Building Regulations are the second system that touches a rooflight job, after planning. They do not ask whether a rooflight looks right in its street. They ask whether the work is safe, whether the roof is still strong, whether heat is kept in and whether people can get out of a room in the roof if there is a fire. The answers depend on what is being done to the roof, and three jobs that look alike from the garden fall into three different categories.
A new opening, where there was solid roof before, is structural work. A like-for-like swap of a complete unit is a notifiable replacement. A new sealed glass unit in a frame that stays put is neither. Each has its own route, and the rules in England are the same for a house in Bournemouth as for one in Lincolnshire, although the body that checks the work differs from place to place.
Each job is taken in turn, with the thermal figures a new rooflight or roof window has to reach, the structure around a new opening, escape from a loft room and the body that receives the notification. Planning permission is covered separately in our planning guide.
Two systems that are easy to confuse
Planning law controls the use of land and the appearance of buildings. The Building Regulations control how a building is constructed, altered or extended. They are made under different Acts, they are applied by different departments, and a house can satisfy one while failing the other.
A rooflight that is permitted development needs no planning application, yet cutting the rafters to fit it still needs Building Regulations approval. In the other direction, a rooflight can have full planning consent and still be refused a completion certificate if the framing around it is undersized. People are caught out by this when a planning decision notice arrives and they assume the paperwork is finished.
The Building Regulations are divided into lettered Parts, each with an Approved Document that explains one way of meeting the rule. Four Parts matter for rooflights:
- Part A, structure: the roof must still carry its loads once rafters are cut
- Part B, fire safety: escape from rooms in the roof, and the spread of fire across a boundary
- Part L, conservation of fuel and power: the thermal performance of the new unit
- Part F, ventilation: relevant where a room is being formed or changed in use
Approved Documents are guidance. A builder may show compliance another way, but following the document is the common route, and building control officers read the work against it.
Which rooflight jobs fall under which rule
The question to put first is what exactly is being removed and what exactly is going in. The jobs we are asked about most are listed here.
| Job | Route |
|---|---|
| New opening cut into a roof | Approval needed: structural alteration |
| Whole rooflight or roof window replaced, frame and glass | Notifiable as a controlled fitting |
| Larger unit in an existing opening, rafters cut | Approval needed: structural alteration |
| Sealed glass unit replaced, frame stays | Not notifiable |
| Blind, motor or rain sensor added to a compliant unit | Check the electrical work separately |
A controlled fitting is a window, rooflight, roof window or door that the regulations name as something the energy rules apply to. When a whole unit is taken out and a new one goes in, the new one counts as a new controlled fitting, so it must reach the figures in Part L and the work must be notified.
The glass-only case works differently because the frame, the part fixed to the building, is not being replaced. A new sealed unit in an existing frame counts as maintenance of the existing fitting, so it falls outside notification. If the frame is cracked, warped or failing, then the sensible answer is a whole new unit, which brings it back into the notifiable group. Our guide to new glass or a whole new rooflight shows how that choice is made on the roof.
A like-for-like swap in the same opening
When the new unit fits the existing opening and no rafter is cut, there is no structural alteration. The job is notifiable only for the energy reason, and the paperwork is lighter. The moment the new unit is wider than the gap between the rafters, a rafter has to be trimmed, and the job moves into the structural category with a more detailed check.
The same applies to a flat roof. Taking out a rooflight and putting a bigger one on a new kerb changes the opening in the joists or deck, and that is a structural change as well. Our answer on replacement and building control covers the swap case in a short form.
The thermal figures a new unit must reach
Part L, with its Approved Document L, sets the limits for heat loss through new fittings in existing homes. The measure is the U-value, in watts per square metre per kelvin (W/m²K). A lower number means less heat escapes. For a replacement or new fitting in an existing dwelling in England the figures are:
| Type of unit | What it must reach |
|---|---|
| Rooflight | 2.2 W/m²K |
| Roof window | 1.4 W/m²K, or a Window Energy Rating of Band B |
The two entries are separate because the regulations treat a rooflight and a roof window as different products. Approved Document L defines a rooflight as a glazed unit installed out of plane with the surface of the roof on a kerb or upstand. Flat-roof rooflights and roof lanterns both fit that description. A roof window, in the same document, is a window set in plane with the roof around it, facing the same way, which is how a window in a pitched slope sits. The line between them affects which target the unit is tested against, so the label on the product, the maker's data sheet and the roof it goes into have to agree.
A Window Energy Rating is an alternative route for windows. It is a letter band, calculated by a method the Glass and Glazing Federation publishes, that takes account of solar gain as well as heat loss, so a window with a slightly higher U-value can still reach Band B. The route is open to roof windows and it saves some argument when a maker publishes the band but a U-value only for the glass.
Reading the figure on a data sheet
Makers quote two U-values. One is for the glass alone, known as the centre-pane value. The other is for the whole unit, frame included. The whole-unit figure is the one the regulations use, because a good pane in a poor frame loses heat through the frame. For a rooflight the whole-unit figure is based on the developed surface area, which is often larger than the roof opening. Where the maker supplies the rooflight and kerb as one assembly with a combined figure, the 2.2 limit applies to the assembly; a kerb built on site should itself reach 0.35 W/m²K. Compare the figure for the entire assembly with the limit and keep the data sheet with your records.
Makers publish the whole-unit figure for each glazing option, and triple glazing goes lower than double. Which glazing suits your roof is a separate question, covered in our guide to rooflight U-values and the double or triple glazed comparison.
The limit for a rooflight is 2.2 W/m²K. A roof window has two ways to pass: 1.4 W/m²K, or a Window Energy Rating of Band B.
Cutting a new opening: the structure
A pitched roof is carried by rafters, each one a timber that runs from the ridge down to the wall plate. They are spaced at regular centres, commonly several hundred millimetres apart, and the roof covering is fixed to battens across them. The rafters are not decoration. They share the weight of the covering, the snow and the wind load between them.
A roof window that is narrower than the gap between two rafters can often be framed without cutting anything. A wider window needs one or more rafters cut, and the load of each cut rafter has to go somewhere. The usual answer is a pair of horizontal timbers called trimmers, fixed above and below the opening, which pick up the cut rafters and pass their load to the full-length rafters at the sides. Those side rafters are often doubled up to carry the extra.
Cut rafters are what make a new opening a structural alteration under Part A. The building control officer wants to see what the roof is made of, which timbers are being cut, how the trimmers are sized and how they are fixed. In some roofs, especially with larger openings or older timbers of uncertain size, a structural engineer's calculation is part of the submission.
Flat roofs
A flat roof has joists in place of rafters, and the deck lies across them. A new rooflight or lantern opening works the same way: joists are cut, trimmers go in, and the kerb stands on a timber frame built around the opening. The kerb itself must be tall enough for the roof covering to turn up it, a height set by the maker and the roof build-up. We cover that in our guide to rooflight upstand height.
What a measurement on the roof gives building control
Our survey records the rafter or joist spacing, the timber sizes where they can be seen, the pitch and the position of the opening against the ridge and the eaves. Those figures go on the survey sheet we give you, and they are the starting point for any drawing a building control body asks for. A guess made at ground level cannot supply them.
Rooms in the roof and escape
Approved Document B covers fire safety. For a rooflight job the part that matters is the escape window, a window large enough and low enough for a person to climb out of if the stair is blocked. The rule applies when a bedroom or other habitable room is created in the roof, for example in a loft conversion.
An escape window only serves a storey that is within 4.5 m of the ground. Past that height a person cannot be expected to get out and down. That sets the limit for what a roof window can do.
| Type of house | Escape from a loft room |
|---|---|
| Bungalow or chalet bungalow | A roof window of suitable size can act as the escape window |
| Two-storey house | A protected stair is needed, a roof window cannot stand in for it |
The height rule explains the second row. On a two-storey house the floor of a loft room is a third storey, above 4.5 m. A protected stair, enclosed in fire-resisting construction, is the route out. Whether a house already has the right stair is something the loft conversion drawings and the building control officer settle, not the rooflight itself.
For a bungalow, the clear opening set by Approved Document B is 0.33 m² or more, with neither side under 450 mm, its lowest edge 1100 mm or less from the floor, and a sash that stays open unaided. The maker's data sheet gives the clear opening of the window you have chosen, and the two are compared before the window is ordered, because a centre-pivot sash sits across its own opening while a top-hung sash swings clear of it. Our answer on how many roof windows a loft room needs covers the light side of the same room.
Approved Document B also deals with how fire spreads across a boundary, and some rooflight materials are limited close to a neighbouring plot. We record the distance to the boundary at the survey and check the unit against the document.
Who checks the work and how notification works
Every notifiable job has to be checked by a building control body. There are two kinds. The local authority service is run by the council; for Bournemouth, Christchurch and Poole that is BCP Council's building control team. The alternative is a private firm of registered building control approvers, the name used since April 2024 for approved inspectors.
The homeowner chooses which route to take, and the choice is open: both bodies work to the same regulations. The process looks like this.
- A building notice or a full plans application is sent to the building control body before work starts. A building notice suits small jobs such as a whole-unit replacement; full plans suit anything with detailed drawings, such as a new opening in a loft conversion.
- The body sets out when it needs to inspect. For a new opening that is normally when the roof is open and the trimmers are visible, before the lining hides them.
- After the work, the body issues a completion certificate or final certificate. Keep it with the house deeds.
The certificate has a practical use. When the house is sold, the buyer's solicitor asks for evidence that alterations were signed off, and a missing certificate for a roof window is a common gap. Getting it right at the time is easier than getting a retrospective regularisation later.
Our job on any rooflight installation is to tell you which of these routes applies to the unit and the opening we have measured, and to supply the figures building control asks for. The reference for the rooflight itself is the roof window installation service if the job is on a pitched roof, or flat roof rooflight installation for a flat one.
Pulling the paperwork together
By the time a rooflight is ordered, the homeowner should hold a short stack of documents. They are not complicated, and each answers one of the questions above.
- The survey sheet, with pitch, rafter or joist spacing, opening size and kerb height as measured
- The maker's data sheet showing the whole-unit U-value or Window Energy Rating band
- A note of which building control body is receiving the notification
- For a loft room, the escape window details from the data sheet
- The planning position for the address, from our check or from the council
The planning item is on the list because the two systems meet there. If the planning answer is that an application is needed, nothing should be cut until it is granted, however far the building control side has progressed.
Once the work is complete, the final items are the building control certificate and the maker's installation guarantee. Keep all of it together. A later buyer, a lender or the next person to work on the roof will want to see it.
faq
building control questions we hear
Do I need Building Regulations approval to replace an old rooflight?
If the whole unit, frame and glass, is replaced it is notifiable as a controlled fitting and the new one must meet the current thermal figure. If only the sealed glass unit changes, it is not notifiable.
Can I choose a private approver instead of the council?
Yes. Registered building control approvers can check the work in place of the local authority service. Both apply the same regulations, so the choice is about timing, cost and who you prefer to deal with.
Does a rooflight have to meet a minimum daylight standard?
No Building Regulations daylight requirement applies to rooflights in an existing house in England. The thermal figure, the structure and the escape rules are what building control looks at.