Question answered
Does a roof window on a side slope need obscure glass?
The short answer
Yes. Under Class C.2, a roof window in a slope on the side of the house must always have obscure glass, however high it sits. The 1.7 m height does not relax that. It relaxes only the second condition, which keeps the window shut unless its opening parts are higher than 1.7 m over the floor of the room.
Where a roof window sits on a slope facing the side of the house, the permitted development rules attach two separate conditions. One of them has no height exception at all.
What Class C.2 actually says
Class C of the permitted development rules covers further alterations to a house roof, and it is the route most roof windows rely on. Condition C.2 adds a rider for the sides of the house. Glass in a slope that faces the side must be obscure, and the window must not open at all unless the opening parts are higher than 1.7 metres above the floor inside.
The two requirements are joined by the word "and". Obscure glass is demanded of every side-slope window without exception. The 1.7 m figure belongs to the second requirement only. A window high enough to open is therefore still obscure, while one low enough to reach is both obscure and fixed shut. Reading the rule as "obscure and fixed unless above 1.7 m" is a frequent error, and it leads people to order the wrong pane.
Which slope counts as a side elevation
The rule turns on the elevation the slope forms, not on the direction the window faces. On a hipped bungalow, the angled end slopes sit on the sides of the house. On a house with its gable end to the road, the long slopes may be the sides. A slope facing the street or the back garden is judged differently from one facing a side boundary, so work out which of your slopes forms a side elevation before you order.
If you are unsure, ask the planning team to confirm the position from a photograph and a simple plan before any money is spent. When we survey a roof we note the elevation of each proposed opening alongside the measurements, so the question is answered before the frame is cut. Roofs with several changes of direction, such as an L-shaped bungalow with a hip and a gable, are the ones most likely to raise a doubt.
Obscure glass in practice
Obscure glazing means glass that is frosted, patterned or otherwise treated so that you cannot see through it. The window maker can supply the pane already treated. A film or a blind added afterwards may not satisfy the condition, because the condition describes the glazing itself. Order the right pane at the start, and keep the delivery note in case you are asked.
Obscure glass still lets light in, which is why it suits a stair, a landing or a bathroom. In a bedroom the loss of view is rarely the issue. The difficulty is the fixed-shut half of the rule, because a bedroom may need an escape route that opens, and an opening window below 1.7 m breaks that second condition. The sizes that apply to escape are set out in what size an escape roof window must be, and the two sets of rules need to be read together.
When the rules change
Class C is a permitted development right, and rights can be removed. Some conservation areas have Article 4 directions that take Class C away for roofs facing a highway, and the directions covering two Adur conservation areas, Old Shoreham and Kingston Buci, are local examples. A listed building needs listed building consent for alterations affecting its character. Flats have no Part 1 rights, so this route is not open to a flat. Conservation area or National Park status does not remove Class C on its own.
If you apply for planning permission instead, the council can set its own conditions, which may or may not mirror C.2. Ask the Adur & Worthing planning team, or the National Park Authority where it deals with applications for your address, before you order a window you might later have to change. Our hub page on skylights and roof windows links the other planning questions.
Practical choices and what to watch
If the room needs an opening window and the only practical slope is a side one, there are alternatives: a window in a different slope, a fixed pane with a vent somewhere else, or a planning application. Weighing them is easier with the roof in front of you, and our survey includes photographs of the actual slope.
Remember the weatherproofing as well. Obscure glass looks the same from the road, and the flashing around it matters just as much; the signs of a failing one are described in why a roof window leaks. Wet glass can also be condensation, which condensation on a roof window explains how to separate from a leak. Frosted glass hides smears and drips, so check the lining and sill from time to time.
Side-slope windows: two follow-ups
Can I have a clear opening window on a side slope if I get permission?
Possibly. Class C.2 is a condition of the permitted development right, not a ban. If you apply for planning permission, the council decides each case and may set its own conditions. Speak to the planning team first, and wait for the decision before the glass is ordered.
Does the same rule apply to a side window in a dormer?
Dormers carry a matching condition for side windows: obscure glass, and no opening unless the moving parts are over 1.7 metres above the floor. The bigger point is that Class B, the right that covers dormers, stops at the edge of conservation areas and the South Downs National Park, so dormer projects there go through a planning application.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.