The short answer
Usually yes on a house, because Class C permitted development covers roof windows. The catch sits in paragraph C.2: glass on a side-facing slope must be frosted or patterned whatever its height. Height only changes the opening rule, so a sash may open if all its opening parts are over 1.7 m above the floor, and otherwise it stays fixed. Flats and listed homes follow other routes.
What paragraph C.2 demands
Class C is the permitted development class that lets a householder alter a roof, and fitting a roof window falls inside it. Paragraph C.2 then attaches a privacy condition to any window on a flank slope, meaning one that forms a side elevation of the house. The glass has to be obscure, and the sash has to stay shut unless every part that opens is higher than 1.7 metres from the floor of that room.
Read it as two separate tests. The glazing test has no height exception, so even a window set very high still needs frosted or patterned glass. The opening test is the only place the 1.7 metres appears. People often remember the rule as obscure and fixed unless high, which wrongly lets height excuse the glass.
Getting this right before you order matters, because a roof window is made to its glazing specification and clear glass cannot be corrected on site. Which slopes count as side elevations depends on how the house sits on its plot, and a corner or end-of-terrace house can be less obvious than it looks, so your council's planners should confirm it for your address.
Limits that sit beside the glazing rule
Under C.1(b), nothing may stand out from the slope by more than 0.15 metres, measured at right angles to the old surface. A window set flush with the tiles stays inside that, while a deep raised kerb round the frame may not. C.1(c) bars work that climbs past the old roof's highest point, which a window set below the ridge never does.
Paragraph C.1(d) takes chimneys, flues, soil pipes, vent pipes and solar equipment out of the class, so those need a different route from a window. All of these tests apply together with the glazing condition, and a design has to pass every one.
The class is written for a dwellinghouse, and the order's definition leaves out a building containing flats. A flat therefore has no permitted development right for the window and needs planning permission instead.
Planning is only one question. Cutting into the roof structure is a Building Regulations matter, which our answer on roof windows and Building Regulations covers. Rafters may need trimming around the opening, and a fit between two of them is the simplest case.
Listed homes, protected areas and Article 4
Permitted development is a starting position and never a promise. A listed house keeps its Class C rights, yet any change touching its character needs listed building consent from the council as well. Our answer on a roof window on a listed building sets out the order of events.
Conservation areas, National Parks and National Landscapes take away the right to build dormers under Class B, but they do not remove the right to fit a roof window under Class C. A local Article 4 direction can still withdraw rights along a particular street. Basingstoke and Deane, for one, has Article 4 directions that bring roof changes under planning control in four of its conservation areas within Basingstoke town. Elsewhere, ask the council whether a direction covers your street.
Where doubt remains, you can apply to the council for a lawful development certificate, which records in writing that the proposal is lawful as drawn.
A side slope often offers the best daylight for a loft room and sits closest to the neighbours. A friendly word before the work starts tends to head off a complaint later.
Choosing and placing the window
Plan the window around the condition. Obscure glass comes in several patterns, and some makers supply roof windows with a frosted inner pane. Ask which of their ranges include one in the opening style you want. If you want the sash to open as well, every opening part must sit above 1.7 metres from the floor of the room.
On a steep slope, the sill of a window can land close to the loft floor, which puts the opening part well under that height. The window then has to be fixed. That affects ventilation and rules the window out as a way of escape, a point our answer on an escape roof window explains for loft conversions.
Before a window is chosen, we measure from the loft floor to the planned sill, then check the position against each Class C test above. Our roof windows and skylights page sets out the full service, though the council always has the final word on whether a given slope counts as a side elevation.
