Can You Fit a Bigger VELUX in the Same Opening?

The opening in your roof is not a freehand cut. It sits between two rafters, with a short timber above and below holding the frame in place. The window you see is the result of those four edges, so the room for a bigger unit is whatever slack the timbers leave.

On a Victorian terrace the rafters are commonly set at spacings that suit the roof covering rather than a modern window size, so the gap can be generous or tight. A 1930s semi tends to be more regular. The only reliable way to know is to measure at the survey, with the old lining and insulation pulled back so the timbers are visible.

Every VELUX size is a letter followed by digits, for example MK04 or CK02. Read the letter for width and the digits for height. Move along the alphabet and the window gets wider; raise the digits and it gets taller. Our page on VELUX size codes sets out how to read them.

That gives you a quick test. Going up in height only is often the easier step, because there is usually room between the cills and the lintel timbers, whereas going wider is the move that meets a rafter. If you want more glass without a structural change, a taller unit of the same width is the first thing to look at.

• Same letter, taller digits: height changes, width stays inside the same rafter gap.
• Wider letter, same digits: the width pushes towards the rafters on either side.
• Both larger: very likely to need a trimmed rafter.

One more check is the depth of the rafters. A bigger frame sits deeper in the roof build-up, and the insulation around it has to be carried across without gaps, so the thickness of the existing timbers and the insulation already in place are both noted at the survey. If the rafters are shallow, the lining may need to be built out slightly so the new unit sits flush inside.

On a roof that has already been stripped for a loft conversion, the picture changes again. Open rafters and a bare roof space make a wider opening far simpler to plan, and it is worth deciding on the larger size before the boards and plasterboard go up rather than after.

If the bigger window will not sit between the existing timbers, we can still fit it. One rafter is trimmed back, and a doubled-up header is fixed above and below to carry the load across the gap. The neighbouring rafters share the added load, and the new unit is framed in snugly.

A bigger opening like this is building work. The rafters need trimming with proper support, and the finished work must meet current regulations. Because a full unit swap is notifiable, Building Control is notified by us when the job calls for it, and the new unit has to meet the roof window standard in Part L: 1.4 W/m²K, or Window Energy Rating Band B.

It also means more work inside. The splayed reveal is rebuilt and finished around the larger frame, which is why the job takes longer than a swap and costs more. Our page on VELUX window fitting cost shows how that difference appears on a quote.

A larger roof window still falls under Class C permitted development for a house in England, provided it does not stand more than 150mm proud of the roof slope and stays below the top of the roof. Making the glass bigger does not change those limits, but it does make a side-facing slope harder to deal with, because a side-facing sash is restricted to obscure, fixed glass unless its opening part sits over 1.7m up.

Flats and maisonettes have no such rights, and listed buildings need listed building consent. In a conservation area the council may have taken the rights away, so ask it before ordering. Bristol City Council covers Clifton and Kingsdown, and Bath and North East Somerset Council covers the World Heritage city.

Before widening, look at what the room needs. A landing or stair void often gets more from a second window than from one oversized unit, and an opening model fitted for fresh air makes a loft easier to live in on a warm night. Our VELUX window installation page shows the options, and opening alternatives appear on every written quote.