Can You Replace a Velux With a Bigger One?

A like-for-like swap drops a new unit into the hole the old one left, with the same rafters, the same trimming and a matching flashing kit. A larger window does not fit that hole. The hole has to grow, and the timbers framing it must be rebuilt so they carry the load.

Rafters are the angled beams that hold up the battens and tiles, running from ridge to wall. They sit at even spacings, and the old window fits between two of them. Wider glass than that gap means one rafter is cut. Short cross timbers called trimmers are then fixed above and below the opening to take the cut ends, and a doubled rafter alongside carries the load. Without that framing, the roof sags over time.

Height matters as much as width. A taller window may need the head and sill trimmers moved, which disturbs the battens and tiles above and below the frame. Our answer on Velux window sizes shows how standard sizes relate to rafter spacing, and why the next size up is not always the easy one.

We begin in the loft, looking up at the underside of the slope. Rafter spacing, rafter size, any purlin or support close by and the trimming already round the window all show there. From that we work out how far the opening can widen before a structural timber gets involved.

Where a rafter has to be cut, or the timbers look undersized, a structural engineer's calculation may be needed, and your building control body may want to see it. That is a job for the owner and a professional to agree, not something to work out by eye. Photos of the timbers go to you, so the framing in the written price is easy to follow.

The covering matters too. Enlarging the opening disturbs a patch of tiles or slates, which are then cut and relaid around a new flashing kit. Our page on replacing a Velux window explains the like-for-like job that this one builds on.

The inside matters as well. A wider opening means new plasterboard linings and a wider reveal, so plan for redecoration around the window.

Altering a roof of a house is dealt with by Class C of the General Permitted Development Order. Two limits matter here. A new window may not rise more than 0.15 metres off the original slope. If it faces a side elevation it needs obscure glass, and the opening part has to stay shut unless it sits over 1.7 metres up from the room's floor. Flats and maisonettes are not dwellinghouses, so those rights do not reach them, and a listed building still needs listed building consent where the change affects its character.

An Article 4 direction, which some conservation areas carry, can alter the picture, so give the planners at your local authority your address and ask before ordering anything.

On the Building Regulations side, roof windows are listed by the Planning Portal as part of a house's envelope. Crawley's own building control team lists newly altered roof openings among work needing approval, which is why a larger opening in the Crawley area goes to them before anything is cut. Elsewhere, ask your own building control body. For a replacement roof window, Approved Document L (2021, Table 4.2) sets 1.4, or Window Energy Rating Band B.

To give a sensible figure we need to see three things: the existing window and its label, the loft side of the opening with the rafters showing, and the size you want. A tape measure across the glass and across the gap between rafters is enough to start.

Checkatrade's October 2026 window fitting guide gives a typical UK average of £1,250 for a new roof window and £1,025 to replace one. A bigger opening is dearer than a swap because of the timber work and the extra covering to cut around, so treat the replacement figure as a floor and not a forecast. Whatever you are offered, find out if VAT and the scaffold or tower are inside the figure, since both can move the total a long way.

After the visit you get one total in writing, backed by pictures of your own roof and its timbers, so nothing arrives as a surprise on the day. Should the structure rule the larger window out, we tell you straight and talk through which size will work.

Because the loft side is the part that tells us most, clear a path to the hatch and move boxes from under the opening. Our answer on Building Regulations for roof windows goes into the approvals in more detail, and the roof windows and skylights page covers the rest of our work.