Rooflights and roof alterations under Class C

The Class C roof alteration rooflight rules come down to two numbers and one exclusion list. A rooflight in the plane of the roof is permitted development on a house if it projects no more than 150 mm beyond the plane of the original roof slope and does not rise above the highest part of the original roof. Side-facing roof windows have to be obscure-glazed and fixed shut below 1.7 m. And Class C does not cover chimneys, flues, soil and vent pipes or solar equipment, each of which is dealt with elsewhere in the Order or needs an application.

What counts as an alteration rather than an addition

Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 splits roof work into two classes. Class B permits an enlargement of a dwellinghouse consisting of an addition to or alteration of its roof, which is the dormer and roof-extension class. Class C permits any other alteration to the roof.

The practical dividing line is volume. If the work adds enclosed space, it is an enlargement and Class B applies, with its volume allowances of 40 cubic metres for a terraced house and 50 cubic metres otherwise, its set-back requirement from the original eaves and its prohibition on anything extending beyond the plane of a roof slope that fronts a highway. If the work changes the roof without adding space, Class C applies.

So a conservation-style rooflight sitting flush in the slope is a Class C alteration. A box dormer is a Class B enlargement. Re-covering a roof in a different tile, inserting a flat-topped rooflight, forming a new valley or installing a sun pipe are all Class C work.

One consequence is often missed. Class B is not available on land described in Article 2(3) of the Order, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. Class C is. In a conservation area you can usually put rooflights into a roof under Class C, but a dormer needs an application.

The 150 mm projection limit

The condition attached to Class C is that the alteration must not protrude more than 150 mm beyond the plane of the slope of the original roof, measured from the perpendicular with the external surface of the original roof.

Three things follow from that wording.

The measurement is perpendicular to the roof surface, not vertical. On a 40 degree pitch, a vertical measurement flatters the figure, and installers who measure vertically can fit a unit that fails.

The reference is the original roof, which means the roof as it existed on 1 July 1948 or as built, if later. A roof already re-covered with thicker tiles does not reset the datum. Where a previous owner has fitted a new roof build-up, the original plane may sit below the current surface, and the 150 mm has to be measured from the original.

The flashing and kerb count. A rooflight advertised as low profile can exceed 150 mm once the upstand, the kerb and the lead apron are installed over a battened and counter-battened roof. Ask the installer for the installed height above the tile plane, not the frame depth.

The highest part of the original roof

Class C does not permit an alteration where the highest part of the alteration would be higher than the highest part of the original roof. For a rooflight in a slope this is rarely an issue. It bites on three other kinds of work.

Raising a ridge, even by a course of tiles, to achieve headroom in a loft. Changing a roof covering to a system with a deeper build-up, where the new ridge sits above the old one. And fitting a rooflight close to the ridge on a shallow pitch, where the upstand can break the ridge line.

Measure the existing ridge height from a fixed datum before the work rather than afterwards. A 60 mm increase is enough to take the whole alteration outside Class C, and the only remedies then are a planning application or a certificate confirming the works were immune.

Obscure glazing and opening restrictions on side elevations

Any window on a roof slope forming a side elevation of the house must be obscure-glazed, and non-opening unless the openable parts are more than 1.7 m above the floor of the room in which the window is installed.

Two points of detail decide whether a scheme complies.

What is a side elevation is a matter of fact about the house, not about the street. On a corner plot, the elevation facing the side road is still a side elevation. On an L-shaped plan, an inner return can be a side elevation even though it faces the garden.

The 1.7 m is measured from the finished floor level of the room, so a loft conversion with a raised floor changes the answer. An openable rooflight at 1.6 m above a new loft floor fails, and the usual fix is a fixed lower light with an opening section above.

Obscure glazing means the glass obscures, not that a film has been applied. Councils treat an applied film as reversible, and enforcement cases on side-facing windows normally end with a requirement to replace the glazing unit.

Chimneys, flues and solar panels are excluded

Class C expressly does not permit the installation, alteration or replacement of a chimney, flue or soil and vent pipe, nor of solar photovoltaic or solar thermal equipment.

That is not the same as saying permission is needed. Chimneys and flues have their own allowance in Class G of Part 1, with a height limit of 1 m above the highest part of the roof and tighter rules on Article 2(3) land. Solar equipment sits in Part 14, with its own projection and siting limits. Soil and vent pipes often fall away as de minimis, though a council in a conservation area may take a different view about a pipe on a front elevation.

But the point is simply to look in the right class. An application refused because someone relied on Class C for a flue is an avoidable cost.

Flat roofs and parapets

Class C applies to a flat roof as readily as to a pitched one, and the 150 mm projection is measured perpendicular to the plane of the original roof, which on a flat roof means vertically. A standard flat-roof rooflight on a 150 mm kerb is therefore at the limit before the upstand detail is added.

Where a parapet exists, the alteration must still not exceed the highest part of the original roof, so a rooflight that sits below the parapet line is comfortable and one that rises above it is not. Raising a parapet to conceal new plant is an alteration that will usually exceed the original roof height and need permission.

And terraces are a separate trap. Converting a flat roof into a usable terrace is not a roof alteration at all, because a raised platform is excluded from the householder classes, and the overlooking issues mean a full application. The curtilage arithmetic in the 50 per cent rule for garden outbuildings is a useful reminder that these classes are read strictly rather than purposively.

Where Class C does not apply

Class C covers a dwellinghouse. It does not apply to a flat or maisonette, so a top-floor flat needs permission for a rooflight, however modest.

The right can also be removed or narrowed:

• By an Article 4 direction, which many conservation areas and some estates have.
• By a condition on the original planning permission removing Part 1 rights, which is common on housing estates built since the 1990s and on barn conversions.
• Where the building is listed, in which case listed building consent is needed for the works regardless of Class C, and the application will usually want a Heritage Statement explaining the effect on significance.
• Where the house is itself the product of a permitted change of use, such as a Class Q or Class MA conversion, which brings restrictions of its own.

Front garden surfacing works the same way, and the limits described for permeable surfaces under Class F are a good illustration of how a condition can quietly remove a right people assume they have.

Recording the works for the file

Permitted development leaves no paper trail unless you create one, and a buyer's solicitor will ask.

Keep a measured drawing showing the original roof plane and the installed projection, dated photographs from the street and the garden, the product data sheet for each rooflight, and the installer's invoice. Where the margins are tight, apply for a certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990 before the work, or under section 191 afterwards. A certificate is the only formal way to confirm that works are lawful.

Illustrative example: a 1930s semi has three conservation rooflights fitted into the rear slope and one into the flank slope facing a side road. The rear lights project 120 mm. The flank light is obscure-glazed with a fixed lower panel and an opening section at 1.75 m above the new loft floor. The ridge is untouched. All four are Class C, and a section 192 certificate obtained before work started closed the question for the eventual sale.

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. If the roof work turns out to need an application, a Planning Statement is the document that carries the case.