Rooflights and the 150 millimetre projection limit

A rooflight is permitted development on a house if it sits no more than 150 millimetres proud of the roof slope and nothing about it rises above the ridge. That is the whole of the size test in Class C of Part 1 of Schedule 2 to the GPDO 2015, and it is why roof window planning permission questions usually come down to the specification sheet rather than the drawing.

Two further conditions attach. Materials have to be of similar appearance to the existing house, and a window in a roof slope forming a side elevation must be obscure glazed and restricted in how it opens.

The 150 millimetre test and how it is measured

The measurement is taken perpendicular to the external surface of the original roof, from the plane of the slope to the highest part of the installed unit. Not from the rafter, not from the tile batten, and not from the inside face.

That matters because the frame and the upstand are what project, and some units sit considerably higher than a homeowner expects once the flashing kit is fitted. Centre pivot and top hung roof windows designed for the market are almost always within the limit. Deck mounted units with a raised kerb often are not.

Two practical points follow. Ask the supplier for the installed projection above the roof plane for the specific flashing type you are using, because the same window projects differently on a slate roof and on a deep profile tile. And remember that original roof means the roof as built, or as it stood on 1 July 1948 for older houses, so an earlier flat roofed extension is not an original roof at all.

Nothing above the ridge

The second limb of the test is that the alteration must not result in the highest part of it being higher than the highest part of the original roof.

Rooflights rarely breach this on a pitched slope. Where it bites is on hipped and mansard roofs near the top, and on additions that sit close to the ridge line on a shallow pitch. Draw the section if the unit is within a metre of the ridge.

Class C also excludes chimneys, flues, soil and vent pipes, and solar equipment, which are dealt with elsewhere in the Order. A rooflight installation that happens to relocate a vent pipe has stepped outside Class C for that element.

Side elevation rooflights must be obscure glazed

The condition catches a window in a roof slope forming a side elevation of the house. It must be obscure glazed, and it must be non-opening unless the openable parts are more than 1.7 metres above the floor of the room.

The 1.7 metre measurement is taken from the finished floor level of the room the window serves, which in a loft conversion is usually the new floor rather than the joists. The obscure glazing requirement has no stated level in Class C, so the sensible reading is that the glass must actually obscure the view. Councils commonly refer to Level 3 or above where they specify, and obscure glazing levels and when they apply covers how that plays out in conditions.

Rear and front roof slopes are unaffected by this condition, which is the reason so many loft conversions put clear glazed windows to the rear and obscure ones to the flank without anybody thinking about it.

Conservation areas do not remove the right

This surprises people, including some designers. Unlike roof enlargements and dormers under Class B, Class C is not withdrawn on the land listed in Article 2(3) of the Order, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites.

So a rooflight on a house in a conservation area is capable of being permitted development, including on the front slope. Three things can still stop it.

• An Article 4 direction, which many councils have made over conservation areas and which routinely removes Class C alongside other householder rights.
• Listed building status, where listed building consent is needed for the works whatever the planning position, and where the answer usually involves a flush conservation pattern unit.
• A condition on an earlier permission removing permitted development rights, which is common on newer estates and on previously converted buildings.

Check all three before relying on Class C. Houses created through the prior approval routes in Part 3, such as an office converted under Class MA or a barn under Class Q, have no Part 1 rights at all.

Where consent is needed in a designated area, a short heritage statement explaining the roofscape, the visibility of the slope from the street and the profile of the proposed unit will carry the application further than a product brochure.

Flat roof lanterns and where they fail

A lantern on a flat roof is a rooflight in ordinary speech and rarely a permitted one in law. Almost any lantern projects well beyond 150 millimetres above the plane of the roof, so the size test fails at the first hurdle.

The same is true of raised kerb units on shallow pitched single storey extensions. Where the extension itself was built under Class A, adding a lantern afterwards is a separate operation that needs its own assessment.

Flats and maisonettes are outside Part 1 altogether. A rooflight in a top floor flat needs planning permission regardless of its projection, and usually landlord consent as well.

Escape rooflights in a loft conversion

Building regulations run alongside all of this and are indifferent to the planning position. Converting a two storey house to three storeys normally requires a protected stairway to a final exit, and an escape window is not a substitute for it.

Where an escape window is the right solution, Approved Document B sets the dimensions. The clear openable area should be at least 0.33 square metres with a minimum clear height and width of 450 millimetres, and the bottom of the openable area should be no more than 1,100 millimetres above the floor. That last figure sits awkwardly beside the 1.7 metre planning condition for side facing windows, which is one reason escape windows are usually placed in the rear slope.

Thermal performance is the other regulated matter. Replacement rooflights have to meet the standard in the current Approved Document L, and the figure quoted by manufacturers can be measured in different planes, so ask for the assessed value and the plane it applies to.

What to draw on the elevations

Half the disputes about rooflights come from drawings that do not show them properly.

1. Show each unit on the roof plan and on every elevation it appears in, at the same scale as the rest of the drawing.
2. Add a section through one unit showing the projection above the roof plane, dimensioned.
3. Annotate the glazing specification and the opening restriction for any side facing unit.
4. Note the material and finish of the frame and flashing, which is what satisfies the similar appearance condition, in the same way as matching materials on a permitted development extension.

Where an application is needed, the roofscape argument belongs in the Design and Access Statement rather than in a covering email.

A Lawful Development Certificate is the only formal way to confirm that works relying on Class C are lawful, and it is worth the fee on any front slope installation in a sensitive area. This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and enter your postcode on PlanScope if you want to see which documents an application would need.