Fire Escape Windows: The Part B Size Rules, Sill Height and When a Loft Needs One

Get the escape window wrong and building control will not sign off the room. The window looks big enough on the brochure, the sash opens, the glass is fine, and then the inspector measures the actual gap with the sash pushed out and finds it short. A room that fails the escape test will not get building control sign-off as a bedroom. The fix is a new, correctly specified window and often a changed sill height, which on a finished dormer means ripping out finished plaster and cladding that went in weeks earlier.

What an escape window is, and what it is for

An escape window, also called an emergency egress window or a means of escape window, is an openable window big enough and low enough for a person to climb out of if the stairs are blocked by fire or smoke. It is a fire-safety requirement under Approved Document B, the part of the Building Regulations that covers fire safety in England. The rule is not about ventilation or daylight. It is about giving somebody a second way out of a habitable room.

A habitable room means a room you live in: a bedroom or a living room, for example. Approved Document B's escape-window provisions exclude kitchens. Where Approved Document B asks for an escape window, every habitable room on that storey needs one that meets the specification, unless the room has direct access to a protected escape route instead.

The whole subject is widely misquoted. Council leaflets and window suppliers still repeat figures that were in the 2006 edition of Approved Document B and are not in the current one. The numbers that matter sit in a single clause, paragraph 2.10.

The decision that comes before the window: escape window or protected stair

This is the point most guides skip, and it is the one that decides whether you need an escape window at all.

Approved Document B treats a storey differently depending on how high its floor sits above the ground. The threshold is 4.5m, and it is measured to the floor of the storey, not to the window or the sill. Get this wrong and you buy the wrong solution.

Below 4.5m, a habitable room can be made safe with an escape window (paragraph 2.2a). Above 4.5m, an escape window on its own is not enough, and the storey needs a protected stairway: a stair enclosed by fire-resisting construction all the way down to a final exit, so that occupants can get down even while a fire burns in a room off the stair (paragraph 2.5a).

What that means in practice depends on the house. In a normal two-storey house, the first floor sits well under 4.5m but the new loft floor sits above it, so the loft room cannot rely on an escape window. Paragraph 2.21 requires the full escape route to be addressed, with fire-resisting doors (minimum E20, which an FD30 fire door comfortably exceeds) and fire-resisting partitions, so the enclosed loft staircase becomes the protected means of escape rather than the window.

In a bungalow, the loft floor is usually at or under 4.5m above the ground. There, the loft rooms can use escape windows (paragraph 2.2a), and a protected stair enclosure may not be needed. This is the case where an escape window genuinely serves a loft room.

So which rooms actually need escape windows? The habitable rooms on storeys at or below 4.5m. In a two-storey house that is the ground floor and the first floor: the first-floor rooms need either an escape window or direct access to the protected stairway, and which one applies depends on the escape route chosen, so confirm it with your building control officer. In a bungalow it includes the loft rooms themselves. Basements take an escape window or a protected stair to a final exit (paragraph 2.16).

The Approved Document B specification, exactly

Paragraph 2.10 is the clause. An escape window should comply with all of the following:

• An unobstructed openable area of at least 0.33m².
• A clear opening at least 450mm high and at least 450mm wide (both, not either). The route through the window may be at an angle rather than straight out.
• The bottom of the openable area no more than 1,100mm above the floor.
• Locks (with or without removable keys) and opening stays with child-resistant release catches are allowed.
• The window must be able to stay open without being held.

The openable area and the two clear dimensions do the work, with the maximum sill setting how low the opening has to sit. Everything else is detail about how they are measured.

If you build in Wales, Scotland or Northern Ireland, these figures are the England ones. Wales has its own Approved Document B, Scotland uses the Building Standards Technical Handbook, and Northern Ireland uses Technical Booklet E. The principle is the same but confirm the numbers under your own regime.

Clear opening is not the frame size

Here is where most windows that fail go wrong. The area and the dimension minimums describe the clear opening: the unobstructed gap measured between the frame and the sash when the window is fully open. They are not the frame size, not the glass size, and not the manufacturer's "clear glazed area". The hinges and the handle eat into the gap, and so does the thickness of the sash itself.

The arithmetic catches people out. A 450mm by 450mm opening sounds like it meets the rule because both minimums are hit. It does not: 0.45 multiplied by 0.45 is 0.2025 square metres, well short of 0.33m². To reach the area at a 450mm width you need a clear height of at least 0.33 divided by 0.45, which is 733mm. CNC Building Control, for example, rounds that to 750mm, which is where the common 750mm clear-opening figure comes from.

The wider the opening, the less height you need. The table shows the minimum clear height for a given clear width to reach the area requirement.

Measure the opening on the actual product data, with the sash at its real opening angle, before you order. On a centre-pivot roof window the sash pivots in the middle and part of it sweeps into the room, so at many sizes the clear escape gap falls short. Check the published clear opening for the exact model rather than assuming any size passes.

Roof windows as escape windows

In a dormer you usually escape through a vertical window in the dormer face. In the sloping part of a roof you escape through a roof window, and here the type matters.

A standard centre-pivot roof window (the common Velux-style GGL or equivalent) pivots on a central hinge. At many sizes the clear opening falls short because the sash intrudes. A top-hung roof window (Velux GPL and GPU ranges, or Keylite's fire escape FE range) is hinged at the top and swings outward, giving a clear, unobstructed gap. Velux top-hung windows open to around 45 degrees, and Keylite's fire escape range is sold specifically as opening to 45 degrees to meet the escape rule.

So if the escape point is a roof window, specify a top-hung escape model, not a centre-pivot. A pitched roof window can be the escape window as long as it is the right type and size, positioned so the sill sits no more than 1,100mm above the floor.

One trap with roof windows is the restrictor. A top-hung restrictor, such as Velux's child-safety restrictor, limits the sash from a maximum of 45 degrees down to a maximum of 27 degrees. Velux's own guidance says this does not normally satisfy the escape requirement or stop a child climbing through. If you need child safety on an escape window, use a stay with a child-resistant release catch, not a restrictor that permanently caps the opening angle.

Do not trust a claim that a particular size code gives the required area. The escape clear opening depends on the model and the opening angle, and the manufacturer's published figure is the only reliable source. Get the clear-opening dimensions for the exact model and size, and check the arithmetic yourself against the table above.

Sill height: the maximum, and the 800mm myth

Approved Document B gives one sill figure: the bottom of the openable area must be no more than 1,100mm above the finished floor. That is a maximum, so a low sill is fine for escape. There is no minimum sill height in the current Approved Document B paragraph 2.10.

So where does the "800mm minimum" come from, repeated in so many council leaflets? It is a leftover. The 2006 edition of Approved Document B cross-referred to Approved Document K (protection from falling), which sets an 800mm guarding height at opening windows, with 600mm allowed for a roof window in a loft extension. The current Approved Document K still carries the 800mm guarding figure, but for roof windows in loft extensions it hands the matter to Approved Document B, and the current Approved Document B gives no roof-window sill figure. The result is a gap that different building control bodies fill differently.

Treat it like this. Approved Document B requires the sill no more than 1,100mm above the floor. Some building control bodies also apply a lower limit around 800mm (600mm for a rooflight in a loft conversion) to guard against falls. Confirm with your own building control officer, and where the geometry allows, design to roughly 800mm up to the 1,100mm maximum so you satisfy both the escape rule and the stricter local practice.

Getting it right on the order

Most failures are specification failures, fixed by what you write down before anything is bought. The clear opening has to survive the hinges, the handle and the hardware. Spell out the following to the supplier:

• The published clear opening: clear width and clear height with the sash fully open, and the openable area, all beating the paragraph 2.10 minimums.
• Escape or "fire egress" hinges and an escape handle, so the sash swings fully open. Standard hinges on some uPVC and timber casements do not let the sash reach the opening angle the escape gap needs. Several suppliers have a "standard or fire spec" option, so tick the fire spec.
• No restrictor on the escape window, and no trickle-vent hardware sitting inside the opening, since either would reduce the unobstructed area.
• An opening stay with a child-resistant release catch if you need child safety, with the window able to stay open unaided.
• The sill height above the finished floor, no more than 1,100mm (and to the local 800mm limit where it applies).
• A drawing showing the handing and opening direction, so the gap opens where a person can actually climb through to a place of safety.

Cost and where to buy

Escape windows cost more than standard ones because of the top-hung mechanism and the escape hardware. For a roof window, a standard centre-pivot in a common 780 by 980mm size runs about £306 – £454 supply-only, while the top-hung fire escape version of the same size runs about £583 – £609, window only and before flashing. The gap between those two figures is the escape premium you pay. These are indicative retail figures from Travis Perkins and Screwfix listings, so check the live price for your size and brand.

Supplied and fitted into a new opening, a roof window is labour-led. Checkatrade data puts it in a wide band:

Scaffold is usually extra, and larger windows cost more. Buy roof windows and escape hardware from the big merchants such as Travis Perkins or Screwfix, or from a specialist roofing supplier, and casement escape windows from a window fabricator who can quote the fire-spec hinge option.

For a side-hung casement with an escape hinge in a wall, there is no reliable current UK figure for the hinge uplift on its own in public retail listings. Older one-off quotes exist but are not a sound guide. Ask the fabricator to price the fire-spec hinge option against the standard unit and judge that quote on its own merits.

Replacing a window that is already an escape window

If you replace a window in an existing habitable room that would need to be an escape window in a new dwelling, and it is big enough, the replacement should keep at least the same potential for escape (paragraph 2.19). If the old window was larger than the escape minimum, you are allowed to reduce the new opening down to the paragraph 2.10 minimum, but no smaller. This catches people doing a straight like-for-like window swap on a first-floor bedroom: fit a smaller or differently hinged unit and you can quietly break the escape route that was there before.

Common mistakes

The pattern repeats. Someone orders a window by frame size, the clear opening comes up short once the sash and hinges are accounted for, and building control refuses to sign off. Or the sill ends up too high because nobody checked it against the 1,100mm maximum during first fix.

Before you commit, confirm which escape route your storey actually needs, measure the published clear opening against the paragraph 2.10 figures, and check the sill height against the 1,100mm maximum plus any local 800mm limit. Confirm all three with your building control officer in writing, because the officer who measures the finished opening is the one who decides whether the room signs off as a bedroom.

Where you'll need this

Escape windows and the protected-stair decision come up whenever a project creates or changes a habitable room, so the question lands at several points. When you set out a loft and choose between a dormer and rooflights, the escape route decides the window type and where it can sit. When you specify the loft staircase, the protected-stair rule is what turns it into the means of escape, backed by the FD30 fire doors that enclose it. When you pick a pitched roof window, the top-hung escape model and its sill height follow from this clause. And at final inspection, the building control officer measures the clear opening with the sash open and signs off the fire strategy against it.

This applies to any project that creates or changes a habitable room, whether a loft conversion, a basement, or a window replacement, not just one kind of build.