Roof Window vs Skylight Cost Difference Calculator

How this calculator works

This calculator compares typical UK supplied-and-fitted price ranges for the two options, drawn from published 2026 industry cost guides. Your inputs scale those base ranges: the calculator multiplies the per-unit range for each option by the number of units and by adjustment factors for the choices you make below, then shows both totals side by side along with the estimated difference between them. Because real quotes vary, everything is expressed as a range from a realistic low to a realistic high rather than a single figure.

A pitched roof window typically costs around to fitted in 2026, while a flat roof skylight runs roughly to , the difference reflecting the kerb upstand and membrane detailing that flat roof units require. In UK usage a roof window is an opening window fitted in line with a pitched roof slope, while a skylight or rooflight is a glazed panel set into a flat roof.

Number of units

Enter how many units you are comparing. In practice your roof type usually decides the product, but extensions with both pitched and flat sections genuinely face this choice.

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

Roof windows benefit from standardised sizes, off-the-shelf flashing kits for common tile and slate profiles, and quick installation between rafters, which keeps costs down. Flat roof skylights need a properly built and insulated upstand and careful integration with the waterproof membrane, which adds labour, and larger units add structural trimming. Roof windows commonly open for ventilation as standard, whereas opening flat rooflights carry a hefty premium. In most projects the roof pitch dictates the answer, and where both exist the pitched roof window usually delivers daylight at the lower cost.

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.