VELUX size codes explained

What the code says

A VELUX size code has two halves. The letters give the width of the frame. The digits give the height. So in MK04, the M is the width and the 04 is the height. Swap either half and you have a different window.

The code does not describe how the window opens, what it is made of or how it is glazed. That sits in a separate type code on the same label, three letters such as GGL or GGU, which stands for the model and its finish. The size code is purely about the outside dimensions of the frame. Treat the two as separate questions: how big is the hole, and what goes in it.

Reading the letters and the digits

The letters step up in width, and the digits step up in height. The table below gives the nominal outer frame sizes for the codes you meet most often on VELUX window sizes in British houses. Wider frames exist beyond S, up to U at about 134. Treat the figures as a guide to the grid and confirm the exact figures on the maker's data sheet for your model.

So MK04 is a frame about 78 centimetres wide and 98 centimetres high. CK02 is the small one, 55 wide and 78 high, the kind that fits between closely spaced rafters. SK06 is wide and tall, 114 by 118, a window for a room that wants a lot of sky. The second letter, K in each of these examples, stays the same across the standard roof window range, so for sizing the first letter and the digits are what you read.

Write the number down with the measurements beside it. A code on its own is a label. A code and a tape measure, used together on site, are a check.

Where to find the code on your window

The code is printed on a label fixed to the window. On most models you open the sash and look at the edge of the sash or the side of the frame. There is also a copy of the label on the packaging and a reference in the paperwork that came with the window, if you still have it.

If the label has worn away, which happens on windows that are many years old, we measure the frame instead. We take the outside width and height of the frame, not the glass, and compare them with the grid. Old windows may be marked in a numbering scheme from before the present one, and the labels for those look different. In that case the measurement on site decides, and the label gives us the model.

It helps to copy the label down before the survey if you can reach it, though it is not essential. At the survey we check the label and the opening together.

Why the code matters for a replacement

Replacing like for like is the simplest job there is, because the existing opening already has the right shape. If the new frame carries the same size code, it fits the same rafter spacing, the same tiling battens and the same internal lining. No timber needs to be cut, no structural approval is needed, and the work is confined to the window and the flashing round it.

Change the code and the job changes. A wider window needs rafters trimmed. A taller one needs the covering opened up the slope and relaid. Either is a new opening in effect, which brings structural work under Part A and a different scale of paperwork. Our page on replacing an old VELUX window follows the like-for-like route step by step.

There is one catch. A like-for-like swap keeps the frame size but not necessarily the flashing. The kit that surrounds the frame is a separate size-coded part, matched to the window and to the roof covering. A new frame goes with a new kit, and we quote and record the pair together.

Spare parts, blinds and the code

The same code is how you buy parts. Blinds are made to the size grid, so a blind for a MK04 is ordered against MK04. Replacement sealed glass units, interior linings and flashing kits all carry the code of the window they belong to. Order the wrong letter or the wrong digits and the part arrives, and does not fit.

That explains why we write the code into the life plan for every window we fit. The plan lists the size, the type and the flashing kit, and says which wear parts can be bought separately later and what each is called. Whoever looks after the window in ten years, whether it is us or someone else, can order against the plan without climbing into the roof to read a label.

Blinds are not strictly part of the roof window, and a blind fitted years ago may be one of several generations. If you are matching an existing blind to a new window, tell us at the survey so we can check that the brackets still fit.

When the code is not enough

A size code tells you the frame, and a frame is only half of what makes a window fit. Four things the code does not cover are worth checking before you order.

The roof pitch. Each model is approved for a range of pitches. A window of the right size can still be the wrong model for a shallow roof.

The roof covering. The flashing kit comes in versions for slate and for tile, and the tile versions are made for different tile depths and profiles. Slate on a Sheffield gritstone terrace and concrete tiles on a 1930s semi need different kits. Our guide to flashing kits explains why.

The rafter spacing. A frame must sit within the gap between rafters, or the rafters must be trimmed. The code names the frame width and the roof supplies the gap. If the two disagree, something has to change.

The type. A centre-pivot and a top-hung of the same code occupy the same opening, but they open differently, and the handle moves from the top of the sash to the bottom. The guide to centre-pivot or top-hung covers that choice.

Size, light and the rules around it

It is tempting to think a bigger code means a better room. A larger window lets in more daylight, but England has no Building Regulations daylight rule for houses, so the size is your choice rather than a requirement. What the regulations do set is the energy performance of whatever you fit. For roof windows in existing houses, the figure to hit is 1.4 W/m²K or WER Band B (Approved Document L, 2021 edition). The code does not change that figure, and we check it against the label of the window we fit.

Where the window serves as an escape route, the size matters in a different way. An escape window needs a clear opening of 0.33 m² with 450 mm each way, and a small code may not give you that. Height decides the rest, so a loft above two floors relies on its stair, whatever size the window is.

Planning is a separate question again. For a house, the permitted development limit on how far a rooflight stands proud of the slope applies to how the frame sits, not to its size code. The detail is on the page about whether VELUX windows need planning permission and in our planning guide.

A worked example from a Sheffield roof

Take a loft room in a stone terrace in Nether Edge, with a single roof window set into a slate roof. The owner wants to know whether a new window will go into the same place. The old label reads MK04. That tells us the frame is about 78 wide and 98 high, and the first thing we do is check that against the tape, because a window that has been in for years may have been trimmed in on site.

The tape agrees. Next we measure the gap between the two rafters either side. A frame of that width sits in a normal rafter gap with a little room for the flashing and the insulation collar. We look at the slate, which will need lifting round the opening, and at the lining inside. The new window is another MK04, so the job stays within the existing opening. If the owner had asked for a PK or an SK in the same place, the rafter gap would have ruled it out, or a rafter would have had to be cut and trimmed.

That conversation, from label to tape to rafters, takes a few minutes on site. It is also the point where we find out whether the owner was really asking about size, or about something else. The real question may be the height of the sill, the amount of daylight, or whether the window can open further. The code gives us a way to talk about those without guessing.

What to do before you order

If you are planning a new window or a replacement and want to get ahead of the survey, three small steps help. Find the label and note both codes. Measure the frame outside edge to outside edge, not the glass. Note the roof covering, whether slate, plain tile or concrete tile, and the rough pitch if you know it.

Do not order from the code alone. A window is a family of parts, and the size is the part that is easiest to get right and the easiest to assume. Orders go wrong when the code is right and the type, the kit or the covering is not. A fixed written quote that names every part removes the guesswork.

What we do with the code at survey

At the survey we read the label, measure the frame to confirm it, and measure the rafter gap, the pitch and the lining inside. We note the code of any window you have already bought, and if that window is not the right size for the opening, we say so before an order is placed. The page on fitting a window you have bought covers that case in full.

When we fit the flashing we also stand behind it, and the flashing work is covered by our 10-year guarantee. The fixed written quote names the code, the type and the kit, and the life plan repeats them, so that every later order is made against a record, not a guess. The installation timeline for a typical job is set out on the page about how long a VELUX installation takes, and Building Control notification is one clause in it, which we handle.