The short answer
Choose a roof window by working out how much daylight the room needs, then fit it to the rafters. As a rough guide, the glazed area should be a good fraction of the floor area, and the width is usually set by the gap between rafters. Standard windows run from about 550 mm to over 1,300 mm wide. A bigger window gives more light but also more heat gain, more cost and more structural work.
Start with the room, not the catalogue
There is no Building Regulations daylight requirement for houses in England, so the size is a matter of comfort and design rather than a legal minimum. Think about what the room is for. A bedroom needs enough light to wake up in but not so much that summer mornings are unbearable. A study wants steady light without glare on a screen. A kitchen or stair landing might want a larger window to brighten a dark core. A good starting point is to stand in the room, picture the window's position and judge how much of the sky it will show from the places you sit and stand.
Window sizes are given as a width and a height of the frame in centimetres, with the manufacturer's code, such as a size code letter pair followed by a number. Common sizes range from narrow units of about 55 cm wide, through the 78 cm and 94 cm widths, up to wide units of 114 cm or 134 cm. Heights vary from around 78 cm to 160 cm. You choose a combination from the maker's chart.
How rafters limit the choice
In most roofs, rafters sit at roughly 400 mm to 600 mm centres, and the cheapest, simplest installation is a window whose width fits between two rafters. That avoids cutting a rafter and building a header. A wider window, or a stacked pair side by side, means cutting one or more rafters and trimming the opening with new timbers. That is quite normal in a loft conversion, but it needs checking by someone who understands structure, and it adds to the labour. Our page on enlarging a roof window deals with the case where a window is already in place.
Whether the rafters are old timbers, trussed rafters or steel is also a factor, because trussed roofs depend on every member and cannot simply be cut. The loft needs inspecting before anyone orders a window.
Pitch, sill height and the view
Window height affects where the sill and top edge fall inside the room. A common aim is for the bottom of the glass to sit low enough to see out while seated or standing, which on a loft room usually means a sill about waist height, with the top reaching near head height. A tall window on a low slope may have its upper edge too close to the ridge, while a very low sill can create a draught at floor level. On a steep pitch, a window can be shorter because the glass faces the room more directly. The slope must also sit within the model's limits, which our lowest-angle guide explains.
If the room is a loft conversion on a bungalow or a chalet-style house, the window may be asked to serve as a way out, which calls for a larger, escape-rated sash. See what an escape roof window is.
Bigger is not always better
More glass means more heat loss in winter and more solar gain in summer. Even a well-rated pane is colder than the insulated roof around it, so the bigger the glass, the more the room feels it. Cost climbs with size, too. Fitted prices published in UK cost guides run from about £680 to £1,300 for a standard window supplied and fitted, with one maker's guide quoting about £1,623 for a replacement and £1,906 for a new opening, including VAT. These are typical UK ranges for a single window, not Beacon prices. Your fixed quote follows the survey.
Two small windows can beat one large one in a long room, since they spread the light and are easier to shade. Our guide to roof window types compares the opening styles as well.
Sizing for two windows or a run
In a long loft room, one window in the middle leaves the ends dim. Two matching windows, set at equal distances from the gable walls, light the room more evenly and let you open one for a cross-breeze. In a gable-end room, spacing them to line up with the furniture layout helps: a window over a desk, one over the bed. A run of three narrow windows, side by side with a small gap, can also look better than a single huge one, and each is easy to shade.
When we survey, we measure the rafters, the pitch and the room, photograph the roof structure and suggest a size that fits without unnecessary cutting. The overall service is described on the roof windows in Basingstoke page.
Every roof is different. A free survey with photos settles it for yours.
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