Loft conversion rooflights: the 150mm projection rule

A rooflight on a loft conversion is permitted development in most houses, provided it projects no more than 150 mm beyond the plane of the existing roof slope and does not rise above the highest part of the original roof. Those two limits sit in Class C of Part 1 of Schedule 2 to the General Permitted Development Order 2015, and they are the rooflight permitted development rules that decide the majority of loft schemes. Flats are excluded entirely. Listed buildings need consent whatever the Order says.

Class C and the 150 mm projection limit

Class C covers any other alteration to the roof of a house, which in practice means rooflights, re-roofing and similar work that is not a dormer. Condition C.1(a) states that development is not permitted if the alteration would protrude more than 0.15 metres beyond the plane of the slope of the original roof, measured perpendicular to the external surface of that slope.

Measure it properly. The 150 mm is taken from the existing roof surface at right angles, not vertically, and it includes the upstand, the flashing kit and any timber packing beneath the frame. A conservation-style rooflight installed flush between rafters is comfortably inside the limit. A standard unit fitted over the tile battens on a shallow pitch, with a proprietary flashing kit and a raised kerb, can exceed it without anyone noticing until an enforcement officer measures.

The phrase "original roof" matters too. Original means as built, or as it stood on 1 July 1948 for older houses. If a previous owner already raised the roof under a full permission, the baseline for your measurement is the original roof and not the one you can see.

Why a rooflight must not sit above the ridge

Condition C.1(b) prevents the highest part of the alteration from being higher than the highest part of the original roof. On a normal pitched roof with rooflights set into a slope, this never bites. It bites on two situations that come up more often than you would expect.

The first is a flat-roof rooflight or lantern on a rear outrigger, where the upstand plus the dome sits proud of the flat roof. Compare it with the original ridge and it is usually fine, but on a single-storey outrigger with a low parapet the measurement is against the original roof of the whole house, so check the geometry rather than assuming.

The second is a rooflight on a mansard or a crown roof, where the flat top of the crown is already close to the ridge line. Adding an upstand there can breach the condition. And on a bungalow with a shallow pitch, the margin is thinner than it looks.

Side-elevation rooflights and obscure glazing

Class C carries a condition as well as its limits. Condition C.2 requires that a window in a roof slope forming a side elevation of the house must be obscure-glazed, and must be non-opening unless the opening parts are more than 1.7 m above the floor of the room in which the window is installed.

Two consequences follow. A rooflight in a side slope that you want to open at cill height fails the condition and needs a planning application. A rooflight installed high in the slope, with the opening mechanism above 1.7 m from the finished floor, complies even if it opens.

The 1.7 m is measured from the floor of the room, so the finished floor level of the new loft room is what counts. Get the joiner to confirm the level before the roofer sets out the opening, because raising a floor by 100 mm for insulation and a new deck has moved more than one rooflight out of compliance.

Conservation areas and the rooflights councils actually accept

One distinction saves a lot of wasted applications. Class B, which covers dormers and roof enlargements, is withdrawn on article 2(3) land, meaning conservation areas, National Landscapes, National Parks, World Heritage Sites and the Broads. Class C is not. So a rooflight can still be permitted development in a conservation area where a dormer on the same roof would need a full application.

That does not make every rooflight acceptable. Councils very often have an Article 4 direction removing Class C on particular streets, and a direction is the one thing that turns a compliant rooflight into unauthorised development. Check the council's Article 4 mapping before ordering.

Where the rooflight is visible from the street in a conservation area, and where a permission is needed for anything else on the scheme, officers will look for a conservation rooflight: a slim, flush unit with a central glazing bar, cast metal or powder-coated to a dark colour, set in line with the tile plane. Specify it on the drawing rather than leaving the model to the roofer, and support it with a short heritage statement explaining the effect on the character and appearance of the area.

Listed buildings and the fabric you lose

Permitted development rights say nothing about listed building consent. Any works that affect the character of a listed building as a building of special architectural or historic interest need consent under section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990, and carrying them out without consent is a criminal offence.

A rooflight in a listed roof means cutting historic rafters, removing handmade tiles or slates and, often, disturbing a lath and plaster ceiling below. Conservation officers assess the loss of fabric, the visibility of the opening and whether the roof void has historic interest of its own, such as early trusses or smoke blackening.

The cases that succeed tend to share three features: the rooflight is on a rear or concealed slope, it is small and flush, and the applicant has shown that the rafters can be trimmed without cutting a principal timber. The cases that fail are usually front slopes and multiple openings.

Illustrative example

An illustrative case: a 1930s semi in a conservation area, loft conversion with two rooflights on the rear slope and one on the flank wall slope facing the neighbour. The rear pair are flush conservation units, 120 mm proud of the tile plane, well below the ridge. Those are permitted development under Class C. The flank unit is 900 mm above the new floor and opens, so it fails condition C.2 and either moves up the slope or becomes an application. Moving it costs less than applying.

Escape, headroom and ventilation in the new room

Planning and Building Regulations are separate consents, and a loft conversion needs both. The rooflight specification is where the two meet.

Approved Document B treats a loft conversion that creates a new storey as a fire safety problem to be solved by the stair, normally by enclosing the stairway from the loft to a final exit with fire-resisting construction and self-closing doors. An openable rooflight is not a substitute for that protected route in a three-storey house, whatever a supplier's brochure says. In simpler cases, an escape window with the required clear openable area and cill height can form part of the solution, and your building control body will confirm which route applies.

Approved Document F covers purge ventilation, which the rooflight usually provides, and Approved Document K sets the headroom over the new stair. Those three documents between them often dictate where the rooflight goes, so settle the internal layout before you fix the roof openings. Working the other way round is how people end up with a rooflight over the stair void and none over the bed.

Confirming the position before the scaffold goes up

If the rooflights are inside the Class C limits and no Article 4 direction applies, you can build. If you want that in writing, apply for a lawful development certificate for proposed use or development under section 192 of the Town and Country Planning Act 1990. It costs less than a planning application, the council decides only whether the works are lawful, and the certificate is conclusive if you sell.

That certificate is also the document a conveyancer asks for. Householders who added rooflights years ago and then hit a sale often find a certificate for existing development is the cleanest fix.

Where the scheme goes beyond Class C, because you want a dormer as well or the house sits on article 2(3) land, the application is a householder one and the same drawings serve both. The measured limits that catch people out on dormers and extensions follow the same drafting style as the rooflight conditions, which is why the rear extension depth limits and the side extension rules repay a read before you finalise a layout.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A lawful development certificate is the only formal way to confirm that works are lawful.

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