The short answer
A roof window needs to work as an emergency escape only where Building Regulations require an escape window for that room, which depends on how high the room is above the ground. Up to 4.5 m, an escape window or a protected stairway is needed. Above that, the rules differ, and a loft conversion over a two-storey house relies on a protected stair instead. Check with your building control body.
The rule that sets the height
The guidance on escape is in Approved Document B, the part of the Building Regulations dealing with fire safety in dwellings. For an upper storey up to 4.5 m above ground level, each habitable room, kitchens aside, needs an escape window, an escape door, or a direct route onto a protected stairway. A protected stairway is a stair enclosed in fire-resisting construction, so it can be used to leave the building even if a fire starts elsewhere.
That 4.5 m figure matters because it is about what a person can reasonably reach from outside, or drop from, with help. Above that height, rescue from a window is not the design assumption, and the guidance relies on the stair instead. Storeys higher than 4.5 m above ground follow separate rules.
So the first question is the height of the room's floor above the ground. A loft room built over a bungalow or a low single-storey extension may be within reach. A loft room built over a two-storey house will not be.
What an escape window has to be
Where an escape window is needed, the guidance sets out minimum sizes. The openable area must be unobstructed and at least 0.33 m². The opening must measure at least 450 mm in height and 450 mm in width, and its lowest edge may be no higher than 1,100 mm above the floor. Those are minimums, and the opening part has to be big enough for a person to get through with the sash held open.
The height-above-floor figure causes most trouble with roof windows. A roof window set high in a sloping ceiling often has its lower edge much more than 1,100 mm up. The window may be generous in size, yet unusable as an escape because nobody could climb to it. Window position, sill height and the headroom beneath it are decisions made at design stage, not afterwards.
A roof window also has to be reachable and operable from inside, with a handle that can be worked without a key in an emergency. Check the maker's literature for how the particular model opens, because designs vary.
Loft rooms on two-storey houses
A loft conversion on top of a two-storey house is the case most people are really asking about. The floor sits above the 4.5 m mark, so the roof window is not counted as the escape route. The room needs a protected stair that leads to a final exit instead, and the designer plans that stair first and the window afterwards.
In practice the stair down from the loft has to be enclosed in fire-resisting construction through the floors below, and the doors along its route need to meet the guidance. That affects the plan of the house: a stair that opens straight into a living room is unlikely to qualify without changes. Building control looks at this at the application stage.
Roof windows still have a use. They give light and ventilation, and they may offer a second way to attract attention from outside. They do not substitute for the protected stair. Your design for a conversion has to satisfy building control on the whole escape route, not just the window.
New and replacement windows
If you are only replacing an existing roof window, the escape position should not get worse. Our guide to replacing a roof window covers what to check. A window that currently serves as an escape route for a room within the 4.5 m limit should keep that function after replacement. Changing the size or the position could reduce it, so measure before ordering.
New openings in a loft conversion are part of a bigger design. The roof window you choose affects light, ventilation and escape, and also the thermal limits in Approved Document L, where the limit for a roof window is 1.4 W/m²K, with Band B on the Window Energy Rating scheme also accepted. Our page on how long it takes to fit a roof window explains the practical side of getting the unit in.
Bear in mind that a few roof window types, such as fixed rooflights, do not open at all. A fixed unit cannot serve as an escape under any rule.
Who decides and what we do
Building control decides. That is the council's building control service, or a registered building control approver, the private alternative that took the place of approved inspectors in April 2024. In Bath, the council route is B&NES Building Control. Ask them about your plan before you order the window, and keep their reply.
We look at the room, check the heights against the figures above, and tell you where a window falls short. Notifiable work comes with a Building Regulations application, and we prepare it. Where a design choice would make the loft awkward, such as a window too high to use, we flag it at survey and suggest an alternative position.
One practical point: if your loft room has a roof window that opens outward on a top-hung sash, check how far it swings and whether a person could actually climb through the gap with the sash held. Some designs open to a limited angle, and the clear opening is smaller than the glass size suggests. Your building control body will want the real measurement from the maker's data.
This page is not legal advice, and the guidance has detail beyond the points above. For fire safety, the building control body has the last word. The roof windows and skylights hub brings together the related questions, and fitting a roof window in winter covers timing.
