Building Regulations and Glazed Extensions

Building Regulations and planning permission are different

Planning permission and Building Regulations are separate systems.

An extension may fall within permitted development rights and therefore not require a conventional planning application, but it can still require Building Regulations approval. Building Regulations deal with matters such as structural safety, insulation, ventilation and fire safety rather than whether the development is acceptable in planning terms.

Most conventional house extensions will involve building-control requirements. Certain conservatories and porches can be exempt where specific conditions are met, but this should be confirmed with the relevant Building Control body rather than assumed. GOV.UK

Energy efficiency and Part L

Part L of the Building Regulations deals with the conservation of fuel and power and is particularly relevant to highly glazed extensions.

Glazing generally loses more heat than an insulated wall or roof, so Building Control considers the extension as a whole rather than looking simply at whether an individual glass unit is energy efficient.

For extensions to existing dwellings, the current Approved Document L sets limiting standards for new walls, roofs, floors, windows, rooflights and doors. It also sets out guidance on the total area of glazing and other openings. GOV.UK

Is glazing limited to 25% of the extension floor area?

The often-quoted 25% rule is useful, but it is frequently oversimplified.

Under the standard approach in Approved Document L, the total area of windows, roof windows, rooflights and doors in an extension should not normally exceed:

25% of the floor area of the extension, plus the area of any existing windows or doors that disappear or are no longer exposed because the extension has been built.

For example, if an extension covers existing patio doors or windows in the original rear wall, the area of those former openings can be added to the basic 25% allowance. GOV.UK

Importantly, this does not mean that extensions cannot contain more glazing.

Where a design exceeds the standard opening allowance, Approved Document L provides alternative routes for demonstrating compliance. These include comparing the area-weighted U-values of the proposed extension with an equivalent compliant extension, or using an approved whole-dwelling energy calculation. GOV.UK

This is why large roof lanterns, extensive sliding or bi-fold doors and other highly glazed designs can still be possible, provided the overall thermal design is demonstrated to comply.

Understanding U-values

A U-value measures how readily heat passes through a building element. The lower the U-value, the better its insulating performance.

For new elements in extensions to existing dwellings, current Approved Document L gives different limiting values for different types of construction. For example, it currently lists 1.4 W/m²K or Window Energy Rating Band B for windows and 2.2 W/m²K for rooflights. GOV.UK

An important distinction is that the glazing U-value (Ug) relates to the insulating performance of the glass unit itself. It should not be confused with the thermal performance of the complete roof lantern, which also depends on the frame, glazing bars, thermal breaks and overall construction.

Approved Document L requires rooflights to be assessed using the appropriate developed surface area rather than simply the dimensions of the roof opening. Where the upstand forms part of a combined rooflight-and-kerb assembly, its thermal performance may also form part of the assessment.

Consequently, a glazing specification of Ug 1.0 or 1.2 W/m²K should not be presented as the U-value of the complete roof lantern.

Safety glazing and overhead glass

Roof lanterns are a form of overhead glazing, so the glass specification also needs to consider how the glazing will behave if a pane is broken.

There is no universal requirement for every domestic roof lantern to incorporate a laminated inner pane. However, relevant British Standards require laminated inner glazing in certain circumstances, including some installations more than five metres above floor level and glazing positioned over water.

In other situations, toughened inner panes may be permitted, subject to the applicable standards and an appropriate risk assessment. The National Association of Rooflight Manufacturers (NARM) nevertheless strongly recommends laminated inner panes because the interlayer helps retain broken glass and reduces the risk of fragments falling into the occupied space below.

Laminated glass incorporates an interlayer that helps retain the glass if breakage occurs, whereas toughened glass is designed to break into relatively small fragments. 

The appropriate glass specification will depend on factors including the roof lantern system, pane size, installation height and use of the space. The manufacturer's specification and relevant glazing standards should therefore be followed.

For more information about toughened and laminated glass, solar-control glazing and thermal performance, see our guide to choosing the best glass for your roof lantern.

Structural design

A roof lantern creates an opening within the roof structure and the loads around that opening need to be safely transferred into the surrounding construction.

On a new extension this can normally be designed into the roof from the outset. Where an opening is being created in an existing roof, additional structural work may be required.

The supporting structure must also take account of the weight of the lantern and glazing together with appropriate wind and snow loads.

The builder, architect or structural engineer should determine the structural design where necessary. Just Roof Lanterns supplies the lantern to sit on a suitably prepared structural kerb or upstand; the supporting building structure itself remains part of the wider construction project.

The roof lantern kerb or upstand

The kerb, sometimes called the upstand, forms the connection between the flat roof and the roof lantern.

It needs to be correctly sized, square, level and structurally suitable for securely fixing the roof lantern. The roof covering and insulation should also be properly integrated with the upstand to maintain weather resistance and thermal continuity.

Different roof lantern systems use different setting-out dimensions. Our timber collections are generally specified from the internal kerb opening, whereas our aluminium collections generally use the finished external kerb size.

Builders should therefore work from the specific upstand information or approved drawing supplied for the chosen roof lantern rather than assuming that the same dimensions apply to every system.

Read our roof lantern kerb and upstand guide →

Ventilation and comfort

Part F of the Building Regulations covers ventilation.

The ventilation strategy for an extension will depend on how the room is used and its relationship with the existing house. Kitchens and other moisture-producing areas can have different requirements from living spaces, while background, purge and mechanical ventilation may all form part of the overall design.

Opening roof-lantern vents can help warm air escape and improve air movement, but they should not automatically be assumed to satisfy all Part F requirements. Ventilation needs to be considered for the room and dwelling as a whole. Current Approved Document F guidance applies in England, with new 2026 guidance scheduled to take effect from March 2027 subject to the transitional arrangements. GOV.UK

Read our roof lantern ventilation guide →

Fire safety

Part B of the Building Regulations deals with fire safety.

For an extension, relevant considerations can include means of escape, the effect of alterations to the existing house, fire spread and the relationship between the extension and property boundaries.

There is not a universal requirement for every roof lantern to use fire-rated glazing. Whether additional fire-resistance measures are necessary will depend on the individual design and location of the extension and should be established by the designer or Building Control where relevant. Approved Document B remains the principal statutory guidance for these matters in England. GOV.UK

Do you need Building Regulations approval?

For most conventional domestic extensions, Building Regulations approval will be required.

Depending on the project, approval may be dealt with through a local authority Building Control body or a registered building control approver. The work is then inspected at appropriate stages and, once completed satisfactorily, a completion certificate is normally issued. GOV.UK

The person responsible for the extension should establish the appropriate Building Control route before work begins.

What does this mean when choosing a roof lantern?

For most homeowners, the important point is that the roof lantern should not be considered in isolation.

Its size and specification need to work with:

• the amount of other glazing in the extension;
• the thermal performance of the roof, walls, floor, windows and doors;
• the structural design of the roof opening;
• the kerb or upstand construction;
• and the ventilation strategy for the completed room.

A larger glazed area does not automatically mean that a design will fail Building Regulations, but it may require the overall extension to be assessed using one of the alternative compliance routes available under Part L.

Your architect, building designer or Building Control body should confirm the compliance requirements for the individual project.

You can browse our full range of roof lanterns or contact us if you would like help with the lantern specification and the upstand information required for your chosen design.

This guide reflects the Building Regulations guidance applicable in England at the time of publication. Revised Approved Documents L and F will generally take effect from 24 March 2027 for non-higher-risk building work, subject to transitional arrangements. Requirements should always be confirmed for the individual project.