The short answer
In everyday British use the two words overlap, but a roof window usually means an opening, framed window set into a pitched roof slope, while a skylight or rooflight usually means a fixed or flat-roof unit that sits on a kerb. The difference matters because Building Regulations give each its own thermal limit, and the fitting method is not the same.
What each word means on a British roof
People say skylight for anything that lets daylight through a roof. Roofers and specifiers are more exact. A roof window is a proper window that happens to lie in the plane of a sloping roof. It has a sash, hinges or a pivot, and it opens, so you can reach it from inside, clean it and let air through. You will find them in lofts, landings, bathrooms and the rear slopes of terraces.
A rooflight is the broader term, and in practice it covers glazing that does not behave like a window: a fixed pane in a pitched roof, a flat-roof dome, a glass box on an upstand, a lantern, or a long run of glazing over an extension. The word skylight sits between the two. In the American trade it nearly always means a fixed unit. In Britain it is used loosely for both, which is why a customer asking for a skylight may really be after an opening roof window.
If you are comparing quotes, ask which one is meant. A description reading skylight with no detail could be a small fixed pane or a large opening unit, and the price and the building work differ a great deal.
How the two are built and fitted
On a sloping roof the window is built into the slope itself. The opening is framed between rafters, with a trimmer above and below, the tiles or slates are cut back, and the window is held in a flashing kit. A flashing is the weatherproof metal and rubber surround that carries water around the frame and back onto the roof covering. On a stone, plain tile or slate roof the covering has to be cut and laid with care, and our guide to the types of rooflight shows how the products differ.
A flat-roof rooflight is different. It sits on an upstand or kerb, a short raised edge built around the opening, and the roof membrane is dressed up the kerb and tucked under the unit. Water does not run off a pitched slope around it, so the detailing of the kerb and its flashing is what keeps the room dry. A fixed rooflight in a pitched roof falls between the two. It is fitted like a roof window but has no sash, so there is nothing to leak at the hinge and nothing to open.
What this means on site
Because the structures differ, the survey differs. For a roof window we look at rafter spacing, the covering and the pitch. For a flat-roof unit we look at the deck, the fall and the upstand height.
Why the regulations treat them differently
The thermal limits for replacement elements in existing homes are set out in Approved Document L, Table 4.2. A roof window has to reach a U-value of 1.4 W/m²K at most; the lowest Window Energy Rating that satisfies the table is Band B. A rooflight is allowed 2.2 W/m²K, measured over its outer developed surface area. Those are different numbers because the products are different: a kerbed rooflight has more surface area than the hole it covers, and the measure allows for that.
In practice, a pitched-roof opening window is held to the tougher figure, while a lantern or flat-roof dome follows the more generous one. When a roofer says the unit meets building regulations, ask which category it is being counted in and what the label shows. Our guide to roof window U-values explains the figures, and the skylight and roof window service page sets out how we survey and price the work.
A U-value is the rate at which heat passes through a material. A lower figure means less heat lost.
Planning and fire escape depend on the type
Under the General Permitted Development Order, Schedule 2 Part 1 Class C, a roof window or rooflight on a house is allowed without a planning application, as long as it stands no more than 0.15 metres beyond the plane of the slope and no part of it rises above the top of the existing roof. A side-facing slope has an extra obscure-glass rule. Listed buildings and homes with a planning condition or Article 4 direction are outside that general position, so check with Stroud District Council's planning team first.
Escape is another place where the type counts. Under Approved Document B, a roof window can serve as an emergency escape only within 4.5 metres of the ground, and it needs an unobstructed opening of at least 0.33 m² with a minimum height and width of 450 mm, with the bottom of that opening no more than 1100 mm above the floor. A fixed rooflight offers no escape at all. Where the loft room sits above two floors, a protected stair is the answer instead.
Which one suits which room
Choose an opening roof window where you want ventilation, a reachable pane or a way out: a loft bedroom, a bathroom, a landing. Choose a fixed unit where you only want light, or where the glass is too high to reach and an opening sash would be a nuisance. Choose a kerbed rooflight for an extension with a flat roof, where a pitched-roof window cannot be used.
On price, For labour and fitting on standard jobs, MyBuilder's 2026 roof window guide quotes £350 to £600, with scaffold around £500. Treat these as national guide ranges, not our pricing; the survey leads to a written quote covering the window, scaffold, building work and making good.
For a view of where to put the opening itself, see our answer on where to place a roof window. If glazing is not the right route at all, read about the alternatives to a skylight. Whichever you choose, the work carries our 10-year workmanship guarantee in writing.