Class C rooflights and the 150 mm projection rule

A rooflight is permitted development under Class C of Part 1 of Schedule 2 to the GPDO 2015 provided it does not protrude more than 150 mm beyond the plane of the original roof slope, and does not sit higher than the highest part of the original roof. Those two numbers decide most loft conversion questions, and the 150 mm one is measured in a way that catches people out: perpendicular to the roof surface, not vertically.

Class C is also the reason rooflights are usually available in a conservation area when a dormer is not.

What Class C covers

Class C permits any other alteration to the roof of a house, meaning alterations that are not enlargements. Rooflights, sun pipes, replacement roof coverings and similar changes fall here. Enlargements such as dormers and hip to gable extensions belong to Class B, which has its own limits and is not available on article 2(3) land.

Three things are expressly excluded from Class C: the installation of a chimney, flue or soil and vent pipe, and the installation, alteration or replacement of solar photovoltaic or solar thermal equipment. Solar has its own rights in Part 14, and flues have their own limits elsewhere in Part 1.

Class C applies to a dwellinghouse. It does not apply to flats or to any building containing flats, so a top floor flat owner adding a rooflight is making a planning application.

The 150 mm rule and how to measure it

The limit 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.

Read that carefully, because it is a measurement taken at right angles to the roof plane. On a 40 degree pitch, a rooflight that appears to stand only a little proud when viewed from the ground can exceed the limit once measured properly.

Two practical consequences follow.

The upstand and frame depth of the product matter. Standard centre pivot rooflights installed with a proprietary flashing kit generally sit within the limit when fitted between rafters in the normal way. A rooflight installed above the tile line on a timber upstand, or fitted with a raised kerb to improve weathering on a shallow pitch, can exceed it.

The condition of the roof matters too. Where a roof is being over-boarded or insulated above the rafters, the finished surface rises, and the 150 mm is still measured from the original roof plane rather than from the new one.

Ask the installer for a section drawing showing the projection at right angles to the slope. If they cannot produce one, that is informative in itself.

The height rule

The alteration must not result in the highest part of it being higher than the highest part of the original roof.

On a straightforward pitched roof with rooflights set below the ridge, this is rarely a problem. It bites on flat roofs, on shallow pitches near the ridge, and on lantern lights over a rear extension where the extension roof is itself the roof of the dwellinghouse. It also bites where someone proposes a rooflight in the ridge line itself.

Original means the roof as it was built, or as it stood on 1 July 1948 for older houses. A roof that has already been raised under a previous permission is measured against the original, not the current one.

Side elevation windows, obscure glazing and 1.7 metres

Class C carries a condition that shapes the internal layout more than any of the limits.

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

Both requirements apply together. A clear glazed side rooflight is outside the Class regardless of its height, and an opening side rooflight with its opening light below 1.7 m is outside it too.

That 1.7 m measurement is from the floor of the room, which on a loft conversion means the new floor level rather than the ceiling joists below. Where a room has a raised platform or a stepped floor, measure from the finished floor at the window.

Nothing stops you applying for planning permission for a clear glazed side rooflight. Many councils grant them where there is no overlooking. The point is that you are then in the application system rather than relying on the Class.

Where Class C does not apply

Check all five before ordering anything.

• Listed buildings. Listed building consent is required for works affecting the character of a listed building, including rooflights, and the fact that planning permission is not needed is irrelevant to that.
• Article 4 directions. Councils commonly remove Class C from front roof slopes in conservation areas, and the direction may apply to the whole area or to named streets.
• Conditions on a planning permission. A condition removing Part 1 rights, or Class C specifically, takes precedence under article 3(4) of the Order.
• Flats and maisonettes, which are outside Part 1 entirely.
• Buildings that are not dwellinghouses, including houses in mixed use with a shop or an office.

Where Class C does apply in a conservation area, the council may still expect a flush fitting conservation style rooflight if you later need permission for anything else, and using one from the start avoids an argument.

Illustrative example: a householder in a conservation area installs three rooflights in the rear slope, which are permitted, and one in the flank slope with clear glazing at low level to light a stairwell. The rear three were lawful. The flank one was not, and a retrospective application followed. Obscure glazing and a fixed light would have avoided it.

Building Regulations and the escape window conflict

Planning is only half the exercise. A loft conversion is notifiable work under the Building Regulations, covering structure where rafters are cut, fire safety and means of escape, thermal performance, guarding and ventilation.

Means of escape is where the two regimes collide. Where a rooflight is the emergency escape window from a habitable room, the guidance sets a minimum clear openable area and minimum opening dimensions, and it requires the bottom of the openable area to be no more than a stated height above the floor. That height requirement sits directly against the Class C condition that a side elevation rooflight must not open below 1.7 m.

The answer is usually to place the escape window in the front or rear slope where the Class C opening restriction does not apply, and to use side slopes for supplementary light only. Resolve it on the plan before the joinery is ordered, because retrofitting an escape window into a different slope after the conversion is expensive.

Where the project needs a planning application after all, a short planning statement setting out the roof slopes, the glazing specification and the overlooking position answers the officer's questions directly, and a simple design and access statement is worth preparing where the property is in a conservation area.

This is general guidance rather than legal advice. A lawful development certificate is the only formal way to confirm that works are lawful, so check the position with your local planning authority or a qualified professional before relying on any rule of thumb about permitted development.

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