The short answer
The right Velux size depends on three things: the gap between your rafters, the height of the room and what the room is for. Each window has a size code that gives its width and length. For a straight swap, use the code on the existing window. For a new opening, choose a size that suits the rafters and, in a bedroom, the escape rules.
Bigger is not always better. The best size is the one your rafters, your ceiling height and the room's purpose will take without extra structural work.
Reading the size code
Roof windows are sold by code, a short group of letters and numbers that stands for the width and length of the frame, and often for the type of window too. The code is on the label on the frame. Ask for it, because every blind, flashing kit and replacement part is made to match it.
Can I replace a window with a different size?
A like-for-like swap keeps the same code, so the hole in the roof stays the same and no framing changes. Going bigger or smaller is possible, but it changes the opening, and that can mean new trimming timbers, new flashings and a revised lining inside. Going smaller is simpler than going larger, since a smaller frame fits inside the old opening, although the gap round it must be filled and sealed neatly.
The flashing kit is chosen to match both the code and your roof covering, so tiles, slates and profiled coverings need different kits. Our page on the Velux flashing kit explains why the match matters, and it is where size and covering meet.
Rafter spacing and the structure
The rafters, the sloping timbers under the covering, run at regular intervals. A window that fits between two neighbouring rafters can go in without cutting any of them, which keeps the structural work light. A window wider than the gap means one or more rafters are trimmed, with new timbers carrying the load round the opening.
How do I find out the rafter spacing?
Look in the loft. Measure from the side of one rafter to the side of the next, in several places, because old roofs are not always regular. Photograph the loft side of the roof and send it to us before we visit. In a roof with trussed rafters, the factory-made timber frames common on newer houses, the structure is more rigid and cutting is a bigger decision that may need an engineer's advice.
That is also why we survey first. Spacing, timber condition and any old repairs shape what size is practical, and we put the answer in your written price. Our answer on fitting a new roof window sets out how a new opening is planned.
Room height, sill height and the view
Where the window sits on the slope matters as much as its size. A window that is too low gives a view of the garden but gets in the way of a bed or furniture. One set too high costs you the view and makes opening it a stretch. The height of the room under the slope often decides the length you can use.
How high should the sill be?
Sit on a chair or stand in the room and see where your eye falls. In a loft room with a low eaves wall, a longer window lets light reach further into the space. A taller person wants the top of the opening higher, and the controls need to be within reach. Think about where the bed, desk or wardrobe will stand before the frame goes in, since moving the window afterwards means opening the roof again and repeating the flashing work.
Do not forget what happens outside. The window must clear the ridge, the valleys and any chimney, and leave room round it for the flashing and for a safe fix to the covering. We check these on the roof itself, since a measurement taken from the ground can be misleading.
Bedrooms and escape windows
If the room is a new loft bedroom, the window may also need to work as a way out. Approved Document B, the fire safety guidance, sets sizes for an escape window: a minimum area of 0.33 square metres, a minimum height and width of 450 mm, and a bottom of the openable area no more than 1100 mm above the floor.
Does every bedroom window have to meet this?
Those sizes apply to escape windows in loft conversions, and a window serves as the escape only for a storey within 4.5 metres of the ground. In a two-storey house the stairs also come into it, since a protected stairway is the usual requirement. Our answer on roof window fire escape explains the detail, and your building control body decides what applies.
On cost, MyJobQuote lists £1,100 for a mid-range Velux and about £100 less for a plain swap (Sep 2026), typical UK ranges that move with size and access. Photos of the room, the loft and any existing label help us plan the visit, and the survey ends in a written total. The main roof windows and skylights page shows how it all fits together.
More on choosing a window size
Is a bigger roof window always better for light?
Not always. A larger pane lets in more light, but it also loses more heat and needs more structural work if it is wider than the rafter gap. A well-placed window of the right size often lights a room better than an oversized one set too low or too high.
Can two small windows replace one large one?
Yes, and it is sometimes the better answer when the rafter spacing is tight. Two windows may sit between rafters without any cutting, though each needs its own flashing and lining. The number of openings changes the work and the price, so we compare both on the survey.
The full picture: Velux roof window and skylight installation in Taunton · More short answers