Guide · Energy Efficient Rooflights
Rooflights and Part L for a replacement
Replacing a rooflight in an existing dwelling in England is controlled work, and the part of the Building Regulations it engages first is Part L, conservation of fuel and power. In practice that means the new unit has to meet a limiting thermal standard, the work has to be notified to Building Control, and a certificate has to exist at the end of it. None of that is difficult. Almost all of the trouble it causes comes from it being skipped.
This page sets out what the limiting figure is, how it is declared, which route the notification takes, what the resulting paperwork is for, and what happens when a previous owner never did any of it. It is the compliance half of the specification discussion on the energy efficient rooflights hub.
What counts as a replacement under the Regulations
The Regulations distinguish between putting a rooflight where there was not one before and swapping a unit in an existing opening. Both are controlled work. The first is usually treated as a new opening in a thermal element and brings the wider set of considerations that go with cutting a roof. The second is a replacement of a controlled fitting, and the requirements are narrower and easier to satisfy.
The line is drawn at the opening, not the unit. Taking out a 900 by 600 dome and fitting a 900 by 600 flat rooflight into the same kerb is a replacement. Taking out the same dome and enlarging the opening to fit a 1500 by 1000 unit is a new opening with a replacement inside it, and the structural and thermal questions both widen.
The limiting U-value a replacement has to meet
Approved Document L sets limiting standards for controlled fittings in existing dwellings, and rooflights carry their own line separate from windows. The limiting whole unit U-value for a rooflight replacement is more generous than the figure for a window, precisely because the test conditions for a unit lying in or near the horizontal plane are harsher.
What matters practically is that the threshold is a ceiling and not a target. Current double glazed rooflights from the mainstream manufacturers clear it comfortably, often by a wide margin, which is why Part L rarely constrains a domestic replacement. It constrains two things only: a very old design still being sold at the bottom of the market, and a bespoke unit where somebody has prioritised a slim sightline over everything else.
Whole unit against centre pane, and which one is assessed
The figure Building Control is interested in is the whole unit U-value, which includes the frame, the spacer and the edge of the glass. Centre pane figures are always better and always irrelevant here, and quoting one against a limiting standard is either a misunderstanding or a sales tactic.
The difference is not small. A unit with a soft coat and argon might read 1.0 W/m²K at the centre of the pane and 1.5 as an assembled rooflight, and on a small unit where the frame occupies a large share of the area the gap widens further. Ask for the whole unit figure in writing at order, because it is what gets recorded and it is what a future assessor or solicitor will look at.
The plane the declaration refers to
A rooflight can be declared honestly at two different U-values because the standards permit assessment in different orientations, and heat moves differently through a horizontal cavity than an upright one. Convection in a near flat sealed unit is more vigorous, so the horizontal declaration is the less flattering figure.
A unit quoted at 1.2 W/m²K in the vertical plane might be nearer 1.6 in the horizontal. Neither is wrong. Comparing one manufacturer’s vertical figure with another’s horizontal figure is wrong, and it happens constantly. For a compliance conversation, establish which plane the declaration refers to before anything else, because the limiting standard is expressed against a specific basis.
Ask for the whole unit U-value and the plane it was declared in, and get both on the order.
Notification routes, and who signs it off
There are two lawful routes for notifiable work. Either a building notice or full plans application is submitted to the local authority Building Control department, or the work is self certified under one of the government authorised registration schemes by an installer who belongs to it and notifies on the homeowner’s behalf.
Sunspire submits to Building Control. The notification is made as part of the job rather than being left to the homeowner, the inspector attends where the authority requires it, and the completion certificate is issued in the homeowner’s name at the end. It is a slightly slower route than self certification and it produces a document issued by the authority, which is the version a conveyancing solicitor prefers to see.
What the completion certificate is actually for
The certificate has no day to day use. It sits in a file and does nothing at all until the house is sold, at which point it becomes the single document that stops a straightforward transaction becoming a slow one.
A buyer’s solicitor asks about alterations. If a rooflight is visible on the survey and no certificate exists, the standard responses follow: a request for regularisation, a request for indemnity insurance, a retention against the price, or a delay while the seller tries to reconstruct what happened eleven years ago. All of that is avoided by a piece of paper that was free to obtain at the time.
Rooflights in unheated and semi heated spaces
The thermal requirements attach to the envelope of a heated building. A rooflight in a detached garage roof, an unheated outbuilding or a carport is not sitting in a thermal element of a dwelling, so the Part L limiting standard for a dwelling does not bite in the same way.
Two cautions apply. A garage that is later converted to habitable space brings the whole envelope inside the heated boundary, and the rooflight is then assessed with everything else, so specifying to the dwelling standard at the outset is the cheaper decision. And structural and safety requirements are unaffected by heating: overhead glazing still needs to be safely specified, and an opening still needs forming properly.
Where a replacement triggers more than Part L
Part L is the part that always applies. Others join it depending on what the rooflight does and where it sits.
| Part | Subject | When it engages on a replacement |
|---|---|---|
| L | Conservation of fuel and power | Always, on any replacement unit |
| K | Protection from falling and impact | Overhead glazing, accessible units, walk-on glass |
| B | Fire safety | Where the unit serves as an escape opening |
| F | Ventilation | Where the unit carried a trickle vent that is being removed |
| O | Overheating | New dwellings, not existing dwelling replacements |
The one people miss is Part F. Where the unit being taken out had background ventilation built into it, removing that provision without an equivalent elsewhere makes the room worse ventilated than it was, and the replacement is expected to maintain it.
Safety glazing overhead, and why Part K rides along
Glass above head height has to be specified so that failure does not drop material into the room. In practice the inner pane of a rooflight is laminated, so a broken pane holds together on its interlayer rather than falling as fragments.
This is standard on modern units and rarely needs asking for, but it is worth checking on a bespoke order and on anything imported at the low end of the market. It also interacts pleasantly with the acoustic specification, since a laminated pane damps rain noise as well as satisfying the safety requirement. On walk-on glass the requirement steps up considerably and the build-up is structural.
Escape openings, and the dimensions that go with them
Where a roof window in a loft room is relied on as a means of escape, the replacement has to preserve that function, and the requirement is dimensional rather than thermal. There is a minimum clear opening area, a minimum unobstructed height and width, and a maximum cill height above the floor of the room.
This catches people out on like for like swaps, because a modern unit of the same external size can have a different clear opening depending on whether it is centre pivot or top hung. Where the existing window was the escape route, the opening geometry of the replacement is checked against the requirement before anything is ordered. It is set out in more detail on the roof window installation page.
Listed buildings and conservation areas
Listed building consent and Building Regulations are separate processes with separate applications, and satisfying one says nothing about the other. Essex has an unusual concentration of protected fabric: Thaxted, Coggeshall, Saffron Walden and the older core of Maldon, with conservation areas across most market towns in the county.
Where a historic building would be unacceptably altered by meeting the full thermal standard, the Regulations allow the requirement to be applied so far as is reasonably practicable, which is the mechanism behind slim framed conservation rooflights sitting flush in a slate slope. The route is a conversation with the conservation officer and Building Control together, before the specification is fixed, not a reason to skip either. The unit itself is covered on the conservation rooflight installation page.
Planning permission, which is a separate question again
Building Regulations govern how the work is built. Planning governs whether it is allowed at all. For most houses a rooflight falls under permitted development, subject to the condition that it does not project more than 150mm beyond the plane of the existing roof slope and does not sit higher than the highest part of the roof.
Those rights are not universal. They are commonly withdrawn by an Article 4 direction in conservation areas, they do not apply to flats or maisonettes, and they are frequently restricted by planning condition on newer estates around Chelmsford and the growth areas at Beaulieu Park. Checking takes very little time and it happens before anything is ordered.
Evidence worth keeping in the file
The certificate is the headline document, but a complete file makes any future question a five minute conversation rather than an investigation. Keep the following together.
- The Building Control completion certificate for the work.
- The manufacturer’s declared whole unit U-value, naming the plane of declaration.
- The glass make-up specification, pane by pane.
- The order or specification document naming the model and external size.
- Photographs of the opening before the covering was made good.
- The workmanship guarantee document.
The photographs are the item nobody keeps and the item that answers the most questions later, because they show the kerb, the insulation line and the flashing laps that are invisible for the rest of the building’s life.
When the previous work was never notified
This is common, particularly on units fitted through the eighties, nineties and the years when the rules were less well understood. The rooflight is there, it works, and there is no paperwork behind it.
Two routes exist. Regularisation is a retrospective application to the local authority, which inspects what it can and issues a regularisation certificate if satisfied. It may require opening up to see a detail, and it carries a fee. Indemnity insurance is the alternative most conveyancers reach for, which covers the risk of enforcement rather than confirming the work is sound. Where a rooflight is being replaced anyway, the cleaner answer is often to notify the new work properly and let the certificate for the current unit stand as the record going forward.
How it runs on a Sunspire job
The notification is handled as part of the installation rather than being handed to the homeowner as an afterthought. The sequence is consistent. The specification with its declared U-value is fixed at order. The notification goes in before work starts. The inspector sees the opening at the stage the authority asks for, which on a rooflight is usually the perimeter detail before the covering is made good. The certificate follows completion.
That ordering matters because the inspection window is short. Once the covering is dressed and the reveal is lined, the parts an inspector wants to see are behind finished work, and demonstrating them means opening something up. Getting the notification in first keeps the whole thing to a single visit, and the workmanship behind it carries the ten year guarantee.
What to ask any installer before signing
Four questions settle the compliance position on any quote, and they take a minute. Which route is the notification going through, local authority or self certification. What is the declared whole unit U-value and in which plane. Who applies, and is the fee inside the quoted figure. And who holds the certificate at the end, and when does it arrive.
A quote that names a unit, a U-value, a notification route and a fee is comparable with another quote. A single figure beside the words supply and fit rooflight is not, and the difference between the two very often turns out to be the paperwork. Indicative figures for replacement work are on the costs page, and the practical side of swapping an old unit is set out under skylight replacement.
Keep reading
More on energy efficient rooflights
Solar gain, overheating and shading
Read the guide →Condensation, ventilation and rooflights
Read the guide →Triple glazing in a rooflight and when it pays
Read the guide →Warm-edge spacers and where condensation starts
Read the guide →Low-emissivity coatings and argon fills
Read the guide →
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