A loft bedroom sounds simple until someone asks how you would get out of it at night with the stairs full of smoke. That question sits behind a whole part of the Building Regulations, and it is the reason a roof window in a loft room is not always just a source of daylight. Sometimes it is also part of the escape plan.
The rules are set out in Approved Document B, the fire safety guidance for dwellings. It does not say that every loft needs an escape window. It sets a height line, 4.5 m above ground, and builds the options around it. Above that line the focus moves to the stairs and the doors on the way down. This guide walks through what the document says, in plain words, and where a roof window comes into it. It covers the sizes an escape window has to meet, how protected stairs and the alternatives work, and why the position of the window on the roof matters for planning as well as safety. None of this replaces a decision from your building control body, who will look at your actual house, but it should leave you knowing what to ask and what to expect on the day the survey is done.
Why a loft room raises a fire question
A loft conversion adds a storey to a house that was designed with fewer. The new room sits at the top, so the way out runs through everything below it. If a fire starts on the ground floor, the stairs become the very route you need and the very route that fills with smoke.
What does the regulation want to see?
Approved Document B looks at the full path from the new room to the front door. Its paragraph 2.21 says the full extent of the escape route should be addressed, which means the landing, the stairs, the hall and the doors, not only the new room. It also says fire resisting doors and partitions should be provided, including upgrading the existing doors where necessary. That is why a loft conversion quote can include work to doors you thought were fine.
Where does the roof window come in?
The roof window is one of the ways out, and in some designs it is the only one apart from the stairs. Approved Document B describes escape windows in its own paragraph, with sizes and a maximum sill height. A roof window that meets those figures can serve as that window. One that does not is still a good window, but it cannot be counted as part of the plan.
It helps to treat the window and the stairs as one question rather than two. Builders sometimes fit a generous roof window first and discover later that the stairs and doors below it need the larger share of the work. Raising the point at the survey stage saves a surprise. We look at how the loft is reached on the very first visit, and our roof windows and skylights page shows where this fits among the other jobs we take on at roof level.
The 4.5 m line and what it triggers
Height above ground is the hinge of the whole section. Approved Document B paragraph 2.5 deals with a storey more than 4.5 m above ground level and gives two ways to meet the rule.
What are the two routes above 4.5 m?
The first is a protected stairway, which means a stair enclosed in fire resisting construction rated for at least 30 minutes. The second is an alternative escape route: a top storey cut off from the lower storeys by construction with the same 30 minute rating, plus a separate way out that leads to its own final exit. A second staircase with its own door to the outside is hard to fit into most family houses, so the protected stair is the route many designers start with.
Does your loft floor sit above 4.5 m?
That depends on your house. The measurement is taken from the ground, up to the floor of the highest storey, so a bungalow, a low two-storey house and a tall Victorian semi will not all give the same answer. Do not assume from the number of floors. Your building control body will measure it, and your designer should do the same before drawing anything. For a house in Crawley the council service is Crawley Building Control, the Local Authority Building Control service for the borough. Elsewhere in our area it may be a different council or a registered building control approver, and it is worth asking early which one is dealing with your job.
If the loft floor does sit above the line, the stairs and doors become the main cost of the conversion, and the roof window is a smaller part of the budget than people expect.
What an escape window has to measure
Approved Document B paragraph 2.10 gives the sizes for an escape window. They are plain numbers, and they are worth checking against the product data before you order, not after the frame is in the roof.
What are the minimum sizes?
The openable area must be at least 0.33 m2. The opening must be at least 450 mm high and at least 450 mm wide. These are minimums for the opening itself, not for the frame, so a window that looks large from outside can still fall short once the sash is open and the hinge, the glazing bars and the edge of the lining are taken out of the clear space. The figures do not say the opening must be a particular shape. A tall, narrow opening and a wide, short one can both pass if every dimension clears the minimum.
How high can the sill be?
The bottom of the openable area is a maximum of 1100 mm above the floor. In a loft this matters more than anywhere else, because the roof slope drops toward the eaves and the floor often sits well below the window. A window set high on the slope may be fine for daylight and fail the sill test. A raised platform inside is one way to meet it, but the platform has to be solid, safe underfoot and big enough to stand on.
What about the type of window?
Top-hung roof windows that swing out from the top edge often have a restricted clear opening unless the sash can be pushed to a stop. Centre-pivot windows rotate and leave two gaps. The data sheet or the maker's help line will confirm the clear opening for your size. Our guide to Velux window sizes lists the common frame sizes to compare.
Protected stairs and the alternatives
For most loft conversions in two-storey houses the stairs are where the real change happens. The window is one piece of a larger plan.
What does a protected stair involve?
A protected stair is enclosed along its length, from the loft down to an outside exit at ground level. The enclosure is built to a fire rating, and Approved Document B paragraph 2.21 asks for doors that hold for at least 20 minutes and partitions rated for 30. In practice that can mean replacing old panelled doors on the ground floor and first floor with fire doors, adding a lobby, and boxing in the stair. It changes how the house feels, which is why people want to hear about it before a quote is signed.
Expect the designer to propose the protected stair for a two-storey house, but remember the document lists other options, so nothing is settled until the drawings are checked.
What is the sprinkler alternative?
Paragraph 2.23 describes a different route. Instead of enclosing the whole stair, sprinkler protection is provided to the open-plan areas, with a 30 minute partition and a 20 minute door separating the ground storey. The loft occupants then get access to a first storey escape window. Some households prefer this when the ground floor is open plan and enclosing a stair would spoil it. Others rule it out on cost. It is a design decision, taken with the building control body, and the escape window forms part of it, which is why its size and position matter.
Whichever route is chosen, the drawings should show it, and the building control body should approve it before the loft floor is boarded.
Roof windows, planning and where the window goes
Where an escape window can go depends on more than fire rules. The planning position on roof slopes also limits your choices, and the two can pull against each other.
Does the roof slope matter for planning?
Class C of the permitted development order, which covers other alterations to a house roof, sets limits on roof windows. Two apply here: the window can stand only 0.15 metres proud of the original slope, and it cannot rise above the top of the original roof. Condition C.2 then covers side slopes. Any window facing a side elevation has to be obscure-glazed, whatever its height. It must also stay shut unless its opening part sits more than 1.7 metres above the floor of that room.
Why does that clash with an escape window?
An escape window needs its openable area no more than 1100 mm above the floor. A side-slope window that opens at that height falls below the 1.7 metre line. So a side-facing window under permitted development cannot be both a working escape window and compliant with C.2. The window would have to go on another slope, or the question goes to your local planning authority. Raise it with them before the design is fixed, because it is cheaper to move a line on a drawing than a frame on a roof.
Houses in a conservation area, in the South Downs National Park or in a National Landscape need extra care, because the rules for dormers change there. A listed house keeps these rights but also needs listed building consent for anything that alters its character. Flats and maisonettes are outside these house rights altogether.
In Crawley, the borough's building control service lists new window openings cut into roofs as work needing approval, so a fresh roof window opening goes through them. Our guide to building regulations for roof windows covers the wider position.
Costs, scaffold and what to send us
The window itself is the smallest line in a loft conversion's cost. For orientation, Checkatrade's window fitting guide, in its October 2026 table, puts a new roof window at an average of £1,250 and a replacement at £1,025, as typical UK figures. Those cover the window and fitting, not the stairs, the doors, the structural work or the fire protection. VAT at the standard rate of 20% may need adding, so check whether any quote includes it.
Who arranges the scaffold?
Fitting a roof window needs safe access at the eaves and at the work area. Poles on a public pavement or road need a licence under the Highways Act, so ask the scaffolder who applies and when. Scaffolding on your own land needs none. Checkatrade's roof replacement guide quotes £700 to £1,300 for scaffold on a new roof as a national figure, and hire time follows the whole conversion, not only the window.
What helps us advise on your loft?
Send the room height, the size of the existing window if there is one, the position of the stairs and a few clear photos of the roof slope from the garden. We can then tell you whether the sill and sizes look workable before anyone visits. A site visit comes first for us, and only then do you get a price in writing.
If your sums show the loft floor is above 4.5 m, speak to your designer about the protected stair first. If the window position looks wrong, our guide to flashing kit types helps you see how a different slope or a different frame changes the weatherproofing. Either way, get the escape route agreed in writing with building control before the window is ordered.
Part of our Velux Roof Windows & Skylights in Crawley
Escape window queries
Can a standard roof window act as a fire escape?
Only if its opening meets the Approved Document B sizes: at least 0.33 m2 of openable area, 450 mm high and wide, with the bottom of the opening no more than 1100 mm above the floor. Many standard sizes pass on width but fail on sill height, so measure the room first and check the maker's data.
Does a loft conversion on a bungalow need the same stairs as a house?
Not necessarily. The stair rules in paragraph 2.5 start at a storey more than 4.5 m above ground, and a bungalow loft may sit below that line. Your building control body measures it and decides which rules apply, so send them the section drawings and ask before you price the stairs.
Who decides if my loft escape plan is acceptable?
Your building control body: the council service, which in Crawley is Crawley Building Control, or a registered building control approver. They approve the design and inspect the work. Send us your drawings and window sizes first, and we can tell you whether the roof side of the plan is workable.
