Part L Building Regulations for Rooflights: Thermal Performance and U-Values Explained
Updated October 2026
As the UK continues to improve the energy efficiency of its buildings, the thermal performance of roof windows and rooflights has become an increasingly important consideration for architects, specifiers and homeowners.
Part L of the Building Regulations sets out requirements for the conservation of fuel and power, including standards relating to heat loss through windows, doors and roof glazing.
With a new edition of Approved Document L published in 2026, understanding the regulations and how they apply to different rooflight designs is essential when specifying products for new-build, renovation and heritage projects.
What has changed under Part L?
The Building Regulations have undergone several significant revisions in recent years, reflecting the Government’s commitment to improving energy efficiency and reducing carbon emissions.
The 2022 changes introduced more demanding thermal performance requirements, while the latest edition of Approved Document L, published in March 2026 and revised in September, sets out updated guidance for dwellings and non-domestic buildings.
The 2026 edition is scheduled to take effect in England on 24 March 2027, subject to transitional arrangements. Until then, the previous requirements continue to apply to relevant projects.
For architects and specifiers, the key consideration is ensuring that the roof windows or rooflights selected meet the requirements applicable to the particular building and project.
The official guidance is available in the 2026 edition of Approved Document L.
What are the U-value requirements for rooflights?
The thermal performance of a roof window or rooflight is measured by its U-value, expressed in watts per square metre kelvin (W/m²K).
A U-value indicates the rate at which heat passes through a building element. The lower the figure, the better its insulating performance.
Importantly, the requirements vary depending on the type of building, whether the work involves a new build or an existing property, and whether the product is classified as a roof window or rooflight.
The following figures provide a useful comparison of relevant standards and specifications:
| Building application | Roof window | Rooflight |
|---|---|---|
| Existing dwellings | 1.4 W/m²K | 2.2 W/m²K |
| Non-domestic buildings | 1.6 W/m²K | 2.2 W/m²K |
| New-build dwellings – notional specification | 1.2 W/m²K | 1.7 W/m²K |
The figures for notional new-build dwellings are reference specifications used in energy performance calculations, rather than universal maximum permitted U-values. The applicable requirements should always be confirmed for the individual project.
Rooflights versus roof windows: why the distinction matters
Although the terms rooflight, skylight and roof window are often used interchangeably, Approved Document L makes an important distinction between them.
A rooflight is a glazed unit installed out of plane with the roof surface, typically on a kerb or upstand.
A roof window is installed in the same orientation as, and in plane with, the surrounding roof.
This distinction is particularly relevant to conservation rooflights, which are designed to sit flush within the roofline to preserve the appearance of traditional and historic buildings.
Although commonly referred to as conservation rooflights, products installed in this way are generally classified as roof windows for the purposes of Part L.
The distinction affects both the applicable thermal performance requirements and how U-values are assessed.
Understanding whole-unit U-values
Not all published U-values provide a like-for-like comparison.
Some manufacturers advertise the thermal performance of the glazing alone, commonly referred to as the centre-pane U-value. However, this does not account for heat loss through the frame, glazing edges and other components.
For an accurate assessment of thermal performance, specifiers should consider the U-value of the complete roof window or rooflight.
The orientation of the product is also important. Thermal performance calculations can produce different results depending on whether a glazed unit is assessed in a vertical, inclined or horizontal position.
For this reason, architects and specifiers should ensure that published U-values are supported by appropriate independent evidence and calculated using the relevant recognised methodology.
Stella Rooflight: independently assessed thermal performance
At Stella Rooflight, we believe that exceptional thermal performance should never come at the expense of architectural authenticity.
Our bespoke conservation rooflights combine slimline 316L stainless steel frames, high-performance glazing and carefully engineered seals with the traditional appearance required for sensitive heritage projects.
Stella rooflights have undergone independent thermal performance assessment through the British Fenestration Rating Council (BFRC), providing architects and specifiers with verified performance information.
Our triple-glazed fixed rooflights can achieve whole-unit U-values as low as 0.92 W/m²K, while double-glazed configurations incorporating genuine glazing bars can achieve 1.4 W/m²K.
Actual performance varies according to the size, glazing specification, opening mechanism and configuration of the rooflight.
This means specifiers can select a product that balances the architectural requirements of a project with the thermal performance needed to support compliance with Part L.
Part L and historic buildings: are there exemptions?
Conservation rooflights are frequently specified for listed buildings, traditional properties, barn conversions and buildings within conservation areas.
Improving energy efficiency in these settings can present particular challenges, especially where modern materials or alterations could compromise the character or historic fabric of a building.
Approved Document L recognises that special consideration may be necessary for historic and traditionally constructed buildings.
In certain circumstances, full compliance with standard energy efficiency provisions may not be appropriate where it would unacceptably alter a building’s character or appearance, or risk long-term deterioration of its fabric.
However, listed status or location within a conservation area does not automatically provide a blanket exemption from Building Regulations.
The appropriate approach will depend on the building, the proposed works and the applicable regulatory requirements.
Where necessary, advice should be sought from the relevant building control body and local authority conservation officer.
Choosing conservation rooflights that meet modern standards
As Building Regulations continue to evolve, selecting roof windows with independently verified thermal performance is increasingly important.
For architects working on heritage and conservation projects, the challenge is often to achieve modern standards of energy efficiency without compromising traditional architectural details.
Stella’s bespoke conservation rooflights are designed to meet this challenge, combining discreet stainless steel frames, genuine glazing bars, hardwood liners and high-performance glazing with excellent thermal efficiency.
By considering the correct product classification, whole-unit U-values and the requirements of Part L at an early stage, specifiers can make informed decisions that support both regulatory compliance and the long-term performance of the building.
Planning a project? Contact the Stella Rooflight team to discuss your requirements or request technical information and independently assessed U-values for your chosen rooflight specification.
