Short answer
Usually yes, if the new window is the same size as the old one. Current VELUX windows are sold in sizes that line up with the older numeric and pre-K codes, so a like-for-like swap can go into the same opening. What needs checking is the flashing generation and the frame depth, because an older opening can need the right flashing kit or an adaptor before a new unit sits properly.
Old VELUX size conversion: how older codes line up
The first VELUX windows were labelled with plain numbers, for instance 104, 206 or 304. Later windows moved to the letter-and-digit system, in which a letter stands for the frame width and the number for its height, as in MK04 or CK02. The older numbers were not random: each corresponds to a current size, and the mapping is published by the manufacturer.
That is why the first job is to read the data plate. Our answer on finding the size code on your VELUX shows where to look. With the old code in hand, the current equivalent can be found, and the survey checks it against a measurement of the frame so that nothing rests on a table alone.
Between the numeric era and the present there was also a period of letter codes with a leading K for the newer frame generation. A window from that period may show a code that looks current but carries a different frame detail. The plate and a tape measure together settle it.
Replacing a pre-2014 velux rooflight like for like
Replacing a pre-2014 velux rooflight with the same size is the simplest case. The old frame comes out, the new frame goes into the same opening between the rafters, and the new flashing dresses over the roof covering. Because the width and height match, the rafters, the lining and the plasterboard around the opening do not need to change.
The new frame is often a slightly different depth from an older one, and the thickness of the roof covering matters as well. Slate, plain tile and profiled concrete tile each have their own flashing, so the roof covering is recorded on the survey. A like-for-like swap in a Victorian terrace with slate is a different kit from one in a 1970s estate house with interlocking tiles.
Swapping the whole unit counts as building work that Building Control must hear about, and this note on notifiable replacements explains why. Where that applies, we send the notice ourselves and the written plan says which body receives it.
Inside, a like-for-like swap leaves the plasterwork and the lining board alone in most cases. Because the new frame has the same outer width and height as the old one, the existing splay and the ceiling finish stay as they were, and the only visible change on the day is a clean new sash. If a blind or a trim was fitted to the old frame, the plan notes whether it moves across or has to be replaced, since blind brackets are specific to a frame generation.
When an adaptor or a different flashing kit is needed
Three situations call for something extra. The first is a frame size that no longer has a direct current equivalent, where the nearest size is a little larger or smaller than the old opening. The second is an old flashing that was designed for a different roof covering, now changed. The third is a pitch or covering that the old kit never anticipated.
In each case the answer is a matching flashing kit for the new window and the covering, and sometimes an adaptor part that bridges the difference in frame depth. These are ordered together with the window, from the survey measurements, so the whole set arrives for the fitting day. The guide to VELUX flashing kits covers the kit types in detail.
- Nearest current size slightly different from the old opening
- Roof covering changed since the first window was fitted
- Frame depth differs between old and new generations
- Roof pitch outside what the old kit was made for
When the opening itself needs to change
Sometimes a homeowner wants to go larger, or the closest current size is wider than the old gap. A wider window may need a rafter trimmed or a trimmer added, which is structural work. The rafter spacing, often around 600mm at centres, sets how wide a unit can go without that.
Neither change is a surprise if it is found early: a wider opening is costed as a separate line in the quote, with the carpentry described, so the extra is visible before you agree to it. A smaller change is possible too. A narrower new window can be fitted in a wider opening with a modified lining and a suitable flashing, though that is more of a rebuild around the frame than a swap. The survey decides which of these applies, and the written quote shows the difference in plain terms. Our size code guide helps when you are comparing options.
How the survey confirms it before anything is ordered
On the free survey we read the plate, measure the outer frame, check the roof covering, record the pitch and look at the rafters either side. Those readings are entered in the written plan alongside the flashing kit, and you receive it with the quote, well before anyone asks for payment.
Where the plate has gone, the survey measurement stands in for it, and we say so in the plan. Either way, the order is placed against a confirmed size, so a wrong opening is found at the survey stage and not on the roof, and the delivery arrives with every part that the fitting needs. Ordering from a number remembered from an old invoice is how the wrong frame turns up, and the sequence is built to avoid it.
If you want to see how the whole job unfolds, the VELUX window installation page follows it from survey to sign-off.