Planning answer
Can a flat rooflight go on a back extension or outrigger?
Yes. A flat roof over a back room suits a rooflight well, though the planning route turns on house versus flat. On Tyneside that difference is a big one.
The answer
On a house, a flat rooflight on a back extension is permitted development under Class C if it projects no more than 150mm beyond the plane of the roof and sits no higher than the highest part of the roof. A Tyneside flat is a flat, with no householder permitted development, so a new rooflight on its outrigger needs planning permission.
What is the rule for a house extension?
Class C permitted development covers the rooflight if the whole installation stays within 150mm of the roof plane and no higher than the highest part of the roof. Check for an Article 4 direction or a planning condition.
A rooflight on a house is the sort of change Class C of the permitted development rules allows without an application. On an extension with a flat roof, the plane of the roof is the roof surface, so the allowance is measured from the covering to the top of the installed unit. The upstand counts too, so a slim unit on a short rim is the one with a chance of fitting.
What can take Class C away
An Article 4 direction, or a condition attached to an earlier planning permission, can remove the right from a house. An extension that was itself built under a planning permission deserves a check, as the decision notice can restrict later alterations. A conservation area on its own does not remove Class C for a rooflight on a house. A listed house keeps Class C on the planning side, but almost any rooflight on it needs listed building consent, and that consent decides the job.
Higher units
A lantern or a tall opening unit usually stands above 150mm and needs a planning application. Our answer on lantern planning permission explains why.
What about the outrigger of a Tyneside flat?
A flat has no householder permitted development, so planning permission is needed. The deeds say who owns the outrigger roof, and the other flat's owner may need to agree first.
The back offshoot of a Tyneside flat is often the flat roof that people want to light. Planning law treats a flat as a flat, upstairs or downstairs, and flats and maisonettes have no householder permitted development. A new rooflight on the offshoot needs planning permission from the council, even if the same unit would be within the 150mm limit on the house next door.
Whose roof is it?
Your title or lease is where the answer sits, naming the roof's owner and any need to get the neighbour's consent first. Read it first, and have that conversation with the neighbour before anything is ordered. A solicitor can untangle wording that is unclear. Our job is to read the roof itself.
The council
The council covering your address receives the application. For each home type in turn, see our guide to rooflight planning permission, while our flat roof rooflights hub deals with the glass.
Which house is yours?
Upstairs flat
- Who decides
- Whoever the title or lease names, and the downstairs owner may have a say.
- Which rules apply
- No householder permitted development, so the council's planning permission is needed for a new rooflight.
Downstairs flat
- Who decides
- Whoever the title or lease names for the offshoot roof. The title may still bring in the neighbour above.
- Which rules apply
- No householder permitted development. A new rooflight starts with a planning application.
House
- Who decides
- The owner of the house, subject to any condition or Article 4 direction on the property.
- Which rules apply
- Class C applies within 150mm of the roof plane and no higher than the highest part of the roof.
Can the extension roof take the opening?
Usually, once the joists are read. The opening is framed with trimmers and doubled joists, and a wide one may need a beam. The work is notified to Building Control and built to Part L standards.
A back extension roof is a set of joists running across the short span, with insulation and a covering above. A rooflight needs a hole cut through all of that, and the joists cut must be picked up by trimmers fixed across the opening.
What we read at the survey
We check the joist size, span and direction, and the walls around the roof, such as a parapet or the house wall behind. From these we learn the widest opening the roof can frame and the margin left for the upstand and covering. Larger openings may need a structural calculation.
Notification and performance
Cutting a new opening in a roof is building work, notified to Building Control, either the local authority or a registered building control approver. Where the work is notifiable, sending the notice is part of our job. For an existing dwelling in England, Part L sets a limiting U-value of 2.2 W/m²K for a rooflight, and 1.4 W/m²K for a roof window.
What the quote carries
Opening, upstand, covering and notification are priced as one job, set out in a written quote after the free survey.
Does it change if the extension is not built yet?
Yes. The rooflight becomes part of the extension design, so its position, its height and its structure are settled together, and planning for the extension covers it where an application is made.
If the extension is still on the drawing board, the rooflight is not a separate job. Its position sets where trimmers and beams go. Its height feeds into the overall roof height. If the extension needs a planning application, the rooflight should be on the drawings that are submitted, so that the decision covers it.
Getting the order right
It helps to choose the rooflight before the joists are laid. The opening can then be framed in from the start, and the upstand and fall can be planned together. Our page on fall around a flat rooflight shows why the two go hand in hand.
If plans change
An extension design often shifts between drawing and build. A rooflight that has been planned with the structure can move a little with it, provided the new position still clears the joists, the parapet and any upstairs window.
Back-room rooflight queries
Does an upstairs flat have a flat roof of its own to light?
Sometimes the roof over an outrigger belongs to one flat and sometimes it is shared. That is written down in the title or lease, so read it first, ahead of any order.
Is a side-facing roof slope treated differently?
On a house, a rooflight in a side-elevation roof slope must be obscure-glazed throughout and non-opening, unless the parts that open are more than 1.7 metres above the floor of the room. A flat roof is a different case, and we read each one at the survey.
Who handles Building Control for a rooflight on an extension?
Our team does, whenever the work is notifiable. The notification goes to the local authority or a registered building control approver, and it is built into the fixed written quote that follows the free survey.