Can a VELUX window be replaced like for like?

When a whole window comes out, the opening in the roof and the rafters either side of it stay exactly where they are. That is the saving over a new opening: no structural trimming, no new hole in the ceiling, and usually no new lining frame.

The parts that change are the window itself (frame, sash and glass) and the flashing that weathers it into the roof covering. A flashing kit is made for one frame and one roof covering, so a new window arrives with its own. Reusing the old flashing around a new frame is false economy, because the two are no longer a matched pair. If you want to see how the kits differ, our guide to VELUX flashing kits explained sets out the three families and the roofs they suit.

The interior trim around the frame is the middle case. Sometimes the existing lining can stay, sometimes it needs adjusting, and the decision is made at survey, not on the day.

Size is the part people worry about most, and it is the easiest to settle. Every VELUX window carries a size code, a letter for the width and two digits for the height, for example MK04 or CK02. A current window of the same code fits the same opening in the roof. The code is on a label on the sash edge or the frame, usually visible when you open the window, and our short answer on how to find your VELUX size code shows where to look.

If the label has faded or been painted over, the opening can be measured instead, and the code worked out from the dimensions. Older windows from earlier ranges may not map neatly onto a current code. In that case the nearest current size is chosen, and the flashing and any trim take up the small difference. That is a conversation to have before ordering, not a surprise to discover with the old window already out.

For a wider look at how codes work, see VELUX window sizes and codes explained.

A like-for-like swap can also keep the way the window works. VELUX windows are either centre-pivot, hinged halfway up the sash so the top swings out and the bottom swings in, or top-hung, hinged along the head and opening outwards like a door on its side. The two behave differently: a centre-pivot window turns over so the outside pane can be cleaned from inside, while a top-hung window gives a wider view out and more headroom at the sill, which suits a low eaves wall.

The product codes follow the same split. GGL is centre-pivot in white-painted pine, GGU is centre-pivot in white polyurethane, GPL is top-hung in white-painted pine and GPU is top-hung in white polyurethane. Polyurethane is the usual choice for kitchens and bathrooms, where steam and splashes are routine. A swap is the moment to change type if the old one never suited the room, as long as the opening size allows it.

Glazing has moved on. A window fitted a couple of decades ago is likely to have a plain double-glazed sealed unit, and the swap is the chance to specify a modern energy-efficient pane, laminated on the inside for good practice overhead, or acoustic glass if traffic or rain on the roof bothers you. Our comparison of VELUX glazing options covers what each choice means in the room.

The new window also has to reach the limiting U-values set by Approved Document L. That is a legal minimum for replacement windows, not an upgrade you have to ask for, and it is one reason a whole-window swap is a more considered job than replacing a pane.

Three situations call for something other than a straight swap.

The window is too small for the room

If you are replacing anyway, a larger window gives more daylight, but a bigger opening means cutting rafters and trimming the ceiling. That becomes a structural alteration, covered by the same Building Control notification, with a different finish inside.

The roof covering has changed

A re-roofed house with a thicker slate or a different tile may need a different flashing family from the one the old window had. The window can stay the same; the kit may not.

Only the glass has failed

If the frame and sash are sound and the sealed unit has misted, replacing the glass unit alone in the existing frame is a smaller job, and it is not notifiable. Whether that suits depends on the age and condition of the frame, which we look at on the survey.

When the whole roof window comes out, frame and glass together, Building Control must be told, because Part L governs windows as controlled fittings. The notification is part of our service. Planning is a separate matter. A like-for-like swap on a house in the same position rarely changes the roof's appearance, and rooflights on houses are usually permitted development. A listed building is different, because external changes need listed building consent, and a conservation area with an Article 4 direction can remove the usual rights. In Exeter that is worth checking on a Georgian terrace, and on the Dartmoor edge you should ask the national park authority. The local planning authority can confirm quickly.

This is the point at which a finish schedule matters. Taking out an old window disturbs the plaster and paint around the lining, however carefully it is done. Old frames are often bedded in with filler and foam, and the new frame sits fractionally differently. The question is what the edge looks like afterwards: a clean line at the plaster, or a gap and a smear of filler.

Each Brightview quote includes a finish schedule. It describes the ceiling and roof construction, what happens to the lining, and the final decoration, down to whether the reveal is left prepared for your own paint or fully painted. For a swap, that means the lining is checked, adjusted where needed and made good before we leave, with dust managed throughout.

Have these to hand: a photo of the room showing the window from inside, a photo of the roof if you can reach a safe view of it, and the size code from the label. If you cannot find the code, send what you have and we will measure on the survey. For the wider picture of fitting these windows, see VELUX window installation, or ask for a quote and we will arrange the survey.