Why a same-size swap normally works
The width and height of a VELUX opening have carried over from one generation to the next, so the gap between your rafters is almost always the right gap for a current window.
When VELUX changed its range in 2001 and again in 2013, it changed the glass, the frame profile, the insulation and the way blinds and flashings attach. It did not change the basic sizes people had already built into their roofs. An M04 from 2008 and a current MK04 belong to the same size family, and the same pattern runs through the range, from the small CK02 to the wide UK04. That continuity is deliberate. It means a window in a loft off Mill Road or a 1930s semi in Cherry Hinton can be renewed without anyone taking a saw to the structure.
So the question is rarely whether the new window is the right size. It is which code you have, how deep the old frame sat, what the roof covering is, and whether anything around the opening has been altered since the first window went in. Each of those is checkable before anything is ordered, which is why we look at the data plate and the lining at the survey rather than guessing from a photo of the outside.
For the wider picture of what changes when a window is renewed (glazing, blinds, finish), see our guide to replacing an old VELUX window. This page sticks to the fit.
Matching your old code to a current window
The size code on the data plate, found on the top of the frame when the sash is open, is the quickest route from your old window to its current equivalent.
Before 2001, codes were numbers only, such as 104, 304 or 606. From 2001 until 2013 they were a letter and two digits, such as C04 or M06. Current windows add a K after the letter, so MK06 or SK06. The letter tracks the width and the digits track the height, which is why the later codes line up so neatly with the earlier ones. Our guide to VELUX window sizes and codes explains how to read a plate in full, and how to identify your VELUX window generation helps if the plate has faded or been painted over.
What you find on the plate points to the likely route:
| Old window | Current match | Usual flashing | Lining |
|---|---|---|---|
| Current code (MK06) | Same code | Standard | Stays |
| 2001 to 2013 (M06) | MK06 | Standard | Usually stays |
| Pre-2001 (306) | Close size, confirm | Replacement type | Stays with the right flashing |
| No plate, odd size | Measure first | Depends | May need work |
A pre-2001 number corresponds to a current size closely enough for a straight swap in most cases, but that is the one generation where we measure the frame as well as reading the plate before ordering.
Depth, flashing and keeping the old lining
A current window can fit the width and height of an old opening and still sit at a different depth in the roof, and the flashing is how that difference is handled.
Windows made before 2001 were designed to sit deeper in the rafters than a current frame. Fit a new window with its standard flashing and it rides at the newer, higher line. Outside, that looks fine. Inside, the plaster reveals that used to meet the old frame now stop short of the new one, leaving a gap to make good. For tiled and slate roofs VELUX makes flashings intended for replacing pre-2001 windows, which let the new frame sit at the old depth so the lining, the plaster and the paint around it stay as they are. Our guide to VELUX flashing types sets out which flashing suits each roof covering.
Windows from 2001 onwards sit much closer to current depth, and a standard flashing for the roof covering is normally right. The frame goes back onto the same rafters, the underlay collar is renewed, and the new window meets the old reveal.
When the old opening will not take a new window as it stands
A handful of situations turn a straight swap into a small alteration, and all of them show up at a survey.
- The old window was not a VELUX. Other brands use their own sizes, so the opening may sit between two VELUX codes and need packing or trimming.
- The rafters or trimmers were altered. A previous owner may have reframed the opening, or a loft conversion may have added steel or a new purlin close to the frame.
- The roof covering has changed. A roof re-covered in slate over what was once plain tile needs a flashing for the covering that is there now, not the one on the original order.
- The battens or timber around the frame need renewing. That is dealt with as part of the replacement, but it affects time on the day.
There is also a choice worth making on purpose. The same opening is not always the right opening. If the old window lit the ceiling and left the desk in shade, or put a south sun patch across a bed on summer mornings, a replacement is the moment to move the window to a better position or fit a bigger VELUX. Our light planning page explains how orientation and position decide where the light lands.
What we check before ordering
Every replacement starts with the plate, a tape measure and a look at the lining, so the window and flashing on the order are the ones the opening needs.
At the survey we read the size code and the type code, measure the outer frame, note the roof covering and pitch, and look at how the reveal meets the frame inside. We also look at the slope's orientation, because the glass choice matters as much as the fit: a south or west slope in Cambridge sees long, unshaded afternoons over flat land, and a solar control pane or an external blind can make the room far more comfortable than the old window ever did. All of that goes into the written light plan that comes with the quote, alongside the flashing type and whether the lining stays. You can book a light survey or message us on WhatsApp with a photo of the plate to start. The main VELUX window installation page covers the full service.
Questions people also ask
Will my old VELUX blind fit the new window?
Generally not. Blinds are made for a specific size code and generation, and current frames have their own brackets and side channels. The VELUX blinds guide helps you choose a new one for the slope and room.
Can I change from centre pivot to top hung in the same opening?
Often, yes, because both types share the size codes. Check the head height and sill height first, since a top hung window opens outwards from the top and its handle sits at the bottom, which changes how it is reached and used.
Can the new window be electric if it goes into the old opening?
Yes. An electric or solar window in the matching size code fits the same opening, and a solar model needs no cable run through the lining. A mains window needs a power supply planned in before the lining is closed.
Does a like-for-like replacement need Building Control?
Replacing a window is normally notifiable work because the new unit has to meet current thermal standards. We handle the Building Control notification where the work is notifiable and install to current Building Regulations.
