The short answer
Class C requires obscure glass in a side-elevation roof window, with no exception for height. Its second condition, that the window not open, is the only one relaxed, and only where the openable parts sit more than 1.7 metres up. A window that breaks either condition falls outside Class C and needs a planning application.
The wording of condition C.2
Class C of Part 1 of Schedule 2 to the General Permitted Development Order 2015 is the permitted development class for roof alterations to a dwellinghouse. Its second condition, C.2, binds any window in a roof slope that is a side elevation of the property. The glass must be "obscure-glazed", and the window must be "non-opening unless the parts of the window which can be opened are more than 1.7 metres" above floor level in the room it serves.
The structure matters. The condition has two limbs joined by "and", so both apply. Limb (a) has no exception, and no height makes clear glazing acceptable. Limb (b) carries the relaxation: the window may open if its openable parts are high enough, which means more than 1.7 metres above the floor.
The common mistake is to read the 1.7 metre figure as a way out of both. It is not. A window high enough to open freely must still be obscure-glazed. Another is to say that side-slope windows must be fixed, which is also wrong when the openable parts are above the line.
The reason is privacy. A side slope often looks toward the next house, so obscure glass blocks the view and the high opening limits overlooking.
What counts as a side elevation
The condition applies where the slope is a side elevation, and that wording is about which way the slope faces. On a simple pitched house with a gable at each end, the front and back slopes face the road and the garden, and the gable ends are the sides. A slope that faces a side boundary is a side-elevation slope.
Hipped roofs, which slope on all four sides, make it less obvious, because the small end slopes face the side. Those end slopes are generally treated as side elevations. On an irregular plan, such as an L-shaped house, one slope can face two directions at different points, and the answer depends on the layout.
We do not rely on a general rule. At the survey we photograph each slope, note which way it faces, and mark it on a simple plan, so the question has a definite answer for your house. If there is doubt, the planning team can confirm which slope is a side elevation.
A rear slope that faces the garden and a front slope that faces the road are not covered by C.2. They are, however, subject to other limits: the 0.15 metre protrusion rule, the height of the original roof, and any Article 4 direction, such as Hart's 1998 directions on frontage roof slopes.
The other limits that still apply
Passing C.2 does not finish the planning check. Class C also refuses an alteration that sticks out more than 0.15 metres from the original roof surface. A roof window that sits flush is within that. A deep frame or a raised kerb may not be.
Class C also does not apply to flats. Under the Order a block of flats, or a single flat inside one, is not a dwellinghouse. A flat therefore has no permitted development rights, and anything beyond like-for-like replacement needs planning permission. Rushmoor makes the same point about flats.
Listed buildings keep Class C but also need listed building consent. A conservation area leaves Class C in place, though an Article 4 direction can take it away. Our answer on roof windows in a conservation area covers those cases.
Building control is separate, because a new opening affects the structure and the glazing has an energy figure to reach. Our answer on roof windows and Building Regulations deals with that.
If the window falls outside Class C
A window that breaks a condition is not automatically refused. It means that permitted development does not cover it, so a planning application is needed, and the council decides on the merits. A clear-glazed side window, or one that opens low, would be in that group.
Councils look at overlooking, and the evidence helps. A clear window that looks onto a blank wall or a garage roof raises no privacy issue, and the neighbours' consent can support an application, though it does not decide it. A short statement and a drawing of the sightlines make the case easier to judge.
It is also worth asking whether a different position solves the problem. Moving the window to the rear slope, or using a rooflight on a low pitch, may avoid the side-elevation conditions altogether. Our answer on dormer or roof window compares the routes.
The planning team at your council makes the decision, and we do not pretend otherwise. We can provide the photographs and measurements an application will need. See roof windows and skylights for the service, and ask before you order if you are unsure.
