Class C rooflights and what counts as an alteration

Class C of Part 1, Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 covers any other alteration to the roof of a house, and a rooflight sits within it provided it does not protrude more than 150 millimetres beyond the plane of the original roof slope and does not end up higher than the highest part of the original roof. The thing that surprises people about class C rooflight permitted development is what it does not exclude: unlike the Class B roof extension right, Class C applies in conservation areas and on the front roof slope.

So a conservation officer may dislike your front rooflight and still have no power to stop it, unless an Article 4 direction has removed the right or the building is listed.

The difference between Class B and Class C

Class B is the roof enlargement right: dormers, hip to gable work, raising the roof, with a cubic content allowance of 40 cubic metres on a terraced house and 50 cubic metres elsewhere. Class C is everything else done to a roof.

Two tests therefore separate them in practice. If the work creates roof space, you are in Class B and the volume matters. If it sits in the plane of the roof and adds nothing to the volume, you are in Class C and the 150 millimetres matters.

A dormer window is always Class B, however small, because it extends beyond the roof plane and encloses space.

The projection limit above the roof plane

The 150 millimetre limit is measured from the perpendicular with the external surface of the original roof slope. In practice that means the upstand of the rooflight, its frame and its glazing all have to sit within 150 millimetres of the tile or slate surface.

Standard conservation-pattern rooflights, which sit flush or near flush with the slate, comfortably satisfy this. Deep-framed proprietary roof windows installed on a thick upstand can exceed it, particularly where the installer adds a timber kerb to make the flashing easier.

Note the word "original". The comparison is with the roof as built or as it stood on 1 July 1948, so if a previous owner raised the roof or added a dormer, the baseline is the earlier form rather than what you see today.

Height relative to the highest part of the roof

The second limitation is that no part of the alteration may be higher than the highest part of the original roof. For a rooflight on a slope this is rarely an issue, since the lights sit below the ridge.

It becomes relevant for ridge-level work: a ridge rooflight, a replacement ridge with a raised profile, or a rooflight on a lean-to that abuts the main roof. Measure before ordering, especially where the roof covering is being changed to a thicker material at the same time.

Side-facing windows and obscure glazing

One condition attaches to Class C. Any window located on a roof slope forming a side elevation of the house 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.

That catches the rooflight people most want: a flank roof window in a loft bedroom overlooking next door. You can have it, but it has to be obscure glazed, and if it opens below 1.7 metres from the finished floor level it falls outside the class altogether.

Order the glazing correctly at the outset. Retrofitting obscure glazing or a restricted opening after a neighbour complains is more expensive than specifying it, and the same control appears as a condition on many householder permissions, as upper floor side windows and the obscure glazing condition explains.

Rooflights in a conservation area

Class C contains no article 2(3) land exclusion, so rooflights on a house in a conservation area, a National Landscape or a National Park are capable of being permitted development. That is a real difference from Class B, where a dormer in a conservation area always needs permission.

Three qualifications matter:

1. An Article 4 direction may have removed Class C locally, and many conservation area directions do exactly that. Read the direction's schedule
2. A listed building needs listed building consent for a rooflight regardless of permitted development, and installing one without consent is a criminal offence
3. Flats and maisonettes are outside Part 1 entirely, so Class C is no help in a converted building

Where an Article 4 direction applies, the application is a householder one and the design matters: a flush conservation rooflight in a slate roof, set in line with the window openings below, is the version that gets permission. A standard white-framed roof window on a street-facing slope generally does not.

Flat roof lanterns and the limits that apply

A roof lantern on a flat roof is the awkward case. The plane of a flat roof is horizontal, so a lantern standing 300 or 400 millimetres above the deck exceeds the 150 millimetre protrusion limit and falls outside Class C.

That does not always mean an application. Where the lantern sits on a rear extension that is itself permitted development under Class A, the extension's own limits govern, and those include an overall height cap of 4 metres on a single storey rear extension and the 3 metre eaves limit within 2 metres of a boundary. The lantern has to fit inside those, measured to its highest point.

Where the lantern is being added to an existing flat roof, measure the protrusion honestly and apply for permission if it exceeds the limit. A householder application for a rear lantern is usually straightforward, and a short Planning Statement addressing overlooking, light spill and the effect on the rear elevation is normally enough.

Checking before installation

Four checks, in order, before anyone orders a rooflight:

• Is the building listed, and is it in a conservation area or other article 2(3) land
• Is there an Article 4 direction covering the property, and what does its schedule remove
• Does the chosen product sit within 150 millimetres of the original roof surface once the upstand and flashing are allowed for
• Is the window on a side elevation roof slope, and if so is the glazing obscure and the opening above 1.7 metres

Where the works sit alongside window and door replacement in the walls below, those follow different rules again, covered in replacement windows and doors: when they are development.

For certainty, apply for a lawful development certificate under section 192 of the Town and Country Planning Act 1990. That is the only formal route to confirm that proposed works are permitted development, and on a conservation area property it is often worth the fee.

If you need an application, PlanScope drafts the Planning Statement and Design and Access Statement from a postcode and a short set of answers. Enter your postcode on PlanScope to see what your scheme needs.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.