Is it easier to replace a rooflight in the same opening than cut a new one?

An existing opening has been cut, framed and finished before. The rafters or joists are trimmed around it, the roof covering is sealed up to the old unit, and the plaster or lining inside is shaped to its depth. When the new unit takes the same position and size, all of that carries over. The survey checks that the trimming still suits the new unit's weight, but nothing has to be cut. On a flat roof the kerb and the flashing around the old unit are looked at too, because they are part of what the new one sits on.

A new opening starts from a sound roof and a blank ceiling, so every one of those stages has to be created.

On a house, a new rooflight usually counts as permitted development, within limits. The full list, including side-facing slopes, is on the planning permission page. A new opening is a new alteration to the roof, so those limits and any local controls are checked for it. A replacement unit that sits no further out than the one it replaces does not change the shape of the roof, but we still check the projection against the original roof at survey.

Both jobs usually come under the Building Regulations. A new opening generally calls for approval: the roof gets cut, the hole is trimmed and supported, the roof has to take the extra weight, and a position near a boundary brings fire performance into it. The unit's energy performance counts as well. A swap of a window or roof window usually needs approval as well, though the structural questions mostly fall away. Replacing only the glass, or a window in its existing frame, is not providing a controlled fitting, so the energy limits do not apply to that work. The Building Regulations guide has the detail. Where the work is notifiable, we handle the Building Regulations notification with the council's building control team or a registered building control approver. That choice between the two is yours.

This is the biggest practical gap. In a new opening, rafters or joists are cut and the loads they carried are picked up by new timbers either side of the hole. The size and spacing of the existing timbers set how big an opening can be and where it can go. Where the opening stays, the trimming is already there, so a new unit of the same size is simply set into it. If the replacement is bigger or smaller, the timbers have to be looked at again, and that is how a swap can turn into something nearer a new opening. The roof must also carry the weight of a heavier unit, such as a larger pane or a triple-glazed one where a maker offers it.

A new opening means a new well from roof to ceiling, with insulation, board and plaster, and a ceiling that needs finishing around it. A swap leaves the existing well in place, though the sides may need touching up where the old frame came out. How far the finishing goes is a question for the survey, whichever job it is.

We do not promise how long a job takes: the roof, the access and the room all play a part. What can be said is that the new opening has more stages: access, cutting, trimming, weathering, lining and finishing, in that order. The swap has fewer, because the opening and the finish are reused. That is why a swap is the smaller job. The details on cost are on what it costs to replace a skylight.

Start with the room. If the light is in the right place but the unit is tired, misted or hard to open, a swap keeps the opening and lets you change how it works: a rod or a motor instead of a hand reach, or a blind made for the room. Who opens it, and how high the ceiling is, should lead the choice. If the light is in the wrong place, a new opening is the answer, and the survey is the point to decide. Pictures of the room and roof sent to WhatsApp 07880 284133 make a start, and the free survey then shows which job you have. On any job, the 10-year workmanship guarantee comes in writing.