building a brand new dormer, including the structure, cladding, roof covering and window, is a different scale of project at roughly to per dormer, with large flat-roof box dormers as part of a loft conversion at the top of that range and beyond.

Windows to replace in existing dormers

Enter how many window units you want to replace within dormers that already exist. This is a normal window replacement job with slightly fiddlier access and weatherproofing, so it costs somewhat more than a like-for-like wall window swap.

New dormers to build (including the window)

Enter how many completely new dormers you want constructed. Building a dormer is roofing and structural work, not just window fitting: it involves cutting the roof, forming a structural frame, cladding, a roof covering, internal finishing and the window itself, which is why the cost is in the thousands per dormer.

Specification level

Two windows that look similar from the street can sit at very different price points. Budget products use simpler profiles, standard hardware and basic glass. Mid-range covers the typical A-rated units most UK installers quote by default. Premium covers high-end systems with upgraded security hardware, better thermal performance, special finishes or designer profiles, and typically adds around 30 percent to the price.

Where you live

Labour rates are not uniform across the UK. Published cost guides consistently show installation prices in London and the South East running roughly 20 to 30 percent above the national average, driven by higher labour costs, travel, parking and congestion charges. The calculator applies an uplift of around 20 percent if you select London and the South East. Product prices themselves are broadly similar nationwide, so the difference mostly reflects fitting costs.

What else affects the price

For window-only replacements, the dormer cheeks and roof junctions are the sensitive areas: the new frame must integrate with the existing flashings and cladding, and access often requires a tower or scaffold, which adds cost on higher roofs. For new dormers, the structural design, the roof covering choice, whether the work is part of a wider loft conversion, and planning constraints all move the price substantially. Rear dormers on houses often fall under permitted development within volume limits, but front dormers usually need planning permission, and building regulations approval applies to all new dormer structures.

Planning permission, building regulations and certification

Most roof glazing falls under permitted development rights, provided the units do not project more than 150 millimetres above the roof plane and sit no higher than the highest part of the roof, but conservation areas, listed buildings and flats have tighter rules, so check with your local planning authority before ordering. Building regulations apply regardless of planning status, covering structural alterations, thermal performance, safety glazing and, in loft conversions, escape and ventilation requirements. Installers registered with a competent person scheme can self-certify that replacement glazing complies with the regulations and issue a certificate you will need when selling the property; for structural roof work, building control involvement is normally required.