The short answer
Yes. Under Class C of the General Permitted Development Order, every roof window on a side-elevation slope needs obscured glass. The 1.7 metre figure is a separate matter: the sash must also be fixed shut unless every opening part sits more than 1.7 metres up from the floor. The obscured glass is needed on that slope whatever the height.
What condition C.2 says, word for word
Roof windows, rooflights and similar changes to a house roof fall under Class C of Part 1 of Schedule 2 to the General Permitted Development Order 2015. Class C comes with a condition, numbered C.2. The condition governs every window set into a side-facing roof plane. Such a window must be fitted with obscured glass. Its sash must also stay fixed, except where each part that opens is over 1.7 metres above the floor beneath it.
Read it slowly and you will see two requirements joined by the word "and". The first, obscured glass, has no height exception at all. The second, non-opening, does. People often merge the two and say that obscure glass is only needed below 1.7 metres. That reading is wrong. The height changes the second rule only.
What counts as a side-facing slope
A side elevation is a wall or roof slope that faces neither the front nor the back of the house in the way the planning rules define them. On a typical semi-detached or detached house with a gable end, the sloping roof planes run front and back, so they are not side slopes. A roof window in the rear slope is a rear window.
The condition bites on hipped roofs, where a third slope turns the corner of the building, and on L-shaped roofs, where a slope may face the neighbour's boundary. A Swindon bungalow with a hip, or a 1930s house with a side wing, can end up with a slope that falls squarely within the condition. If you are not sure which slope is which on your own house, Swindon Borough Council's planning team, or your own council, can confirm it before you order.
What obscure glass is in practice
Obscure glass is textured, frosted or patterned glass that lets light through but blurs the view. Manufacturers sell roof windows with an obscured pane as a standard option, usually a frosted or etched finish applied to the inner face, so a flat sheet of ordinary glass with a film stuck on is not what you are buying.
The planning rule does not set a numbered privacy grade. It says only that the glass must be obscured, so the practical test is that someone looking in or out cannot see recognisable detail. Ask the supplier to confirm in writing that the pane is obscured by manufacture, and keep that note with your records. It helps if the council or a later buyer's solicitor asks.
Obscure glass also changes how a room feels. It diffuses sunlight and removes the sky view, so it suits a bathroom, a shower room or a stairwell much better than a bedroom with a window you hoped to look out of. Think about that before you choose the slope.
The 1.7 metre height and the opening rule
The height only matters for opening. Above 1.7 metres, measured up from the finished floor, the sash may open. Anywhere lower, it has to stay fixed.
On a loft conversion, floors and sloping ceilings mean the answer varies across a single window. A tall window on a steep slope may have a top edge well above 1.7 metres and a bottom edge much lower. What counts is the opening part, so the supplier needs the measurement from the finished floor to the lowest point where the window can open.
A fixed window can be a problem for escape from a loft room. A bedroom on an upper floor may need a window that opens wide enough to climb through; Approved Document B asks for an unobstructed openable area of at least 0.33 square metres, at least 450 mm high and 450 mm wide, and the bottom edge of that opening no higher than 1100 mm. A fixed side-slope window cannot do that job, so the escape window usually goes on the rear slope.
When the permitted development route does not apply
Class C is a permitted development right, and permitted development can be taken away. A listed house is a different case: consent for alterations is separate, and the usual rights may not apply. An Article 4 direction can withdraw Class C in a conservation area, and so can a condition on an earlier permission.
Conservation areas and National Landscapes do not remove Class C by themselves. Dormers are a different matter: they do not qualify under Class B on that land, which we explain on the page about planning for roof windows.
Homes in the Railway Village, Old Town or a village conservation area near Swindon deserve an early question to the council. Ask before you buy anything, because only the council can settle it.
How to handle it at quote stage
When we survey a loft or a roof for new windows, we note which slopes face the side and tell you which windows will need obscure glass and a fixed sash. The written quote then names the windows to be supplied, so you are not surprised when the delivery arrives.
If you want a window that opens on a side slope with clear glass, the route is a planning application, not permitted development. Whether that would be granted is for the council to say.
While you plan, our pages on electric roof windows, roof window sizes and how long fitting takes will help you pick a model and a plan. If you already have a roof window on a side slope that is clear and opens low down, it is worth asking us or the council how that came about and whether it needs attention. Our roof window replacement page covers swapping one for another, and the skylights hub lists the rest.