A roof window in a spare loft is a skylight. A roof window in a bedroom is part of the fire escape. The difference matters, because the Building Regulations treat a sleeping room in a roof space the same way they treat a bedroom on any upper floor: there has to be a safe way out if the stairs are blocked by fire. In some lofts a roof window is that way out. In others it cannot be, however large it is.
This guide is about the loft conversion case only. It sets out the figures in Approved Document B (the fire safety volume of the Building Regulations), explains what counts as an escape window, and walks through the choices that follow: the height of the sill, the position on the slope, the glazing standard and the permission route. Choosing a roof window by size alone, for a room that is not a bedroom, is a different question, covered in our guide to roof window sizes. For everything else we do with roof windows, start at the roof windows and skylights page.
When a loft roof window has to be an escape window
Approved Document B, in the edition published on GOV.UK in 2026, sets out the escape arrangements for a dwelling. Paragraph 2.2 deals with upper storeys where the floor is no more than 4.5 m above ground level and the house has only one stair. In that case every habitable room above the ground floor, kitchens excepted, needs one of two things. Either it has an emergency escape window or external door, as described in paragraph 2.10, or it opens directly onto a protected stairway, meaning a stair enclosed in fire-resisting construction.
A loft bedroom is a habitable room, so the rule reaches it. Where the new floor sits no higher than 4.5 m above the ground, a roof window that meets paragraph 2.10 can serve as the escape route. Where it sits higher, which is usual once a loft is added to a two-storey house, a roof window cannot be relied on and the loft room needs a protected stair instead. The stair is a structural and fire-protection job in its own right, so it belongs in the plan before anyone orders glass.
The 4.5 m test is measured to the floor of the new room, not to the glass. A tall house with a low-pitched roof can fail it even when the roof window looks close to the eaves.
What makes an escape window: 0.33 m2, 450 mm and the sill
Paragraph 2.10 of Approved Document B describes the window in four measurements and one behaviour. The openable area, meaning the clear opening you could actually climb through and not the size of the glass, must be a minimum of 0.33 m2. Within that, the opening must be at least 450 mm high and at least 450 mm wide. The bottom of the openable area must be no more than 1100 mm above the floor. And the window must be capable of remaining open without being held.
| Test | Figure in Approved Document B |
|---|---|
| Openable area | Minimum 0.33 m2 |
| Clear height | Minimum 450 mm |
| Clear width | Minimum 450 mm |
| Bottom of the opening above the floor | Maximum 1100 mm |
| Staying open | Must stay open without being held |
What the area figure means in practice
The three dimensions interact. A window that is 450 mm wide would have to be roughly 733 mm high to reach 0.33 m2, because 0.33 divided by 0.45 is about 0.733. A window 600 mm wide needs about 550 mm of clear height. The two 450 mm minimums stop anyone achieving the area with a narrow slot, so a tall, thin opening fails even if its area works out on paper.
Why the glass size is the wrong number
The openable area is smaller than the glass. The sash frame takes up room on every side, and on a centre-pivot window the sash swings on its middle axis, so part of the opening is blocked by the sash itself. A roof window that is generous on the order sheet can leave a clear opening that is tight. The manufacturer's published data for the exact model and size is what the building control body will want, and it is what we check before quoting, not the catalogue headline dimension.
The requirement to stay open unaided means a friction stay or a restraint that holds the sash, not a hand. It has to remain open for a person who is leaving in a hurry.
Setting the sill height on a pitched roof
The 1100 mm figure is a limit on how high the bottom of the opening can be above the floor. A roof window set high on a steep slope can break it. The geometry is simple: the higher the window sits up the rafters, the higher the sill sits above the floor, and a floor-to-sill height beyond 1100 mm means a person has to climb to get out.
On a loft conversion the floor level is fixed by the existing joists and the new floor build-up, so the working variable is the position of the window on the roof slope. Moving the window down towards the eaves lowers the sill and also changes the headroom beneath it and the view out. In a steep roof a low position often reads well from the room. In a shallow roof there may be little choice, and the room layout can drive the answer.
Checking before the rafters are cut
The sill position has to be settled before the rafters are cut for the opening, because the trimming around it depends on where the window lands. On a survey we record the floor level, the pitch and the rafter positions, then draw the window on the slope and test the sill height against the 1100 mm limit. If the room has to be reworked, it is far cheaper to find out on paper.
Reaching the window: headroom, furniture and a safe exit
Approved Document B tests the window. It does not describe the route to it, but that route decides whether the window is any use in a real emergency. A bed pushed up under the sill, a built-in wardrobe next to the opening or a desk beneath it all make the exit harder, even where the window itself passes every measurement.
When we measure a loft with you we look at what will stand in front of the window and what a sleeping person would have to climb over. Plans that put the bed head on the wall below the window usually change at this point. Where the window is the escape route for a bedroom, a clear floor area beneath it is worth more than a few extra centimetres of glass.
Getting out onto what?
An escape window is only as good as the place it lets you out onto. Building control will ask what lies outside: a roof slope that is steep, slippery with moss or ending in a drop is hard to reach safely. On many houses the answer is simply that the window gives access to a roof slope from which a firefighter's ladder, or the person themselves, can reach the ground. We note the eaves height, the type of covering outside the window and whether a gutter or an extension roof sits below, and pass that on with the drawing.
Insulation and energy values for a new loft window
A loft conversion turns an unheated roof space into a heated room, and the windows go into the thermal envelope. The energy volume of the regulations, Approved Document L (2021 edition), limits heat loss through a new or replacement roof window in a house that already stands to 1.4 W/m2K (Table 4.2), with Energy Rating Band B accepted as the alternative. Flat rooflights have their own cap of 2.2 W/m2K. The lower the number, the less warmth escapes through the glass.
Most manufacturers publish both numbers in their data sheets for each glazing option. We specify from those sheets and keep them with the job file, so the building control body can see them when the work is inspected.
The flashing and the insulation around the frame
The glass is only part of the thermal story. Foam or mineral wool has to fill the space round the frame, with an airtight seal, and the insulation in the sloping ceiling must reach the frame without a break. This is also where condensation shows up in the first winter if the job is rough, which is why we cover it separately in our guide to condensation around roof windows. The weatherproofing at the top, sides and bottom of the opening is a separate topic again, set out in the guide to roof window flashing kits.
Permission, building control and the dormer question
Class C of the General Permitted Development Order lets most pitched roofs take a roof window without a planning application, within limits: no more than 0.15 m proud of the existing slope, and nothing higher than the original ridge. On a side slope the glass has to be obscure in every case, and the window has to be fixed shut unless anything that opens sits over 1.7 m above the floor. Class C has no restriction tied to conservation areas or the Cotswolds National Landscape, but an Article 4 direction, a planning condition or listed status can remove the right. Check your own address with Stroud District Council's planning team before ordering.
A loft conversion that also adds a dormer is a different matter. Dormers fall under Class B, which is not permitted development on Article 2(3) land. That land includes conservation areas and the Cotswolds National Landscape, which covers just over 50% of Stroud District according to the council's own profile. In those places a dormer needs planning permission, while a pure roof window conversion often does not.
Building Regulations are separate
Planning and Building Regulations are different systems. A loft conversion is a change of use of the roof space and needs a Building Regulations application whatever the planning position. Within Stroud District, applications go to the Gloucestershire Building Control Partnership (a service shared with Gloucester City); the private alternative is a registered building control approver. Floor structure, stair and fire separation sit with your designer or builder. For the roof openings, we explain the consent and handle the notification.
Fitting the roof windows in a conversion: sequence and cost
On a loft conversion the roof windows go in at a specific point. The roof is stripped locally, the rafters are trimmed to form the opening, the window is set and flashed, and the covering is made good around it, all before the room is boarded and plastered. The aim is to have the openings weathertight early, so the internal trades are not working under a tarpaulin.
MyBuilder's 2026 pricing for a standard roof window is about £350 to £600 for labour and fitting, plus roughly £500 for scaffold when one is needed. In a conversion with several openings, one scaffold serves them all. After the survey, the written quote prices scaffold, windows, trimming, flashing kits and making good as separate items.
What we need to start
Photographs of the slope and the loft, the room plan if there is one, and the model you have in mind. We bring the escape figures to site and test your preferred position against them.
Our workmanship guarantee runs for 10 years, in writing, and waste leaves the site with us.