Short answer
What it costs to replace a rooflight depends on which job you have: new glass in the existing frame, a like-for-like unit in the existing opening, or a different size that means altering the opening. The survey settles that, and a fixed written quote follows.
Free survey / fixed written quote / Building Control notification handled / 10-year workmanship guarantee
Three jobs that all get called a replacement
People ask for a replacement when a unit has misted, when it has failed, or when it is simply tired. Underneath, those are three different amounts of work. A misted unit in a sound frame sits at the first rung, and a unit whose frame has given up sits at the second. A change of size or type is a decision rather than a fault, and it is priced as a new opening because it is one.
From least to most
- New glass, same frame The sealed unit changes and the frame stays, where the frame is sound.
- Whole unit, same opening The unit is swapped for one of the same size, so the opening and the roof around it stay as they are.
- Different size or type The opening is altered, and the roof structure and the surround change with it.
Reading what is already in the roof
The unit already in the roof tells us a good deal. With a VELUX roof window, swing the sash open and run your eye along its top edge: a metal plate there, fitted to every VELUX made after 1968, carries the size marking, and the size code on it tells you the width and height. The guide to VELUX size codes reads the marking, and the page on a like-for-like VELUX swap follows it through.
On a flat roof the unit stands on its own kerb or upstand, so the survey measures what is on the roof. If your unit has misted, the guide to sealed units or a whole rooflight helps you choose between the first two jobs, and misted rooflight glass cost looks at the glass alone.
Where a swap saves work, and where it does not
A unit that goes into an opening already cut can skip the structural work a new opening needs. That holds while the frame, the flashing or kerb, and the roof around them are sound. If the surround has been damaged, or the new unit is a different size from the old one, the job grows and the figure with it. A bigger unit in the same position can mean cutting joists that were never cut before.
Flat roofs raise a further question. Where water lingers on a flat rooflight after rain, the fall around it needs looking at, and the answer on water sitting on a flat rooflight explains why. The survey notes it before a new unit is priced.
What Wales asks of a replacement
Welsh guidance sorts the work by what changes. Where a window or roof window is swapped like for like, that is building work; a repair is not. Glass-only changes in a frame that stays are not counted as building work by the energy rules.
The limits for the new unit sit in Approved Document L (Wales). A replacement roof window should be no worse than 1.4 W/m²K or Window Energy Rating band B, and a replacement rooflight no worse than 2.2 W/m²K. Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver.
| Topic | What the guidance says |
|---|---|
| Ventilation | Trickle vents no smaller than the old ones, controllable by the occupant or automatically |
| Escape | Where a new house would need an escape window there and the old one was big enough for escape, the new opening should give at least the same escape potential |
| Glass in reach | Approved Document N covers glazing safety in Wales |
Planning, and the quote
A swap in the same position raises a smaller planning question than a new opening, though a unit that stands further out from the roof than the old one can bring it back. The 150 mm projection limit and the protected-land conditions are in the planning guide, and the Building Regulations guide covers the rest.
Nothing is priced before we have seen the roof. You get a free survey, then a fixed written quote, with the workmanship guaranteed for 10 years in writing. The costs page shows how pricing works more widely, and the answer on fitting a new skylight covers the other side of the coin. Ring 01792 929062 or use the survey request.