Part L of the Building Regulations deals with the conservation of fuel and power, and for a house its guidance sits in Approved Document L, volume 1. Windows, roof windows, rooflights and doors are all controlled fittings under that document, which is why a rooflight can fall foul of a heat-loss test before anyone has asked how much light it gives. The document describes one accepted way of complying with the regulations. The building control body judges the finished work against the regulations themselves.
What Part L asks of a house, and what it leaves alone
A fitting you add or replace has to reach a limiting U-value. A fitting you leave alone does not. Nothing in the guidance obliges you to upgrade a rooflight that stays where it is, so a tired unit over a landing remains your decision until the roof is opened or the glazing is changed.
Part L starts to bite when glass goes into the roof, and from then on it caps how much heat that glass may let through.
Why roof glazing gets its own figure
A wall is solid for most of its area, and a window is a small part of it. A rooflight is glass and frame all the way across, set in the layer of the house where warm air gathers. The guidance treats roof glazing as its own case, and draws a line between two kinds of roof glazing in its glossary.
The limit applies to the unit as installed
The ceilings are whole-unit values, with glass, spacer bar and frame counted together. A fabricator can supply a good pane in a heavy frame and still miss the figure. For that reason we keep the data sheet for the exact size that goes on your quote, and the sheet is what we hold up against the limit.
Quick orientation
A new or replacement roof window takes 1.4 W/m²K, or Window Energy Rating band B at the least. A rooflight or roof lantern takes 2.2 W/m²K. Both figures are for the whole unit, frame included.
Roof window or rooflight: the word decides the number
Approved Document L carries its own glossary, and the sorting happens there. For a rooflight it says a glazed unit standing on a kerb or upstand, out of plane with the surface of the slope or deck. For a roof window it says a unit that shares the orientation of the surrounding roof and lies in plane with it.
That wording puts a glass panel over a flat-roofed utility and a lantern over a kitchen extension on one side, because each stands on an upstand. A window set into the slope of a loft or a chalet bungalow, lying level with the tiles round it, is on the other.
For work on an existing dwelling, Table 4.2 of the 2021 edition gives the ceilings. The window limit of 1.4 W/m²K goes to a new or replacement roof window, with Window Energy Rating band B as the alternative route. A rooflight or roof lantern is allowed 2.2 W/m²K.
Odd cases
A unit that stands proud on a kerb is a rooflight by the glossary's test wherever it sits, so a glass panel on a kerb in a pitched garage roof takes the 2.2 figure. A flush conservation-style window lying in the slope takes the window figure. We name the category on the quote so you can see which limit we worked to.
| What goes in the roof | Glossary term | Limit for an existing dwelling |
|---|---|---|
| Window in a loft slope, lying with the tiles | Roof window | 1.4 W/m²K, or band B on the Window Energy Rating |
| Glass panel on a kerb over a flat roof | Rooflight | 2.2 W/m²K |
| Roof lantern on an upstand | Rooflight (a lantern is one) | 2.2 W/m²K |
| Flush conservation-style window in a pitched slope | Roof window | 1.4 W/m²K, or band B |
Work that brings Part L into play
A new hole in the roof is the plainest trigger. It counts as building work, and it draws in more than energy: the structure round the opening, fire protection and, in some rooms, escape. Part L is the energy strand of that package, and the unit has to meet the figure for its type from the day it goes in.
Replacement is the second trigger. When an old rooflight comes out and a new one goes into the same opening, the new unit is a controlled fitting being provided, and it meets today's ceiling. A lantern fitted before the current edition appeared may have been fine in its day and still fall short of what a replacement must reach.
A glass swap in a frame that stays
Changing only the sealed glass in a frame you keep is a question people raise with misted units. The answer turns on how the work is classed and on what the old frame can carry, so we settle it at the survey and write the route on the quote. Our guide to new glass or a whole new unit covers the practical side.
Extensions and conversions
A lantern in a new extension roof is part of a wider energy assessment of the extension. A loft or garage conversion brings requirements for its roof, floor and walls alongside the glazing. We raise those at the survey so the rooflight is specified with the whole project in view.
Who checks the work
Where the work is notifiable, we handle the Building Control notification. The local authority's building control team or a registered building control approver, the title for private bodies since April 2024, inspects it and signs it off. Your council's planning team is a separate door, and the skylight planning guide deals with that side.
The Part L side of a job
- Survey and category We decide whether the unit is a roof window or a rooflight, and note the limit that goes with it.
- Notification We handle the Building Control notification where the work is notifiable.
- Inspection The local authority or a registered approver inspects as the work proceeds.
- Completion The data sheet for your unit and the sign-off paperwork are gathered into one file.
The evidence that proves a unit complies
Paperwork for the energy ceiling comes in layers. The manufacturer's data sheet gives the whole-unit U-value for a named size and glazing build-up. For a roof window judged on the Window Energy Rating route, the rating band is the proof. Our written quote then names the product, the size and the glazing, so anyone who opens the file later can match sheet to unit.
Reading a data sheet
A sheet may print a figure for the middle of the glass, another for the frame and a third for the unit as a whole. The ceiling applies to the third. Where a supplier gives only the centre-pane value, we ask for the whole-unit figure before we price the job, because a number that leaves out the frame cannot be set against 1.4 or 2.2.
A bespoke lantern has no catalogue entry
A lantern made to measure has no stock listing for its exact size. The fabricator supplies a figure for that build, and we check the claim against the glass build-up and the frame section before the lantern is accepted for the job.
Why size shifts the answer
The frame takes up a larger share of a small unit than of a large one, so two sizes in the same range can return different whole-unit values. A catalogue code on a roof window, such as MK04, points to a specific size: the letters give the width and the digits the height.
Four questions for any data sheet
Is the figure for the whole unit? Is it for the size on the quote? Which glass build-up and frame does it assume? Does the product named on the quote match the product named on the sheet?
Planning, listing and conservation sit beside Part L
A lantern that meets 2.2 W/m²K still has to get through planning. Its upstand and glass usually stand more than 150 mm proud of a flat roof, which puts it outside the Class C permitted development that usually covers roof windows on houses, so it normally needs an application unless the council confirms otherwise. A flush roof window is the reverse case: usually straightforward on planning, and held to the tighter 1.4 ceiling.
Flats, converted buildings, houses with a condition removing their rights and the rule for side slopes are all set out in the skylight planning guide, and the lantern planning and Building Regulations page takes the lantern case further.
Old houses and special consideration
Approved Document L makes allowance for buildings of special architectural or historic interest, so the energy figure may be balanced against the character of the building. In that situation the council's conservation officer becomes part of the conversation, and the energy limit is argued with the building in view. A listed house keeps its Class C rights, yet an altered or new roof window almost always requires listed building consent, as the guide to rooflights on listed buildings explains.
Conservation areas on their own
Living in a conservation area does not strip Class C away by itself; an Article 4 direction does, and the page on Article 4 and rooflights shows how to find out whether one covers your street.
Myth
A replacement rooflight only has to match the performance of the one it replaces.
Fact
The new unit is measured against the current ceiling for its type, and a unit installed years ago may not reach it.
Choosing a unit that clears the limit
When a unit you like misses its ceiling, there are levers to pull before the idea is dropped. A different size can return a better whole-unit figure. A change of glazing build-up, with a low-emissivity coating or a gas fill, lifts the glass. A frame with a thermal break warms the inner face. We try these on paper at the survey, and the quote shows the version that passes.
The kerb round a rooflight
The ceiling covers the unit and not the roof round it, but a cold upstand undoes some of the benefit. On a garage conversion with a flat rooflight, the insulation in the kerb joins the insulation in the roof, and we note the state of that junction when we measure. Our page on upstands and kerbs goes through the build-up.
Where to read further
The numbers behind U-values are worked through in rooflight U-values explained, and the effect of material is covered in glass or polycarbonate for heat. The wider picture of heat, glare and glass lives on the energy-efficient rooflights page.
For a loft room, a chalet bedroom or a bungalow hall where you want to know which ceiling applies, book a free survey. You get a fixed written quote that names the category and the whole-unit figure, and a 10-year workmanship guarantee in writing.
