Short answer
Yes, a rooflight can be fitted into an existing flat roof. The roof must be opened up and, where needed, strengthened, the glass must satisfy the energy rules, and the planning and Building Regulations position has to be settled before any work starts. Some Welsh houses need a planning application and some do not, so the survey establishes which applies to yours.
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What the roof has to allow
A flat roof is the easiest place to put a rooflight, since there is no slope to cut across. The opening still means work underneath. Cutting a hole is a structural change: the Welsh Government notes that the opening will usually mean altering the roof's structure. The weight of the new unit has to be borne by what is left, so any joists that are cut get framed round their ends, and the roof is reinforced where it needs to be.
Water is the other half. A flat roof is only flat to the eye, and the covering falls gently towards an outlet. A rooflight sits on a kerb or upstand above that surface, so the survey checks that rain leaving the glass has a clear route across the roof. Rooflight makers publish the pitches their products suit. A VELUX flat roof dome, for instance, is intended for pitches from 0 to 15 degrees.
Planning in Wales
Welsh permitted development rules for roof alterations differ from England's, and the difference matters for houses on protected land. On an ordinary house, a rooflight can be permitted development if it stands no more than 150 mm proud of the original roof plane and no higher than the highest part of the original roof. Where a house sits in a conservation area, the Gower AONB, Bannau Brycheiniog National Park or a World Heritage Site, a new rooflight on a roof slope is not permitted development and needs planning permission.
A rooflight on a kerb stands proud of the roof, so how far it projects is one of the figures we take on site. Our page on flat rooflight planning and Building Regulations in Wales gives the full conditions.
| Your house | Planning position |
|---|---|
| Ordinary house | Can be permitted development inside the projection and height limits |
| Conservation area, Gower AONB, National Park | Needs a planning application |
| World Heritage Site | Needs a planning application |
| Flat or maisonette | No householder permitted development rights |
| Listed building | Listed building consent where the work affects its character, through the local planning authority |
Rights that can be taken away
An Article 4 direction lets a council withdraw permitted development, and a planning condition set when the property was first approved can strip them too. Neither shows from the street. When you cannot tell, you can ask the planning department at your council, and a Lawful Development Certificate turns the reply into a formal one.
Listed buildings are separate again. Permitted development does not cancel the need for listed building consent, which is required where the work would affect the building's character. Consent reaches fixtures and details indoors as well as the outside, and your local planning authority is where you apply for it.
Building Regulations and the glass
Fitting a rooflight is building work. Approval under the Building Regulations will generally be needed for a new one, and where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver. If the rooflight is close to a boundary, how the unit performs in fire has to be weighed too.
The energy rules sit alongside. Approved Document L (Wales) says a new rooflight in an existing home should have a U-value no worse than 2.2 W/m²K, and it defines a rooflight as a glazed unit on a kerb or upstand. A roof window set in the plane of the roof should meet 1.4 W/m²K instead. Our guide to choosing a flat roof rooflight compares which type suits which roof.
When the roof is already tired
An old flat roof can complicate a new opening. A tired covering, a soft kerb on an earlier rooflight or a roof that has held water in one corner each change what the job involves. Our page on why water sits on a flat rooflight looks at what a puddle tells you, and the cost page explains what moves the price of a flat roof skylight.
To have the roof looked at, call 01792 929062 or request a survey online. The flat roof rooflights page shows the full range, with a written guarantee on our workmanship that lasts 10 years.