Building Regulations for Skylights: Part L & Part K Explained

Almost every new or replacement skylight must meet building regulations, even when it does not need planning permission. The two parts that matter most are Part L, which sets energy efficiency and U-value standards, and Part K, which covers protection from falling and safe glazing. This guide explains both in plain terms.

Planning permission and building regulations are different systems. Planning is about whether you can do the work; building regulations are about whether the work is safe, efficient and properly constructed. A skylight can be permitted development and still need to comply with building regulations.

Part L: energy efficiency

Part L deals with the conservation of fuel and power. For rooflights, the key requirement is that the glazing achieves a good enough U-value — a measure of how much heat is lost through it. A lower U-value means better insulation. When you replace or install a rooflight, the unit generally needs to meet a maximum U-value to comply.

Modern double and triple-glazed rooflights are designed to meet these standards. The exact target figure and how it is measured are covered in detail in our Part L rooflight U-value requirements guide. Choosing the right glazing also keeps your rooms comfortable and your heating bills sensible — see our energy-efficient skylight installation page.

It helps to understand how Part L treats a rooflight in the wider context of a project. For a replacement, the approach is usually a “controlled fitting” one: the new unit must meet the standard set for a replacement rooflight, and you keep evidence of its performance. For a new rooflight in a new opening, or for rooflights forming part of a larger project such as an extension or loft conversion, the energy performance is assessed alongside the rest of the work — the overall building has to satisfy the energy targets, and the glazing is one input into that calculation. There is also a sensible balance built into Part L between letting useful daylight in and limiting heat loss; rooflights bring excellent daylight per square metre, but a large glazed area still has to be specified well so the room does not overheat in summer or leak heat in winter. A good installer thinks about glazing specification, orientation and shading together, not the U-value in isolation.

One frequently misunderstood point is that the U-value quoted for a rooflight can be measured in different ways — for the glass alone, for the whole unit, and at different reference pitches — so two products quoting similar-looking numbers may not be directly comparable. What matters for compliance is the figure relevant to how the rooflight is actually installed. Where a project is being assessed by building control, it is the whole-unit performance in its real orientation that counts.

Part K: protection from falling, collision and impact

Part K covers safety. For skylights, two aspects come up most often:

Safety glazing

Glazing in critical locations — broadly, areas where someone could come into contact with it — must be either toughened or laminated safety glass, or otherwise protected, so that if it breaks it does so safely. Overhead glazing in particular is expected to be specified so that, in the unlikely event of breakage, glass does not fall into the room. Reputable rooflights are supplied with appropriate safety glass as standard.

Protection from falling

Where a rooflight is openable and there is a risk of someone falling — for example a low-level or walk-on rooflight, or an opening rooflight on an accessible roof — Part K considerations around guarding and safe openings apply. For most domestic roof windows fitted in a pitched roof, this is straightforward, but it is something a good installer checks.

Walk-on and floor-level rooflights deserve particular care, because they raise their own questions. A walk-on glass unit set into a flat roof terrace must be specified as a structural, load-bearing assembly with the appropriate laminated build-up, so it can safely carry foot traffic and fail safely if it ever did break. A glazed floor panel or a rooflight set low in a wall or floor can also create an edge people could fall from or against, so guarding may be needed. None of this is exotic, but it is the kind of detail that should be designed in from the outset rather than discovered on site.

Other regulations that can apply

Depending on the job, other parts of the building regulations may also be relevant:

• Structure (Part A): cutting an opening into a roof can affect the rafters, so the structure must be properly supported and, where needed, trimmed.
• Fire safety (Part B): in a loft conversion, escape and egress rules become important — see our loft conversion building regs guide.
• Ventilation (Part F): rooflights can contribute to background and rapid ventilation in a room.

How compliance is demonstrated

For a replacement rooflight in an existing opening, the work usually still needs to satisfy building regulations, and you should keep evidence that the products and installation meet the standards. For new openings, especially in loft conversions, building control involvement is normal and a sign-off is issued when the work is complete. Our guide to building control sign-off explains when this is needed.

There are broadly two routes through building control. The first is a full plans application, where drawings and details are submitted and approved before work starts — common for loft conversions and larger structural jobs. The second is a building notice, a lighter-touch route for simpler work where the job is inspected as it proceeds rather than approved in advance. Either way, the work is checked against the regulations and a completion certificate is issued at the end. You can use your local authority’s building control service or a registered building control approver (an approved inspector); both assess the same regulations. The completion certificate is the document that matters when a property is later sold, so it is well worth having the paperwork in order from the start.

What this means for your skylight

In short: choose a properly specified rooflight with the right U-value and safety glazing, make sure the opening is structurally sound, and confirm whether building control sign-off is required for your particular job. Get those right and your skylight will be safe, efficient and compliant.

How Aspect Skylights helps

We install and fully replace rooflights to the required standards, specifying glazing that meets Part L and Part K and making sure openings are properly formed and supported. We can guide you on what applies, but we are not a building control body and do not claim to be approved, registered or certified as one. Explore our services or get a quote after a free survey.