Part L and replacement rooflights

Part L of the Building Regulations covers the conservation of fuel and power. For dwellings the guidance sits in Volume 1 of Approved Document L, and 2021 is the edition in force. It is guidance rather than law, but following it is the usual way to show the work complies with the regulations themselves.

A rooflight is a thermal element in the roof. Glass loses heat far faster than an insulated roof build, so the regulations treat windows, roof windows and rooflights as a category of their own, with a limiting U-value they must not exceed. U-value measures how much heat passes through a material: the lower the figure, the better it holds warmth in. If you want the measure explained from the beginning, our guide to rooflight U-values covers it.

The regulations also use a term that matters here: controlled fitting. Windows, roof windows, rooflights and doors in the external envelope are controlled fittings, and fitting a new one is controlled work. That is why a rooflight is regulated even when no structure is touched. The glass and frame are the thing being controlled, not just the hole they sit in.

The rules differ depending on what the job is, and it is worth being clear about which one you are having.

A new rooflight in an existing roof

This is building work in two senses. Cutting or trimming rafters is a structural alteration, and the new unit has to meet Part L. It needs Building Control approval, and a completion certificate once it is signed off.

Replacing the whole rooflight

Taking out the old frame and glass and fitting a new unit is also notifiable, even when the new one goes into the same opening. The replacement is a controlled fitting, so it has to meet Part L, and the work has to be notified to Building Control or self-certified by an installer registered with a competent person scheme.

Replacing only the sealed glass unit

If the frame stays and only the double glazed unit inside it is changed, the work is not notifiable. This is the usual route for a unit that has misted between the panes while the frame is still in good order. It does not release you from good sense: the new unit should perform at least as well as the old one, and ideally better, which is easy with modern glass. There is a fuller look at that choice on replacing just the glass in a rooflight.

The dividing line is the frame. Frame in, notifiable. Frame stays, glass only, not notifiable. Where a job sits near that line, for example a frame that looks sound but is being swapped for a different opening method, treat it as notifiable and say so on the quote.

Approved Document L sets a limiting U-value for a replacement window, roof window or rooflight, and a tighter one for a new element in a new dwelling. The numbers change between editions, so we do not print one here. The figure for your job is in the current Approved Document, and it is the figure your installer should be designing to.

What matters for choosing glass is how a rooflight reaches the limit. There are four things that move the number:

• the number of panes, double or triple glazing;
• a low emissivity coating on one pane, which reflects heat back into the room;
• the gas between the panes, usually argon, which conducts less than air;
• the spacer bar at the edge of the glass, where a warm-edge spacer cuts the cold bridge around the perimeter.

Frames count too. A rooflight is rated as a whole, frame and glass together, so a very good pane in a poor frame does not deliver the headline figure. Ask for the whole-unit U-value, not just the centre-pane value that sometimes appears in brochures. The centre-pane figure is always the flattering one, because the edges and frame lose more heat than the middle of the glass.

Double or triple is a decision worth reading on its own. We cover it in double or triple glazed rooflights, including what the extra pane does to weight and to the frame it has to sit in.

There are two routes for a notifiable replacement, and the paperwork differs.

The first is to involve Building Control: a Building Notice or Full Plans application to the local authority, or an initial notice from a registered building control approver (the private-sector route, renamed under the Building Safety Act 2022). A surveyor may visit, check what has been fitted, and issue a completion certificate once satisfied. This route needs a little notice, and the work cannot be hidden behind new finishes before it has been inspected, which matters when a lining and plaster skim are going on afterwards.

The second is self-certification by an installer who is registered with a competent person scheme for window installation. They declare the work compliant themselves, and a certificate follows through the scheme. The route depends on the installer, and not every firm uses it.

Either way, the paper trail is the point. When you sell the house, a solicitor may ask for the certificate, and a missing one for a replacement rooflight can mean a retrospective application or an indemnity policy. We handle the Building Control notification for you where the work is notifiable, and the certificate belongs in the same folder as your finish schedule and guarantee.

It is also worth knowing what the notification does not do. It confirms the unit and installation meet the regulations. It does not assess the plaster finish or the paint line on the inside. That is why the finish schedule exists as a separate document, so what happens in the room is written down as well.

Approved Document L allows for some situations where meeting the standard in full would be wrong for the building.

Listed buildings, and buildings in conservation areas or of traditional construction, have special considerations. The guidance accepts that a replacement may need to be a different specification where the standard unit would harm the character of the building or its historic fabric. That is a conversation to have early, with the conservation officer at the local planning authority as well as with us. It is also worth reading about conservation rooflights, which are made to sit flush and slim, and which come with glazing options that still reach decent thermal performance.

Listed buildings need listed building consent for external changes, whatever Part L says. In conservation areas, permitted development for rooflights usually applies, but an Article 4 direction can remove it, so check with the local planning authority before ordering anything. National parks, such as Dartmoor and Exmoor, can have some permitted development rights restricted.

The other case people ask about is a rooflight in an unheated space, such as a porch, a lean-to or a garage that is not part of the living accommodation. The thermal rules apply to the thermal envelope of the dwelling, so whether a particular space counts depends on whether it is inside it. That is a fact about your house, not a general rule, and it is the kind of detail we note down at the survey.

Meeting the limit is the floor, not the target. Once a rooflight sits on the right side of the U-value limit, the useful questions are about the room under it.

A south-facing kitchen extension with a lantern can overheat in summer even with a very good winter U-value. That is a solar gain problem, and the answer is solar control glass, not a lower U-value. We cover it in solar control glass for rooflights. A north-facing landing may want the clearest glass and the best insulation it can get, because it has nothing to gain from the sun and everything to lose through the pane.

Laminated inner panes are worth considering overhead. They are good practice rather than a Part K requirement, because Part K4 covers glazing in critical locations such as doors and low-level panels, not rooflights as such. But laminated glass holds together if it breaks, which is a sensible thing to want above a bed or a dining table.

A few other choices often get settled at the same time:

• warm-edge spacers, which help the edge of the glass stay warmer and reduce the risk of condensation forming along the sill;
• acoustic laminate, where rain on glass is a concern, as a glazing option rather than a headline feature;
• self-cleaning coatings, which suit a flat rooflight you cannot easily reach.

None of these change the Part L result on their own, but they all go on the specification the notification refers to, so it is worth deciding them before the order is placed.

A replacement rooflight meets the regulations at the glass, and the room only sees the lining. So Part L work and the finish are bound together more than they seem.

A new frame is often slightly different in size or depth from the one it replaces, which leaves a gap or a step at the edge of the old lining. Better insulation around the frame is part of reaching the thermal standard, because a cold lining is where condensation shows first. That means stripping back to see what is behind the plaster, wrapping the frame in insulation, lapping a vapour control layer onto it, and then re-forming the edge. A lining that was cold for decades is a common reason to rebuild rather than patch it, and the result is better for it.

In an older house, the ceiling around the opening may be lath and plaster, and the making good wants lime rather than gypsum. In a 1930s semi or a newer extension it is plasterboard and skim, which blends in more easily. The thing the notification cannot tell you, and the finish schedule can, is how the edge will be formed and painted. Our guide to how a rooflight is finished inside goes through the options, and replacing an old skylight covers the roof side.

A quote that mentions Part L properly is easy to spot. These are the things worth looking for before you sign:

• whether the job is a whole-unit replacement or a glass-only change, and which of those the price assumes;
• who notifies Building Control, and whether the cost is included;
• the whole-unit U-value of the rooflight being offered, rather than a centre-pane figure;
• how the frame is insulated and sealed against air leakage, including the vapour control layer;
• what happens to the lining and ceiling around the opening, and how it is finished.

Our written quotes come with a finish schedule covering the last of those. If a quote does not say who handles the notification, ask. A short answer is a good sign, and a vague one is not. If you want to see what a Part L-compliant rooflight looks like in the range we fit, the energy efficient rooflights page lists the options and how they suit different rooms.