Fire Escape Window Regulations UK: The Three Numbers, and the 4.5m Rule That Overrides Them

Three dimensions decide whether a window counts as an escape window, and one storey height decides whether an escape window counts at all.

Clear openable area: minimum 0.33m².

Clear dimensions: minimum 450mm high and minimum 450mm wide. The route through may be at an angle rather than straight through.

Cill: the bottom of the openable area a maximum of 1100mm above the floor. There is no minimum.

The override: escape windows serve storeys up to 4.5m above ground level. Above that, Approved Document B wants a protected stairway.

Approved Document B, paragraph 2.10: all three must be met Dwellinghouses. The area test and the two dimension tests apply to the same opening. Finished floor level Clear openable area minimum 0.33 m² width: minimum 450 mm height: min 450 mm maximum 1100 mm to the bottom of the opening. No minimum. Then check the storey Up to 4.5m above ground: window will do. Above 4.5m: protected stairway instead.
Diagram by Vertigo Interiors, from Approved Document B Volume 1 (Dwellings), 2019 edition, paragraphs 2.1 to 2.10.

The fire escape window regulations that apply to homes in England sit in Part B of the Building Regulations, and the working detail is in Approved Document B Volume 1, which covers dwellings. The 2019 edition, incorporating the 2020, 2022 and 2025 amendments, is the current document. Paragraph 2.10 is the one everybody quotes, and it is short enough to quote in full.

Windows or external doors providing emergency escape should have an unobstructed openable area of at least 0.33m², at least 450mm high and at least 450mm wide, with the bottom of that openable area no more than 1100mm above the floor. People escaping should be able to reach a place free from danger from fire. Locks and opening stays may be fitted. The window should be capable of remaining open without being held.

The three dimensions, and where they go wrong on site

All three tests apply to the same opening, at the same time, and they are measured on the clear opening rather than on the frame or the sash. That distinction is where most failures come from.

  • 0.33m² is not achieved by 450mm x 450mm. A 450 by 450 opening is 0.2025m². The minimum dimensions and the minimum area are separate tests, and a window that scrapes both will be roughly 450mm by 733mm at the smallest.
  • The hinge and the stay eat the opening. A side-hung casement on standard hinges does not open flat, and the sash itself intrudes. Escape hinges, sometimes called easy-clean or fire escape hinges, exist to recover that clear width, and are usually the difference between a compliant and a non-compliant window in the same aperture.
  • The route may be at an angle. Approved Document B explicitly permits this, which is what allows a top-hung window to work: the person goes out and up rather than straight through.
  • 1100mm is a maximum, not a target. There is no minimum cill height in the current guidance, and the widely repeated 800mm figure is not in it. Guarding where there is a drop is governed separately, by Approved Document K, and our guide to Part K stair and guarding dimensions covers that side.

The rule that catches loft conversions

This is the part that changes projects, and it is not about the window at all. Approved Document B sets escape provisions by storey height:

  • Ground storey. Every habitable room except a kitchen needs either an opening directly onto a hall leading to a final exit, or an emergency escape window or door.
  • Upper storeys a maximum of 4.5m above ground level. Where the house is served by only one stair, every habitable room except a kitchen needs either an escape window or external door, or direct access to a protected stairway.
  • Upper storeys more than 4.5m above ground level. The house should have a protected stairway: a stair separated by fire resisting construction, minimum REI 30, at all storeys, running to a final exit or to two separated ground level final exits.

A typical two-storey London terrace puts the second floor of a loft conversion above 4.5m. At that point an escape window in the new bedroom does not discharge the requirement. What the guidance asks for is a protected stairway, which in practice means fire doors to every room off the stair enclosure, fire resisting construction to the enclosure, and often a new fire-rated ceiling or upgraded floor. Cavity barriers or a fire resisting ceiling, minimum EI 30, are also expected above the protected stairway enclosure.

That is why a loft conversion is never confined to the loft. Our page on when a home renovation needs fire doors covers the doorset side, and the wider Building Regulations picture for a London renovation sets out what else gets pulled in.

Go higher again and there is a further layer. Where a house has two or more storeys more than 4.5m above ground level, an alternative escape route is needed from each storey more than 7.5m above ground, or a sprinkler system to BS 9251 throughout. That catches tall Victorian houses and any conversion adding a fourth floor.

Inner rooms, and the two-rooms-one-window allowance

An inner room, meaning a room whose only escape route is through another room, is permitted in a limited set of cases: a kitchen, a laundry or utility room, a dressing room, a bathroom, WC or shower room, a gallery meeting the relevant provision, or any room on a storey a maximum of 4.5m above ground level provided with an emergency escape window.

That last case is worth reading twice, because it is the one that lets an open-plan first floor work. Above 4.5m it is not available, and a bedroom reached through another room becomes a problem rather than a design choice.

The allowance most often forgotten is the shared window. Two rooms may be served by a single escape window, provided a door between them gives access to it without passing through the stair enclosure, and both rooms have their own access to the internal stair. In a plan with a rear bedroom facing a lightwell, that can be the route to a compliant layout without moving a structural opening.

What to specify, practically

  1. Fix the storey first. Measure from ground level to the finished floor of the highest storey. Above 4.5m, stop looking at windows and start pricing a protected stairway.
  2. Design the clear opening, not the aperture. Give the window supplier the clear opening you need, in millimetres, and ask them to confirm it with the hinge and stay specified.
  3. Check the cill against the finished floor. Not against the screed, and not against the joists. A 25mm engineered board and an acoustic underlay have both moved the 1100mm dimension before now.
  4. Check what is outside. People escaping have to reach a place free from danger from fire. A window over a fully enclosed courtyard is governed by a separate provision on the size of that space relative to the height of the house.
  5. Get it signed off. Escape provision is a building control matter and cannot be self-certified under a competent person scheme. Window replacement itself can be, which is a different thing entirely and covered in our replacement window U-values guide.

The source document is free: Approved Document B on gov.uk, Volume 1 for dwellings. If you are planning a whole-house project in London or the South East, Vertigo Interiors works through this alongside the design rather than after it, and our page on planning permission for internal alterations covers the consents that run in parallel.

Frequently Asked Questions

What size does a fire escape window have to be?

Three figures have to be met at once. The unobstructed openable area must be a minimum of 0.33m², with a minimum height of 450mm and a minimum width of 450mm, and the bottom of that openable area must be a maximum of 1100mm above the floor. Approved Document B allows the route through the window to be at an angle rather than straight through, which is what makes a top-hung casement workable.

Is there a minimum sill height for an escape window?

No. Approved Document B sets a maximum of 1100mm above floor level for the bottom of the openable area and gives no minimum. The 800mm figure that circulates widely is not in the current guidance. A low cill is fine for escape, though Approved Document K guarding requirements apply separately where there is a drop on the other side.

Do I need an escape window in a loft conversion?

Usually you need something else instead. Escape windows serve habitable rooms on storeys a maximum of 4.5m above ground level. A loft conversion over two existing floors normally puts the new floor above 4.5m, and at that point Approved Document B calls for a protected stairway with fire resisting construction to a final exit, not a window. That is the requirement that turns a loft conversion into a whole-house job.

Which rooms need an escape window?

All habitable rooms except kitchens, on the ground storey and on upper storeys up to 4.5m above ground level, unless the room opens directly onto a hall leading to a final exit or has direct access to a protected stairway. Bathrooms, WCs, utility rooms and dressing rooms are not habitable rooms for this purpose.

Can two bedrooms share one escape window?

Yes, within limits. Approved Document B allows two rooms to be served by a single window, provided a door between them gives access to the window without passing through the stair enclosure, and each room has its own access to the internal stair. It is a useful allowance in a plan where one bedroom faces a lightwell.

Can escape windows have locks?

Yes. Locks with or without removable keys, and opening stays with child-resistant release catches, may be fitted to escape windows. The window also has to be capable of remaining open without being held, which rules out a sash that drops shut and any friction hinge tired enough to swing back.