A roof window looks like a small job. In regulatory terms it is several jobs at once. Cutting an opening in a roof alters its structure. The new window is a heat-loss element, so the energy rules apply. If it lights a loft bedroom, the fire escape rules apply. And someone has to certify that all of this was done correctly. This guide sets out what the Building Regulations ask of a roof window in England and how the paperwork works. Planning permission is a different system, covered in our page on planning permission for a skylight, and this page does not restate those rules. We do not give legal advice. Your building control body decides what applies to your property.
Why a roof window is notifiable
The Building Regulations apply to building work, which includes altering a structure in a way that could affect its stability, thermal performance or safety. A new roof window cuts through rafters and insulation, so it is building work. A replacement window in an existing opening is also covered by the energy rules for replacement windows. That is why a window installer has to either notify building control or be registered with a competent person scheme that allows self-certification of certain work, mainly the energy aspects of window replacement. Where a new opening needs structural change, the structural side is not covered by those schemes in the same way. On any notifiable window job the application and the inspection booking are ours to look after, and the right certificate reaches you at the end.
Part A: structure and the trimmer
A pitched roof is a set of rafters spaced at regular intervals, each carrying its share of the roof load down to the walls. A roof window wider than the gap between two rafters needs one or more rafters to be cut. Their load has to go somewhere, which is why timber trimmers are fitted across the opening, one above and one below, and the cut rafters are supported on them. The trimmers pass the load to the uncut rafters on either side. Those side rafters then carry more, so they may need doubling.
The size of the timbers depends on the span and the load, and Approved Document A, which covers structure, sets out how to check them. For a small window between two rafters no cutting may be needed, which keeps things simple. For a wide window or a bank of two or three, the calculation grows, and an engineer's input may be needed. This is also the stage at which roofs of unusual construction, such as trussed rafters in a modern house, show their limits. Trussed rafters are engineered as a whole, and a truss must not be cut without a design from an engineer or the truss maker. That is why the type of roof is the first thing we check, and why our answer on whether you can put a roof window in any roof exists.
Part L: the energy figures
Part L covers the conservation of fuel and power. For existing dwellings the 2021 edition, in Table 4.2, sets targets for new or replacement windows. The target for a roof window is either a U-value of 1.4 W/m²K at worst, or a Window Energy Rating in Band B. A rooflight, which in this context means a glazed element in a flat or low-pitched roof, should achieve 2.2 W/m²K. The U-value is the measure of heat flow, so a lower number is better. The Window Energy Rating takes account of heat loss, solar gain and air leakage together.
A current double-glazed roof window from a mainstream maker usually meets the figure, but the data sheet should say so, and we check before ordering. Older windows and secondhand units may not. Part L also expects new windows to be fitted so that gaps are sealed and insulation is continuous around the frame, which is where an insulation collar earns its place. A poorly insulated surround is the usual cause of condensation, as set out in our guide to condensation on roof windows.
Part B: escape from a loft room
Approved Document B covers fire safety. If the window serves a bedroom or habitable room in a loft, the question is how people get out in a fire. Height is the limit: a roof window qualifies as an escape only for a storey no more than 4.5 metres above ground level, which rules it out as the escape from a loft on a typical two-storey house. A loft conversion on a two-storey house needs a protected stairway leading to the ground floor and a final exit, not a roof window. Where a roof window does serve as an escape, it has to be large enough, with an unobstructed opening and a sill at a suitable height. Our answers on what an escape roof window is and roof windows for loft conversions cover this in more detail.
Fire spread also matters near boundaries. A window close to a neighbouring property can be limited in how much unprotected area it provides, and the covering around it may need to have a suitable fire rating. Building control looks at such cases as they arise.
Weather-tightness and the workmanship side
The regulations also cover resistance to weather. Approved Document C deals with site preparation and resistance to contaminants and moisture, and it expects a roof to keep rain out. A roof window therefore has to be installed with flashings and an underlay detail that keep water out for the life of the covering, following the maker's instructions. That is why we use the maker's own flashing kit for the model and the roof covering, and why we never substitute sealant for a proper lapped detail. Inspectors do not usually test the flashing, but a leak later is the real inspection, and ours comes with a written 10-year workmanship guarantee.
Wherever a roof window meets insulation, the work should avoid a gap between the window and the roof insulation, because that gap is a route for cold air and moisture. A properly fitted insulation collar closes it. Where the roof has been insulated with spray foam, the foam must be trimmed back cleanly and the junction handled so that the foam does not cover the frame fixings. The RICS consumer guide from March 2023 cautions that foam may hold damp against the timbers and hide them from inspection, and lender attitudes to foamed roofs differ from one to another. If you have foam in your roof, tell us at the start.
Ventilation and daylight
Part F covers ventilation. A habitable room is expected to have ventilation, and a roof window with a trickle vent or a ventilation flap can supply it. Bathrooms and kitchens need extraction as well as, or instead of, opening windows. Daylight is a point people often ask about. There is no Building Regulations daylight requirement for houses in England, so you are not obliged to provide a particular glazed area, though good design usually does. Planning authorities sometimes look at daylight for neighbours, but that falls under planning, not building control.
Glazing safety and fall protection
A window that is low enough for someone to fall against needs safety glazing, usually laminated or toughened glass. Roof windows above a staircase, or in a position where a person could stumble into them, need to take that into account. In many roof windows the inner pane is laminated as standard, and the outer is toughened. Opening restrictors and child-safe catches may be wise where children use the room, although the regulations do not always require them. For a window that opens low in a wall as well as in a roof, guarding rules for openings can apply, and we flag them at the survey.
Who inspects and who certifies
When building control is involved, there are two routes. The first is the local authority's building control team at your council. The second is a private building control body, which since April 2024 is called a registered building control approver. Either route involves an application with details of the proposed work, inspections at set stages, and a completion certificate at the end. For a roof window the inspection is often at the point when the opening is framed and before the lining closes it up, because after that the timbers are hidden.
Some window installers are registered with a competent person scheme and can certify the energy part themselves. That route is for replacement windows in existing openings and does not cover structural alteration. We tell you at the survey which route applies. If you are selling, the buyer's solicitor may ask for the certificate, so keep it with the house papers. If you have a window without one, ask your council about regularisation.
Specific cases that change the answer
A flat is different. The roof is often not yours to alter, and a lease may forbid it. A listed building adds consent requirements, and a conservation area limits the position and style of windows, as our guide to conservation rooflights explains. A loft conversion brings the full set of regulations at once: stairs, escape, insulation, floor strength and fire protection. And a roof with spray foam insulation under the rafters can make cutting an opening and sealing the surround more involved, since foam may need cutting back first. Each of these changes the paperwork more than the window.
What the regulations mean for cost
The regulations add three costs: the structural timber and labour, a better-insulated window and collar, and the building control fee if a notice is needed. A modest part of the price of a fitted window. As a guide, one trade source gives a typical UK range of £680 to £1,300 for a standard centre-pivot window supplied and fitted, and manufacturer figures are £1,623 to replace and £1,906 to add a new one, including VAT. Your fixed quote follows the survey and itemises any building control charge. Our roof window installation cost page explains the usual cost drivers.
For the whole picture of the service, see our roof windows page, or ring 01256 213956 to book a visit where we measure the roof, check the rafters and explain which of these rules apply.
