Class C rooflights and the 150 millimetre rule

A rooflight is permitted development under Class C of Part 1 of Schedule 2 to the GPDO 2015 if it protrudes no more than 0.15 metres beyond the plane of the original roof slope, measured perpendicular to the roof surface, and if no part of it sits higher than the highest part of the original roof. That is the whole of the dimensional test, and 150 millimetres is less generous than it sounds once you add the upstand, the kerb and the frame of a standard unit.

What Class C covers beyond rooflights

Class C permits any other alteration to the roof of a dwellinghouse, meaning an alteration that does not enlarge the roof. Class B deals with enlargements such as dormers and hip-to-gable conversions. Class C picks up everything else.

So Class C covers rooflights, re-roofing in a different material, changes to the roof pitch where no enlargement results, and the insertion of ventilation terminals other than those dealt with elsewhere.

Three things are carved out expressly. Chimneys, flues and soil and vent pipes fall under Class G rather than Class C. Solar photovoltaic and solar thermal equipment falls under Part 14. And anything that enlarges the roof space is Class B, with its cubic content allowances and its own conditions.

Class C applies to a dwellinghouse. Flats and maisonettes have no Part 1 rights at all, which is covered in the note on why flats and maisonettes have no Part 1 permitted development.

The 150 millimetre limit and how it is measured

The measurement is taken perpendicular to the external surface of the original roof, not vertically and not from the tile line.

That perpendicular measurement is where installations fail. A conventional pitched rooflight sits on a timber upstand, which raises the frame above the rafter line before the unit itself is fitted, and the flashing kit adds a further build-up. Take the height of the upstand, the frame and any external kerb together and a standard installation on a slate roof can clear 150 millimetres without anyone intending it to.

Two practical responses. Specify a flush-fitting or conservation-pattern rooflight designed to sit within the tile plane, which most manufacturers now produce in a range of sizes. Or draw the detail at 1:5 before ordering, showing the rafter, the upstand, the frame and the finished external face against a line set perpendicular to the roof.

The same limit applies to re-roofing in a thicker material. Adding a layer of insulation above the rafters and re-tiling raises the whole roof plane, and where the increase exceeds the limit the work is not permitted development under Class C.

Not exceeding the highest part of the original roof

The second limit stops alterations that raise the ridge. No part of the alteration may be higher than the highest part of the original roof.

Note the word original. Throughout Part 1, original means the dwellinghouse as it was built, or as it stood on 1 July 1948 if it is older than that. So a house whose ridge was raised under a 2005 planning permission still has its pre-2005 ridge as the reference point for Class C, which surprises people.

This limit rarely bites on a rooflight in a main roof slope. It bites on ridge-mounted units, on roof ventilators near the apex, and on alterations to a lower roof where the designer measures from the wrong roof.

Side elevation windows and obscure glazing

One condition attaches to Class C. Any window located on a roof slope forming a side elevation of the dwellinghouse must be obscure-glazed, and non-opening unless the parts that can be opened are more than 1.7 metres above the floor of the room in which the window is installed.

Obscure-glazed means glass that cannot be seen through. Councils commonly refer to level 3 or above on the standard obscuration scale when they write the equivalent condition on a planning permission.

The practical consequence for a loft conversion is that a rooflight in the side slope facing the neighbour has to be obscure and either fixed shut or set high. That affects the layout, because a bathroom rooflight set 1.7 metres above a low loft floor may sit above the collar. Decide this at design stage rather than at first fix.

Designated land, Article 4 directions and listed buildings

Class C is not excluded on article 2(3) land in the way Class B is, so rooflights can in principle be permitted development in a conservation area, a National Park or a National Landscape. That is a distinction worth knowing, and it is one of the few places where the rules are more generous than people expect. The wider position on designated land is set out in the guide to article 2(3) land and permitted development in protected areas.

Two qualifications matter. Many conservation areas are covered by an Article 4 direction that removes Class C, or removes it on elevations fronting a highway, and directions vary street by street within the same authority. Check the direction itself, not a summary.

And listed building consent is a separate consent that permitted development rights do not touch. Inserting a rooflight into the roof of a listed building affects its character as a building of special interest, so consent is required, and it is a criminal offence to carry out the work without it. The application needs a Heritage Statement explaining the significance of the roof and the effect of the alteration, and professional review of that statement is worth having on a graded building.

Rooflights in an extension you have just built

A common sequence is a rear extension built under Class A, followed by rooflights added to its flat or shallow-pitched roof.

Two issues arise. Where the extension was granted planning permission, a condition tying the development to the approved drawings may mean adding rooflights needs a non-material amendment. Read the decision notice.

Where the extension was permitted development under Class A, the position is less tidy, because Class C measures against the original roof and an extension built in 2019 is not part of the original roof. Authorities take different views on how to apply the test to a later roof. That uncertainty is a good reason to apply for a certificate rather than to argue it afterwards.

Recording the installation for a future sale

Nothing in permitted development creates a record. The rooflight goes in, the scaffold comes down, and there is no document saying the work was lawful.

A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works are lawful. For a rooflight the application is modest: a location plan, a roof plan, an elevation, and a section at 1:5 through the rooflight showing the perpendicular projection against the original roof plane. That section is the document the council actually assesses.

Where the work is already done, section 191 covers an existing use or operation on evidence of what was built and when. Keep the installer's specification sheet and dated photographs, because the manufacturer's data on upstand height is the best evidence there is.

If the scheme falls outside Class C and needs an application, enter the postcode to see which supporting documents the council is likely to expect. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.