The short answer
Often, yes. The limit is the roof structure, not the window. A larger opening means cutting back rafters and building new trimmers to carry the load, and that structural work needs Building Regulations approval. Planning permission is normally not needed on a house if the window stays within the permitted development conditions, but flats and listed buildings are different.
Bigger is usually possible, with conditions
If your old window feels small for the room, you are not stuck with its size. Many homeowners swap a modest window for a wider or taller one, or combine two small ones into a pair. Whether it works depends on what lies behind the plaster, because the window can only be as large as the roof structure will carry.
Two different situations get mixed up here, so it helps to separate them. One is changing like for like, where the new window fits the existing opening. The other is enlarging the opening itself. Only the second involves new structure.
Like for like is the easy case
Each VELUX window has a size code, and a new window of the same code goes into the same opening with little disturbance to the roof. If you want to see how the codes read, our page on VELUX window sizes and codes explains the letters and digits. The code is printed on the data plate on the frame, which you can see when the window is open.
Stepping up one size in height sometimes still fits between the existing rafters. That can be simple. Stepping up in width is where the structure starts to matter.
What changes in the roof
A pitched roof is carried by rafters running from the ridge down to the wall plate. A window opening interrupts one or more of them. Where an opening is made wider than the gap between two rafters, a rafter must be cut and its load handed to new timbers on either side of the window.
- Trimmers are the horizontal timbers above and below the opening. They frame the hole and take the cut rafters' load.
- Doubled rafters sit either side of a wider opening to carry what the trimmers pass to them.
- Lining and insulation are reformed around the new, larger reveal so the room stays warm and the finish is tidy.
Some old roofs, particularly those with small-section timbers or purlins in awkward places, leave less room for change than they appear to from the room below. We check this at the survey, not on the day with the roof open.
Building Regulations apply
Widening the hole is a structural alteration, and the Building Regulations treat it as such, looking at the timber and at heat loss. The glazing target for the new unit is 1.4 W/m²K, or Window Energy Rating Band B. The notification is ours to make, and the certificate ends up in your handover folder.
If the structural change is larger than a simple trimmer, a structural calculation may be needed so that the Building Control body can sign off the design. We tell you at the survey if that is likely.
Planning permission
Size of glass is not what planning looks at. The tests for a house concern how far the unit protrudes (no more than 150mm from the roof plane) and whether it rises above the ridge or highest point. A wider pane passes or fails on exactly the same terms as the narrow one it replaces. Facing a side slope adds a condition: obscure glazing and no opening, unless the opening section is over 1.7m from the floor.
A flat or maisonette cannot rely on permitted development here, so an application is needed. Listed buildings call for listed building consent, and a conservation area may be covered by an Article 4 direction on particular streets. The skylight planning permission guide lays out each case, and your council has the final word.
Bigger windows and bigger rooms
A larger window brings more daylight and a wider view, and in a small loft room it can change how the whole space feels. It also changes how the room behaves. More glass means a little more heat can leave in winter and enter in summer, so we often suggest a blind at the same time. Our page on VELUX blinds and accessories covers the options.
Enlarging the window of a sleeping space raises escape questions. Approved Document B only credits an escape window within 4.5m of the ground, which suits a bungalow or chalet bungalow with rooms under the slope. Above that height, such as a loft conversion over two storeys, the answer is a protected stair, and no size of roof window changes that.
Combining windows
Where one tall window is not practical, two windows side by side or one above the other can bring in the same light. Each opening is framed separately, and the pair shares a flashing kit designed for that arrangement. This can suit a roof with a chimney breast or an awkward rafter run.
What happens at the survey and the fit
Our free survey starts with a tape on the slope and a look at rafter spacing from inside the loft, then a talk about where the extra light should fall. Costs come back itemised, alongside a plan for each day of the fit. Dust sheets go down over floors, stairs and furniture, and a screen goes up around the room if the layout allows. Rain cannot get in overnight because the opening is closed in every evening. We clear our own rubbish, and the same crew stays from the first cut to the handover walk-through.
You can read about the wider service on our VELUX window installation page.
We measure the slope and check the rafters at the free survey, then tell you how big the opening can go.
Get a free quote