The short answer
Building Regulations reach a roof window in two ways. Cutting a new opening alters the roof structure, so building control should be asked first. Fitting a replacement in an older home brings in the energy standard for windows, 1.4 W/(m²·K) or Band B. Roof windows are also left out of the legal definition of a thermal element.
A new opening alters the structure
Cutting a fresh opening interrupts rafters, which are the sloping timbers carrying the roof. A header and trimmers have to pick up that load, and the junction with the covering has to stay watertight. All of that is building work, and building control may want to inspect it.
The Planning Portal's re-roofing test, under 25 per cent of the building envelope and under 50 per cent of the roof, is written for changing a covering. It does not settle whether a structural opening is exempt, so the safe course is to ask rather than assume. Our answer on cutting rafters for a roof window explains the frame.
Which office answers depends on the address. Rushmoor and Hart run a single team, the Hart and Rushmoor Building Control Partnership, with 01252 398715 as its number. Surrey Heath's service is Surrey Heath Building Control Consultancy, Wokingham's is Building Control Solutions, while each remaining council in our area runs its own team.
A private alternative also exists: a registered building control approver, a regime running since 6 April 2024. Whichever you pick, the end product is a completion certificate where one applies, which a purchaser's legal adviser will probably request.
Where the window lights a loft room, Approved Document B on stairs and escape adds a further layer.
Swapping an old window for a new one
Energy rules govern replacement windows in homes that already exist. Approved Document L Volume 1, in the 2021 edition with 2023 amendments, has a window row in Table 4.2 asking for 1.4 W/(m²·K), or at least Band B for Window Energy Rating, and its note includes roof windows. That edition began on 15 June 2022.
A new edition renumbers the tables as 3.2 and 3.3 and starts on 24 March 2027 for most work. Until that day the old tables rule. If your job falls near the changeover, ask your building control body which edition it will apply to your address.
Rooflights have their own row at 2.2, so a flat-roof unit and a pitched-roof window must not be compared against one number. Our answer on a roof lantern or a flat rooflight looks at the flat-roof product.
Even a true like-for-like replacement has to reach the figure, so an old single-glazed unit cannot go back in as it was. The supplier's data sheet gives the U-value or rating for the whole window, and we file it with your paperwork.
If only the glass has failed, the job is lighter, and the sealed unit may be the part to renew.
Why a roof window is not a thermal element
Building Regulations 2010, regulation 2(3), limits the term thermal element to walls, floors and roofs, with windows, doors and rooflights excluded by name. So for energy purposes, glass in the roof answers to a standard of its own and sits outside the roof proper.
Renovation of a thermal element means a new layer or a replaced layer in it. That matters for the roof around the window rather than the window itself. Approved Document L sets 0.35 W/(m²·K) as the trigger for existing roofs and 0.16 as the improved level, with some give where full upgrading is impractical. For renovation and loft conversions it is 0.16, never 0.15.
In practice, a lone roof window does not oblige you to insulate the roof afresh. A re-roof or loft conversion that includes one may.
Planning is a different system again. Class C covers roof alterations to a house provided nothing projects beyond 0.15 metres from the slope, while a conservation area, a listed house or an Article 4 direction can change the position. Our answer on roof windows in a conservation area picks that up.
What we do about it on a job
Our survey establishes whether you need a swap or a fresh opening. We measure the pitch and the rafter sizes and look at the loft side, then tell you which rules are likely to bite. Where we cannot be certain, we name the question to put to building control.
The written scope names the window model with its published U-value or rating, lists any structural work, and states who is dealing with building control. A wide opening that needs an engineer is flagged before you commit. For a listed or conservation-area house, the consent route is settled first.
Photographs of the framing and the insulation collar are taken before the lining hides them, because an inspector may ask to see them and a later buyer's surveyor certainly will. They are yours, with the data sheet and any certificate.
None of this is legal advice. Decisions on building control and planning belong to the councils, and the right step is to check with them. Our job is to hand you the facts about your own roof so that the question you ask is specific.
For the stages of a job from first visit to finish, see roof windows and skylights.
