A loft conversion lives or dies on its windows. The roof slopes cut down the usable floor area, the eaves leave little wall to put glass in, and the room is a long way from the front door. Roof windows are the usual way of bringing in light and air, and for a house with a plain pitched roof they are often the simplest. They also come with rules. If the loft is to be a bedroom, the question of how people get out in a fire has to be answered before the first window is ordered. This guide covers that, along with sizing, position, headroom, ventilation, insulation and the roofing work that goes alongside. The planning rules for adding a window are in a separate guide that we link to, not repeated here.
The escape question comes first
Approved Document B sets the fire safety rules for dwellings. When a loft is converted into a habitable room, the question is how someone upstairs gets out if the stairs below are on fire. For a house of up to two storeys, one answer is a window large enough to climb through, reachable from inside and opening onto a place from which a person can reach safety. But there is a limit on height. The ceiling on this is set by distance from the ground: only a storey that is 4.5 metres or less above it can use a roof window as the escape.
The consequence matters. A converted loft on a bungalow, or on a low chalet-style house, may be within that height, and a suitably sized roof window can serve as the escape. A loft on a standard two-storey house is higher than that, so a roof window cannot count. The Building Regulations then require a protected stair: the stair from the loft down to the ground-floor exit is enclosed in fire-resisting construction, with fire doors, so that it remains usable while the rest of the house burns. A loft conversion on a two-storey house needs a protected stair, not a roof window. This is a design decision made before the windows, and building control will want to see it on the drawings. The opening size and sill height needed for the cases where one is allowed are set out in our short answer on escape windows.
How many windows and how big
Daylight is a comfort matter, not a legal one. There is no Building Regulations daylight requirement for houses in England, so building control will not tell you how much glass to fit. What decides the number is the use of the room. A bedroom is happy with one or two medium windows. A study that you work in all day wants more light, perhaps a pair. A bathroom wants one. A rule of thumb used by many designers is to compare the glazed area with the floor area, but with a sloping roof the floor area is smaller than it looks, and even a modest window gives a lot of light, since a window in the roof sees the open sky.
Window size is limited by the rafter spacing. A window that fits between two rafters needs no rafter to be cut, which keeps the structure simple. A wider window means cutting one or more rafters and fitting trimmers, and the load goes to the uncut ones beside it. Our guide to roof windows and Building Regulations explains the structural side. Our answer on what size roof window do I need goes through common sizes.
Position, headroom and view
Where the window sits in the slope controls three things. First is headroom: a pivot window swings inward at the bottom, so if you place it low in the slope you will bump into it when it is open. Placing it higher, or using a top-hung window, avoids that. Second is the view. A sill about a metre above the floor lets someone sitting down look out, whereas a window set too high shows only sky. A sloping ceiling gives a natural eye level that depends on the pitch, and standing in the empty loft with a tape measure is a better guide than a drawing. Third is furniture. Beds, wardrobes and desks are going to sit under the eaves, and the window needs to work around them, not behind them.
On the outside, windows placed in a line, aligned with the windows below and spaced evenly, look right from the street. Random positions look like afterthoughts. Our guide to roof window types helps choose between pivot, top-hung and fixed styles for each position. In a conservation area, the front slope is often protected, and a conservation rooflight on the back may be the answer, as in conservation rooflights.
Dormer or roof windows
Roof windows are the lightest way to convert a loft, but they do not add floor area or standing height. A dormer, a box built out from the slope with vertical walls, does both. Dormers are a bigger project: they usually need their own structure, a flat or pitched roof of their own, cladding, flashings and often a planning check. Many people use roof windows for the rear and side slopes and add a dormer only where more headroom is wanted, usually for a stair or a bathroom. A mix is quite common. Whichever route you take, the roof has to be sound and watertight around every opening. Our guide on when to replace a roof is worth reading before you build on an old covering.
Ventilation and overheating
Loft rooms are the hottest rooms in the house in summer. The roof catches the sun, heat rises into them and the ceiling is the roof. Windows are your best answer. Opening windows high and low create a through-draught, and roof windows with a ventilation position let air move while the sash stays nearly closed. Part F of the regulations expects a habitable room to have ventilation, and a window that opens can supply it. Solar control glass cuts the heat that gets in, external blinds block it before it reaches the glass, and an internal blind helps with glare. See do roof windows make rooms hot for the options. Keeping the bathroom door closed and using an extractor fan also keeps moisture from drifting into a bedroom and misting the glass, as explained in our guide on condensation on roof windows.
Insulation and the energy rules
A loft conversion changes a cold space into a heated one, so the roof becomes part of the thermal envelope. Insulation goes between and over or under the rafters, and the windows have to meet the energy standard. Under Part L 2021, a replacement or new roof window in an existing home passes either on heat loss (1.4 W/m²K) or on a Band B Window Energy Rating, while a rooflight is held to 2.2 W/m²K. A mainstream double-glazed roof window meets that, and our job is to make sure the insulation collar continues the roof insulation round the frame, with no gaps. A gap at the window is the commonest cause of cold patches and mould in a converted loft. Where the roof has been sprayed with foam, the junction needs particular care, and RICS guidance on foam in roofs explains the risk of damp being held against timbers.
The roofing work
Each window is a break in the roof, and the roofing side has to be right. The work goes in this order. The tiles or slates are stripped from the opening and a margin around it. The rafters are cut and trimmed if needed, and the underlay is cut and dressed to the frame. The window is fixed and the flashing kit is fitted from the bottom up, with the underlay collar sealed. The tiles are cut and re-laid around the flashing. Inside, the lining is fixed and the insulation completed. The choice of flashing depends on your covering, as set out in roof window flashing kits. Several windows are cheaper to fit together than one by one, since the scaffold and set-up are shared.
If the roof covering is old, fit the windows at the same time as renewing it. Cutting into a worn roof, then re-roofing it three years later, means paying twice. We tell you at the survey if the roof is sound enough to take the windows.
A typical sequence for a loft conversion
The windows are one item in a longer sequence, and their timing matters. Design and drawings come first, with the staircase, the escape route and the window positions all settled together. Building control is notified, and where a structural engineer is involved their calculations cover the floor, any steel and any wide window openings. The roof comes next: the scaffold goes up, the windows are fitted and the roof is made watertight, before the interior work begins. Only then do the floor, the stairs, the partitions, the insulation, the electrics and the plastering follow. Doing the roof first means the building stays dry while the expensive interior finishes go in. A conversion that fits windows late, or leaves openings covered with temporary sheeting through a wet spell, risks damp in the new floor and walls.
Tell us early if a conversion is planned, even if the windows are months away. We can survey the roof, check the covering and the rafters, and flag any problems with the structure before the architect finalises the layout. A roof that needs renewing is best dealt with first, because every window you fit depends on it.
What it costs
The only sourced prices are for windows in general. Fitted prices for a standard window run, according to one trade guide, from £680 to £1,300, and a manufacturer puts adding a new roof window at £1,906 with VAT. A loft conversion adds the wider project: stairs, floor strength, insulation, plastering, electrics and heating, which are a separate matter and outside what we quote. Our fixed written quote covers the roof windows and the roofing work, after the survey. For the detailed breakdown of a window price, see how much a roof window costs to fit.
Where to start
Before ordering windows, get three facts. What height is the existing loft floor above the ground, and is the house more than two storeys? What are the rafter size and spacing, and is the roof made of rafters or factory-made trusses, which cannot be cut without a design? And is the property in a conservation area, listed or a flat? The answers decide what is possible and which permissions are needed. Our planning permission for a skylight guide covers the permitted development side. Our roof windows page lists everything we fit, and 01256 213956 books a visit where we look at the roof, check the rafters and talk through the layout.
