The short answer
Yes, a roof window can serve as an escape window in a loft room if it meets Approved Document B. Paragraph 2.10 sets three tests: an openable area of at least 0.33 m2, 450 mm as the smallest height and width, and 1100 mm as the greatest height of the opening's lower edge above the floor. Only storeys within 4.5 m of the ground can rely on one.
Turning the loft of a semi or a terrace into a bedroom raises the question. The sizes in the guidance are firm, so the window and its height above the floor are planned together.
What Approved Document B asks of the window
Approved Document B is the fire safety guidance that sits behind the Building Regulations. Volume 1 covers dwellings, and GOV.UK lists the current edition as the 2019 edition with later amendments. The rules for escape windows sit in paragraph 2.10.
What size does the opening need to be?
The guidance asks for a minimum area of 0.33 m2, with 450 mm as the smallest height and the smallest width. Both tests apply. A tall, narrow opening can have enough area and still be too slim to climb through, and a wide, shallow one can be wide enough without enough height.
Measure the part of the window that opens, not the glass or the frame. A roof window with a large pane but a deep frame, or one that opens only a short way on a restrictor, can come out smaller than its catalogue size suggests. Check the clear opening with the sash fully open, and work from that measurement, not from the size on the order form.
Raise the question with the supplier when choosing the type of window, because the way a roof window opens, whether it tilts or pivots, changes how much clear space is left.
Where the window sits above the floor
The second test is height. The same paragraph puts a ceiling of 1100 mm on how far above the floor the bottom of the openable area may sit. In a loft room with a steep pitch, that decides where the window goes on the slope.
Why does the sill height matter on a sloping roof?
Because someone has to reach the opening and climb out, possibly in smoke or with a child. A window set high in the slope, with its lower edge above waist height, fails the test however large the glass. A window set lower, closer to the eaves, is more likely to pass, though it may also shorten the head height inside.
Roof windows come in tall and wide formats, and the choice interacts with the rafter spacing and with the position of any existing beams. A good design puts the window where the floor, the pitch and the rafters all agree. We measure the loft at the survey, work out where the lower edge of the openable area would fall, and tell you plainly if the position you have in mind will not work.
That is a first look rather than a design. Your building control body checks the final layout.
Why 4.5 m matters in a loft conversion
Only storeys within 4.5 m of the ground can rely on an escape window. That explains why a converted loft above two floors takes more than a window. Paragraph 2.5 deals with a storey higher than 4.5 m above ground, and gives two options: protect the stairway, or provide another escape route.
Where does the stair come in?
With two floors below, the new loft room normally calls for a protected stair, though the guidance allows alternatives. One of them, in paragraph 2.23, is fitting sprinklers in open-plan areas. The details depend on the layout of the house, so the plan is agreed with building control before work starts.
That is why a roof window alone does not settle the matter for a second-floor room. In a low house, where the loft room sits within 4.5 m of the ground, the escape window can be the right answer. For a taller house, the stair arrangement comes first and the window follows.
Our page on Building Regulations for roof windows covers when approval is needed. Somerset Building Control can be reached on 0300 303 7790, and the overview of roof windows and skylights has the wider options.
What to settle before you order the window
Escape rules are easy to meet when planned early and expensive to put right once a window is in. A few steps, taken in order, avoid that.
Who checks that the window passes?
Your building control body: Somerset Building Control for Somerset addresses, or a registered building control approver if you choose a private one. They look at the drawing before work and inspect afterwards. Ask them to confirm the escape arrangement for the room in writing, because they, not the window supplier, give the answer on compliance.
Then settle the window type. Ask the supplier for the clear opening dimensions with the sash open, not only the frame size, and for how the window opens. Compare them with the 0.33 m2, 450 mm and 1100 mm figures above. Work out where the lower edge of the openable area falls against the planned floor level, allowing for the final floor build-up.
Finally, look at what is outside. A window that opens on to a slope with a gutter, a valley or a roof edge below it may need a safe route for the person climbing out. We look at that during the survey. A written price follows, and if the window cannot meet the guidance in the position you wanted, you hear about it first, with a position that does work. For other window choices, see types of roof windows.
More on escape windows in lofts
Does every loft room need an escape window?
Not as a stand-alone rule. What the room needs depends on the house and how the new storey connects to the stairs. With two floors below it, a loft room normally needs a protected stair, and the guidance allows alternatives. Your building control body confirms the escape plan for your layout before work begins.
Can a fixed roof window serve as an escape window?
No. The guidance is about an openable area, so the part of the window that opens has to meet the size and height limits. A fixed rooflight lets in light but cannot be an escape route, so a loft room that depends on a window for escape needs one that opens.
The full picture: Velux roof window and skylight installation in Taunton ยท More short answers