The short answer
In England, a new first-floor bedroom usually needs an escape window whose clear opening is 0.33 m² or more, 450 mm or more in each direction, with its lowest edge 1,100 mm or less above floor level. The other route is a protected stair down to an outside door.
A bedroom on the first floor of an extension is a new place for someone to be asleep when a fire starts downstairs. Building regulations deal with that in one of two ways: a window big enough to climb out of, or a stair that stays free of smoke long enough to get down it. For most two storey extensions, the window is the simpler answer.
The rules below are the ones in England, from Approved Document B, volume 1. Wales has its own version of the document, and Scotland uses its technical handbooks, so check the local wording if you are building there.
When an escape window is needed
In a house where no floor is more than 4.5 m above the ground, which covers almost every two storey extension, each habitable room on the first floor needs either:
- an emergency escape window, or
- direct access to a protected stairway, meaning a stair enclosed by fire-resisting walls and doors that leads straight to an outside door.
A habitable room here means a bedroom, a living room or a study. Bathrooms and landings do not count. Kitchens are treated differently.
Most houses have an open-plan hall or a stair that comes down into a living room, so a protected stairway would mean new fire doors and partitions throughout. That is why the escape window is the usual choice in a new bedroom.
The size of an escape window
The figures that matter are about the opening, not the glass:
- The clear area you can climb through is 0.33 m² or more.
- That opening measures 450 mm or more in height and in width.
- Its lowest edge sits 1,100 mm or less above the floor.
All three must be met together. A 450 mm by 450 mm opening is only 0.2 m², so a square opening needs to be nearer 575 mm each way, or you can use a taller, narrower one such as 450 mm wide by 750 mm high.
“Unobstructed” means the clear gap once the window is fully open. Side-hung casements that open to 90 degrees are the easiest way to get it. Tilt-and-turn and top-hung windows can work, but only if the clear gap meets the figures; restrictors that stop the sash opening fully defeat the point unless they can be released easily.
Where the window can be
The window has to lead somewhere you can be rescued from or walk away from. Someone climbing out of a first-floor window needs to reach ground that is clear, or a place where a ladder can reach them. A window above a glass roof, a steep drop into a basement light well or an enclosed courtyard with no way out is not a safe route.
On a rear extension, this usually means the escape window faces the garden. On a side extension, it normally faces the front or back rather than the neighbour, which also suits planning, because side-facing windows upstairs often have to be obscure-glazed and fixed shut.
Locks and child safety
Escape windows can have locks and stays, but a stay needs a release catch so the window can be opened fully without searching for a key. The catch can be child-resistant. Keep keys for any lockable handle in the room, not in a drawer downstairs.
Smoke alarms come with the bedroom
Adding a habitable room above the ground floor brings in fire detection as well. Expect to fit mains-powered smoke alarms, linked together, in the hall and landing of the house, not only in the extension. Your electrician and building control will confirm the layout.
Roof windows and bedrooms in the eaves
If the upstairs room is partly in the roof, a roof window can serve as the escape window, as long as it meets the same opening sizes and the bottom of the opening is within reach. Measure the distance from the floor to the bottom of the opening along the slope with care; it is easy to end up too high.
Getting it signed off
Building control checks the escape window at the plan stage and again on site. Put the window size and opening type on the drawings early, and give the window supplier the clear-opening figures rather than just the frame size. With a two storey prefab extension, the panels have their openings cut before they leave the factory, so the window size has to be right on the drawings before the panels are made.