Keeping the same opening: what stays and what changes
Matching the existing size keeps the roof structure untouched, so the new skylight arrives as a straight upgrade of glass, frame and seals.
On a flat roof, the old unit comes off its kerb and the new one sits on the same upstand, or on a new timber upstand built to the same internal dimensions if the old one is low, soft or below the 150mm the trade expects above the finished roof. On a pitched roof, a roof window of the same size drops into the same gap between the rafters, and the internal lining can usually stay. Older VELUX windows fitted before 2001 used a different size and depth system, and the replacement flashings made for them let a new window sit at the old installation depth without cutting into the plaster.
Same size does not mean same light. Modern units have slimmer frames, clearer glass and low-E and solar control coatings the old one lacked, so the sun patch on the floor may be a little larger and cooler in summer. Our page on whether a new rooflight will be brighter than the old one explains where that gain comes from and when a coating takes some of it back.
Going bigger: more glass, more structure
A larger replacement rooflight means a larger hole, and that means cutting and trimming the joists or rafters that carry the roof around it.
On a flat roof over a Mill Road side return or a 1970s rear extension, joists usually run at 400mm or 600mm centres. Widening the opening across them means cutting one or more and adding trimmers either side to carry the load, sized so the roof stays stiff. Lengthening it along the joists is simpler, because nothing is cut across the span, only the kerb is rebuilt. On a pitched roof, a wider roof window means cutting a rafter and trimming the gap, while a taller one of the same width often fits between the existing rafters with only the battens and tiles or slates reworked.
Because this is a structural alteration, it is normally notifiable, and an engineer may be asked to confirm the trimming. The ceiling below is opened wider and the reveals are rebuilt, plastered and decorated. None of it is unusual work, but it turns a one-day swap into a longer job and it should be priced and drawn before the roof is opened.
Going smaller: less common, sometimes the better answer
A smaller skylight suits rooms where the old one gave too much glare or summer heat, or where a larger standard size was forced in years ago and never looked right.
Reducing a flat rooflight means building the kerb in from the old edges, adding timber noggins or joist sections to close the spare width, and insulating and covering the new solid area so it meets current Part L standards. Inside, the ceiling is extended to the new reveal and made good. On a pitched roof, a narrower window leaves a gap to infill with timber, insulation, battens and tiles, and a new lining inside.
It feels counterintuitive to fit less glass, but a south facing loft bedroom in a Romsey terrace can be a far better room with a smaller, well placed roof window and an external awning blind than with a large one that bakes it every afternoon. The light planning page shows how orientation and season set that balance.
Standard sizes, made to measure and the gap between them
Sometimes the old unit was a size nobody makes any more, so the question becomes how close a modern unit can get without structural work.
Pitched roof windows come in fixed sizes; a VELUX MK04 is 78 by 98cm and a CK02 is 55 by 78cm, for example. Flat rooflights and lanterns are sold both in standard sizes and made to measure. Where a standard unit is a little smaller than the old opening, the upstand can be packed in; where it is a little larger, it may still sit on a slightly widened kerb without touching a joist. The table sets out the usual outcomes.
| Situation | Usual route | Structure touched? |
|---|---|---|
| Same size available | Direct swap | No |
| Slightly smaller unit | Pack the kerb in | Kerb only |
| Made to measure match | Order to the kerb | No |
| Much larger opening | Trim joists or rafters | Yes |
| Much smaller opening | Infill and insulate | Yes |
For older flat rooflights, the guide to replacing an old flat rooflight covers upstand choices in more detail, and replacing dome rooflights covers the domes found on many 1960s and 1970s flat roofs.
Planning and Building Control when the size changes
A like for like swap rarely raises planning questions, while a change of size can, especially in a conservation area or on a front slope.
Under permitted development, a rooflight must not project more than 150mm from the roof plane or sit higher than the highest part of the roof, and that limit applies whatever the size. In a conservation area covered by an Article 4 direction, on a listed building or on a flat, a larger or differently shaped unit may need consent where an identical one would not. Greater Cambridge Shared Planning handles this for the city and South Cambridgeshire. Our answer on planning permission for a replacement rooflight goes further, and the guide to Building Control for replacement rooflights sets out when the work is notifiable. Where it is, we handle the notification.
Questions people also ask
Is it cheaper to keep the same size?
Usually, yes. A same size replacement avoids structural timber, extra plastering and in some cases an engineer, so labour and time are lower. The cost drivers for a replacement are set out separately.
Can I move the rooflight as well as change its size?
Yes. Resizing and moving often go together, since both mean a new opening. The guide to repositioning a rooflight when replacing it explains how the old opening is closed and the new one formed.
Does a bigger skylight need scaffolding?
Not always, but larger openings and heavier glass make proper access more likely, particularly on a pitched roof above the first floor.
How do I decide which size is right?
Start with the room, not the old unit. At the survey we look at the slope, the sun path and where you want light to fall, then set the size in the written light plan that comes with the quote. To start, book a light survey or read more about misted and failed rooflight replacement in Cambridge.
