Short answer: Roof window means of escape is possible only where the room floor is 4.5m or less above the ground outside, so in practice only a bungalow or chalet bungalow with rooms in the roof qualifies. Under Approved Document B, volume 1, it needs at least 0.33m² of unobstructed openable area, an opening at least 450mm high and at least 450mm wide, and the bottom of that opening no more than 1100mm above the floor. Higher up, where a loft conversion sits over two full storeys, the way out has to be a protected stair, and an escape skylight in a loft conversion is not accepted in its place.
Where the 4.5m line falls
The whole question of roof window means of escape turns on one measurement: the height of the floor of the room above the ground outside. Storeys more than 4.5m above the ground cannot rely on an escape window under Approved Document B. Below that, someone can climb out and be helped down, or wait at the window for a ladder. Above it, a window is no longer counted as a safe route.
A bungalow converted upstairs sits comfortably inside that line. A chalet bungalow also falls inside it. Bedrooms built into a steep roof from the start define the type, and it is common on Wirral estates, around Formby and in the villages towards Ormskirk. For those rooms, a roof window of the right size and position can serve as the exit.
A typical two-storey house is different. Stack the ground floor, the first floor and then the new loft floor, and the loft ends up well above 4.5m. Whatever size of window goes into that loft, it does not count as the way out.
A loft on a two-storey house: the stair is the escape
When a conversion turns the loft into a third storey, Approved Document B expects a protected stair. That means the escape route runs from the loft down through the house to an exit door, enclosed in fire-resisting walls and ceilings, with fire doors onto it from the rooms it passes. If a fire starts in the kitchen at night, the people in the loft walk down a stair that smoke and flame cannot reach for long enough to get out.
This is the part of loft conversions where roof windows get oversold. An escape-style roof window in a third-storey loft can still be a fine window, and it opens wide for fresh air. It does not replace the protected stair, and nobody should leave a survey thinking it does. The stair design belongs to whoever designs the conversion, and the job's Building Control body signs it off. We fit the roof windows to suit their drawings.
What "unobstructed" means on a roof window
The 0.33m² figure is the clear opening when the sash is open, not the size of the glass or the outside of the frame. On a roof window that difference is big. A centre-pivot sash rotates about its midpoint, so even fully open the sash sits across the hole, and the gap left either side is too small to climb through. That is why makers sell dedicated escape roof windows, usually top-hung or side-hung, that open far enough to leave the frame clear.
The three figures work together. A window can pass on area and fail on width, so a tall, narrow window that clears 0.33m² is still wrong if the clear width is under 450mm. And the 1100mm limit on the height of the opening above the floor has to be checked on the slope as built. On a shallow pitch, or above a raised knee wall, the bottom of the opening can end up higher than people expect.
| Window | Opening | As an escape window |
|---|---|---|
| Centre-pivot | Sash turns in the middle | Not made for it |
| Top-hung escape | Opens out from the head | Made for it, in sizes that clear the limits |
| Side-hung escape | Opens like a casement | Made for it; check the size |
| Fixed or vented | Does not open fully | Never |
Placing an escape roof window
A roof window that meets the sizes on paper still has to work in the room. The person climbing out needs to reach it, so nothing permanent should sit in front of it, such as a fitted wardrobe or a bed built into the eaves. The opening needs to lead somewhere a person can wait or be reached, which is why we look at what is below the window outside: a garden, a drive, or a flat roof over a back extension.
- Floor of the room within 4.5m of the ground
- Escape model, not a standard pivot
- Clear opening checked on the maker's data
- Bottom of the opening measured on the slope
- Nothing fixed in front of the window
- Signed off by the Building Control body
On the Flintshire side of our area, Wales publishes its own Approved Document B, and the building control body, either Flintshire or Wrexham council or a registered building control approver, confirms what applies to your house. We check with them before the window is ordered.
For the wider choices in a loft, from window size to headroom, see the roof window installation hub and our loft conversion window layout guide. Escape gets checked during the job rather than found out at the end, because for any notifiable roof window we fit, the notification to Building Control comes from us. Get a free quote
Escape windows in the roof: the follow-up questions
Can my existing bungalow roof window be swapped for an escape one?
Often, yes. If the rafters allow a window that clears the escape sizes, the old one comes out and an escape model goes in, sometimes in a slightly larger opening. It is controlled work, so the swap is notified like any other replacement.
Who decides whether my loft room needs an escape window?
Working from Approved Document B, the Building Control body makes the call, and that means either the council or a registered building control approver. We raise it at survey and price the right window from the start, so the decision is not left until the inspection.
Does a dormer window count instead of a roof window?
It can, for a room within the 4.5m height. The same rules apply to any escape window, so a dormer window that meets the opening sizes and the sill height can serve the room, and the roof windows are then free to be chosen for light and view.