Class C roof alterations and the 150mm rule in practice
Class C covers alterations that stay close to the roof plane, and unlike Class B it is not switched off in conservation areas. Flat roof lanterns are where it usually fails.
Rooflight permitted development under Class C allows any other alteration to the roof of a house, provided the alteration does not protrude more than 0.15 metres beyond the plane of the original roof slope, does not rise above the highest part of the original roof, and does not include a balcony, a raised platform, a chimney or flue, or solar equipment. Those few words cover most rooflight installations, a good deal of re-roofing, and rather more than householders assume.
The part that surprises people most is that Class C, unlike the roof enlargement right in Class B, is not switched off in conservation areas and National Parks. A rooflight on a front roof slope in a conservation area can be permitted development, unless an Article 4 direction says otherwise.
What Class C permits on any roof slope
Class C is the residual roof right. Class B covers enlargement, meaning anything that adds volume such as a dormer. Class C covers alterations that stay close to the existing roof plane.
In practice it covers rooflights and roof windows, re-covering a roof with a different material where that materially affects appearance, replacing a roof with one of similar form, fitting ventilation terminals within the 150 millimetre limit, and alterations to a flat roof that do not create usable outdoor space.
Two exclusions apply before you reach the dimensions. The right is not available where the house exists only because of a prior approval conversion under Part 3, such as an office or agricultural building converted to a dwelling. And flats and maisonettes have no Part 1 rights at all, so a rooflight in a top floor flat needs an application.
An Article 4 direction can remove Class C in a designated area, and many conservation area directions do exactly that. Check the council's Article 4 register before relying on the right.
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 surface as it stands today.
That distinction has two consequences. On a pitched roof the allowance is measured at right angles to the slope, so a rooflight installed on a shallow pitch protrudes less in plan than the same unit on a steep one. And the reference plane is the original roof, meaning the house as built or as it stood on 1 July 1948, so a roof that has already been re-covered with a thicker system does not reset the datum.
A standard centre-pivot roof window fitted between the rafters with the frame sitting proud of the tiles will typically be well within 150 millimetres. A conservation style rooflight fitted flush or near flush is comfortably inside it. What can breach the limit is a unit installed over an existing covering, a raised upstand on a flat roof, or a proprietary flat roof lantern.
Flat roof lanterns are the common failure. A dome or pyramid lantern on a kerb frequently stands well over 150 millimetres above the roof surface, so it is not permitted development under Class C even though it feels like a rooflight. Measure it before ordering.
Not exceeding the highest part of the original roof
The second dimensional test is simpler: no part of the alteration may be higher than the highest part of the original roof.
On a pitched roof with a rooflight partway up the slope this rarely bites. It bites on three things. A rooflight or ventilation terminal installed very close to the ridge, where a 150 millimetre protrusion perpendicular to the slope can still put the top of the unit above ridge level. A lantern on a flat roof lower than the main ridge, where the test is against the highest part of the original roof of the house rather than the flat roof itself. And any alteration to a roof that has already been raised, where the comparison is with the original.
Where the house is a bungalow or a single-storey flat-roofed dwelling, read this test carefully, because the margin is small.
Front roof slopes and conservation areas
Here is the point that is worth the price of admission. Class B, the enlargement right that allows dormers, is expressly unavailable on designated land: conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. Class C carries no equivalent exclusion.
So rooflights on any roof slope, including one facing the highway, can be permitted development in a conservation area. A great many people assume otherwise and apply for planning permission they do not need.
Three cautions, though. An Article 4 direction may have removed the right, and directions covering householder alterations are common in conservation areas. A listed building needs listed building consent for the works regardless of Class C, and that is a criminal matter rather than a planning one. And where the roof forms part of a building of some sensitivity, a flush conservation rooflight will be considerably easier to live with, and is usually a better design answer, even where a standard unit would be lawful.
The equivalent dormer question on designated land is a different matter entirely, and is dealt with in Class C rooflights and the 150 millimetre rule alongside the volume tests in Class B roof enlargement and the volume allowances.
Flat roofs, parapets and what counts as a platform
Alterations to a flat roof sit within Class C, subject to the same two dimensional tests, and to the exclusion of verandahs, balconies and raised platforms.
A raised platform is defined for these purposes as a platform more than 0.3 metres above ground level, which means a flat roof terrace is a raised platform and is not permitted development under Class C. Neither is a balustrade forming part of one. Changing a flat roof covering, adding or altering a low parapet within the height limits, or installing a rooflight within 150 millimetres are all capable of falling within the right.
Access is the giveaway. If the work creates a surface people are expected to stand on, with a door or a balustrade serving it, the council will treat it as a terrace whatever it is called on the drawing, and overlooking will become the issue.
Solar panels are outside Class C altogether. The installation, alteration or replacement of solar photovoltaic or solar thermal equipment is expressly excluded, and is dealt with under the separate renewable energy provisions in Part 14 with their own limits and conditions. Chimneys, flues and soil and vent pipes are excluded on the same basis.
Windows on a side elevation
One condition attaches to Class C. Any window on a roof slope forming a side elevation of the house must be obscure-glazed, and must be non-opening unless the parts that open are more than 1.7 metres above the floor of the room.
This is a continuing condition rather than a one-off requirement at installation. Replacing that obscure unit with clear glazing later takes the alteration outside Class C, which makes it unauthorised development rather than a minor breach.
Side elevation means the elevation of the house, so a roof slope facing a neighbour across a side boundary is caught even though it is a roof rather than a wall.
Where Class C ends and Class B begins
The line is enlargement. If the work adds volume to the roof, it is Class B and the cubic content allowances apply, along with the exclusion on designated land and the eaves set back condition. If the work stays within or close to the plane of the existing roof, it is Class C.
A dormer is an enlargement. A rooflight is an alteration. A hip-to-gable conversion is an enlargement. Re-tiling is an alteration, if it amounts to development at all, since works that do not materially affect the external appearance of the building are not development in the first place.
Where a scheme does both, each element is tested against its own class, and a project that combines a rear dormer with front rooflights needs the dormer to satisfy Class B and the rooflights to satisfy Class C. If the dormer fails, the rooflights can still be lawful.
Permitted development is not a permission, and the only formal way to confirm that works are lawful is a lawful development certificate. Where a flat roof lantern or a near-ridge rooflight sits close to the limits, the fee is worth paying. If an application is needed, PlanScope drafts a Planning Statement structured to UK planning policy and marked where measurements are still required.
Reports are AI-assisted drafts that should be reviewed before submission, and we do not guarantee planning approval. This is general guidance, not legal advice: permitted development depends on the facts of your house and on any Article 4 direction, so check with your local planning authority before starting work.
Frequently asked questions
How is the 150 millimetre limit measured?
Perpendicular to the external surface of the original roof, not vertically and not from the roof as it stands today. The reference plane is the house as built or as it stood on 1 July 1948, so a roof that has been re-covered with a thicker system does not reset the datum for the measurement.
Are rooflights permitted development in a conservation area?
Class C carries no exclusion for designated land, unlike the Class B enlargement right, so rooflights including those on a front roof slope can be permitted development in a conservation area. Check first whether an Article 4 direction has removed the right, as many conservation area directions do.
Is a flat roof lantern permitted development?
Often not. A dome or pyramid lantern on an upstand frequently stands more than 150 millimetres above the roof surface, which takes it outside Class C even though it feels like a rooflight. Measure the finished height above the roof plane before ordering the unit.
Can I create a roof terrace under Class C?
No. Verandahs, balconies and raised platforms are excluded, and a raised platform means one more than 0.3 metres above ground level, which a roof terrace always is. A balustrade serving such a surface is caught too, and councils treat access and a door as the giveaway whatever the drawing calls it.
Do solar panels fall under Class C?
No. The installation, alteration or replacement of solar photovoltaic or solar thermal equipment is expressly excluded from Class C and is dealt with under the separate renewable energy provisions in Part 14, which carry their own limits and conditions. Chimneys, flues and vent pipes are excluded on the same basis.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.