A loft room needs three things from its roof windows. They must bring in daylight, they must let the room breathe, and in many houses one of them must also serve as the way out if the stairs are blocked. The first two are a matter of taste and budget. The third is a safety rule, and it decides which windows you can choose, how high they sit and how big the opening has to be.
That rule depends on the house. A converted loft in a bungalow sits low enough for a window to act as the escape route. A loft above a two-storey house is higher than the rule allows a window to serve, so the escape route has to be the stairs themselves, protected from fire. Either way, the work alters the roof structure, which brings in building control, and the windows must meet the thermal limits in Part L.
This guide takes those points in order: the escape window dimensions, the protected stairway, the U-value limits, the planning rules for roof windows and for dormers, and the practical choices of size and position. The wider range is in our skylights and roof windows section.
How the height of your loft decides the escape route
Approved Document B Vol 1 treats escape from upper storeys according to how high the floor stands above the ground. Where the highest floor is no more than 4.5 metres up and the house has a single stair, each habitable room other than a kitchen needs one of two things: an emergency escape window or external door, or a way straight onto a protected stairway.
Bungalows and low lofts
In a bungalow the loft floor usually falls under that height, so a roof window can be the escape route. It has to meet the opening sizes in the next section, and it should be placed so that a person can reach it and climb through.
Two-storey houses
Above a two-storey house the loft floor is higher than 4.5 metres, and storeys above that height follow separate provisions. In practice the stair serving the loft becomes a protected stairway, and the rooms below open onto it in a way that keeps the route to the front door clear of fire. A roof window then provides light and air but does not stand in for that stair. Your building control body confirms the detail for your plan.
Measure from the right place before designing anything. The height runs from ground level to the top storey's floor, and ground levels vary on sloping Sussex plots, so a house that looks like a regular two-storey can sit differently at the back.
Settle this with whoever is drawing the plans before you pick a window, because the stairs, the landing and the first-floor doors all follow from it. Window sizes and positions come second, and they are far easier to change on paper than in the roof.
The escape window: size, height and how it opens
Where a roof window is the escape route, paragraph 2.10 of Approved Document B Vol 1 sets four tests. The openable area must be at least 0.33 square metres. The opening must be at least 450 millimetres high and 450 millimetres wide. The bottom of the openable area must be no more than 1100 millimetres above the floor. And the window must be able to stay open without being held.
Openable area, not frame size
The 0.33 square metres is the clear gap when the sash is open, not the glass or the outer frame. A centre-pivot window turns on a bar, so the clear gap at each end is smaller than the pane. A top-hung window lifts out from the bottom edge and leaves a different shape of gap. The maker publishes the clear opening for every model and size. Ask for that figure, and check the smaller of the two dimensions against the 450 millimetre minimum.
The 1100 millimetre height
In a loft with a sloping ceiling, the window sits some way up the slope, and the bottom of its openable area can end up far above the floor. If that point rises past 1100 millimetres, the window fails as an escape window however big it is. The remedies are a lower position on the slope, a taller unit that reaches down, or a small change to the kneewall in the design.
Staying open unaided matters because nobody in smoke wants to hold a sash up and climb through at the same time. Most roof windows have a restraint that does this, but a poorly hinged unit may not.
For motorised units, read our guide to electric roof windows before choosing one for an escape position.
Protected stairs and what building control looks at
A protected stairway is a stair enclosed in fire-resisting construction, so smoke and flames cannot reach it quickly and people can leave by it. In a loft conversion the enclosure usually means fire-resisting doors on every room that opens onto the stair, with the surrounding walls and ceilings built to hold back fire for the required time. Details differ from house to house, and your designer or building control body specifies them.
Many older houses make this awkward. A 1930s semi often has a staircase running straight into the hall, with doors on the landing. A Victorian terrace may have a stair rising through the middle with bedrooms either side. Adding a loft room can mean replacing doors, forming a lobby or changing the route to the front door, and the plan should price these in at the start.
Why the roof work comes under building control
Cutting openings in a roof, fitting trimmers and strengthening rafters are structural alterations, and the Planning Portal names structural alterations among the reasons re-roofing work needs approval. Sign-off is given by either a registered building control approver or the council's building team, and inspections take place as the job progresses, not after it.
Adur & Worthing Councils state that the owner has to notify building control as each named stage on site is completed. Where our roof work is notifiable we handle the Building Regulations notification, while the designer deals with the stairs and the rest of the drawings. Keep the completion certificate with the house papers, since a buyer's solicitor will ask for it.
Part L: insulation and the roof window limits
A loft conversion turns an unheated space into a heated room, and that raises the stakes on the roof's thermal performance. Table 4.2 of Approved Document L Vol 1 (2021 edition incorporating 2023 amendments) lists the limiting U-values when an element is new or replaced in an existing home. The U-value is a heat-loss figure, so lower is better.
For the roof element the limit is 0.15 W/(m²·K). For windows, and the table's note says these include roof windows, the figure is 1.4, or Band B at least on the Window Energy Rating scale, applying in full since 15 June 2023. A rooflight, meaning a unit on a kerb out of the plane of the roof, is held to 2.2, so check which category a product falls under. A roof window in a pitched loft slope is the 1.4 kind.
What this means in the roof
The windows are only part of it. The sloping roof around them must be insulated to the standard, and the insulation has to meet the window frame without a gap. A cold ring around the frame is a common cause of condensation and of black mould on the lining, so the insulating collar supplied with the window is worth fitting properly and checking.
Ventilation is the other half. Insulation fitted tight between the rafters still needs air moving above it, in a way that suits the underlay. Moisture trapped in roof timbers does damage for years before anyone sees it, so we look at airflow at the survey and your designer should confirm it in the specification.
If an older conversion needs its windows renewing, replacing an old roof window covers the like-for-like swap.
Planning: roof windows, side slopes and dormers
Roof windows are usually the simpler planning question in a loft conversion, and dormers the harder one.
For roof windows, Class C of Part 1 of the General Permitted Development Order applies. A window may stand no more than 0.15 metres proud of the original slope and may not top the original ridge line. Class C has no article 2(3) exclusion, so it can apply in a conservation area or the South Downs National Park, unless an Article 4 direction takes it away or the building is listed. Several directions in Adur and Worthing do remove it on slopes that face a street.
A window in a side slope must be obscure-glazed, whatever its height. It must also stay shut unless its opening parts are more than 1.7 metres off the floor. That figure relieves only the shut-fast rule, so a side-slope window used for escape would still need obscure glass. Flats do not have Part 1 rights.
Dormers are a different class
A dormer is a roof addition under Class B, with its own limits: no part above the existing highest point, nothing forward of a front slope that faces a highway, and extra volume capped at 40 cubic metres for a terrace or 50 for any other house. On article 2(3) land, which takes in conservation areas and the National Park, a dormer is not permitted development at all. Our guide to choosing between a roof window and a dormer compares the routes.
On a period roof in a protected area, slim flush units may be easier to justify, as conservation rooflights show. Check with the Adur & Worthing planning team before the layout is drawn around windows that might need permission.
Choosing the number, size and position of the windows
Once the escape route is settled, the choices are practical ones about light, headroom and cost.
How many, and where
A single large window gives a different feel from two smaller ones side by side. Pairs tend to light a room more evenly and leave the roof balanced when seen from the garden. Put them where you will actually stand: a window over the bed is wasted by day, while one at the head of the stairs brightens the whole landing. A window set high gives sky and no view, and one set lower gives a view when you stand or sit, though if it is the escape route it must still meet the 1100 millimetre rule. A balcony-style unit, which opens at the top and folds out at the bottom, adds headroom and a view in one position at a higher price.
Fitting and cost
Every window needs a flashing kit matched to the covering, and the opening is trimmed with timbers so that the rafters keep carrying the load. MyBuilder (updated 21/08/2026) gives a typical UK range of £350 to £600 for installing a standard roof window, and about £500 for scaffolding. A conversion is a bigger job than a single swap, so the size of the openings, the covering, access and the survey findings set the real figure.
We survey the roof, photograph what we find and follow with a written quote at one total. The written quote names the roof windows and flashing kits, and the waste leaves with us. If the roof is also due for work, doing both together can spare a second scaffold. For a flat section of roof, flat rooflights are the matching choice.
