Mon Jun 29
Window specification sits at the intersection of design, performance and compliance. Get it wrong and you risk abortive work, delayed sign-off from building control or – on listed building schemes – planning enforcement. Get it right and the window specification supports the broader regulatory strategy for the project from the outset.
This guide sets out the key requirements of the UK building regulations affecting windows in England, covering the approved documents most likely to affect your specification decisions.
Note: building regulations are subject to ongoing revision. Always consult the current approved documents and seek advice from your building control officer for project-specific guidance.
The approved documents that affect windows
Several parts of the building regulations in England have direct relevance to window design and specification. The most significant are:
- Approved Document L – conservation of fuel and power (energy efficiency)
- Approved Document F – ventilation
- Approved Document Q – security (new dwellings only)
- Approved Document B – fire safety
Each imposes distinct requirements. On most projects, windows will need to satisfy more than one simultaneously.
Part L: Energy efficiency and U-values
Approved Document L is the part of the building regulations with the most direct impact on glazing specification. It sets minimum thermal performance standards for windows, measured primarily through U-values – the rate at which heat passes through a material. Lower U-values indicate better thermal performance.
Current U-value requirements in England
For replacement windows in existing dwellings, the maximum permitted whole-window U-value is 1.4 W/m²K. For new build dwellings, the requirements are set within the context of the overall building’s Target Emission Rate (TER) and Target Fabric Energy Efficiency (TFEE) calculation, rather than as a fixed window U-value in isolation.
The 2022 update to Part L represented the most significant revision in nearly a decade, requiring at least a 31% reduction in emissions for new dwellings compared to previous standards. It is an interim step toward the Future Homes Standard, which is intended to move all new homes to a net-zero-ready specification.
For architects, the practical implication is that window specifications for new residential buildings need to be considered as part of the whole-building energy model – not specified in isolation.
What counts as a replacement window?
Where an existing window is removed and replaced, the new window must comply with Part L as a legal requirement. A certificate of compliance is required to confirm this, typically provided by a FENSA-registered installer or via a building regulations application to the local authority building control.
Part F: Ventilation
Part F of the building regulations governs ventilation, and its requirements interact directly with window specification in ways that are easy to overlook at design stage.
The key considerations for windows under Part F are:
- Background ventilation – replacement windows in dwellings should include trickle ventilators unless it can be demonstrated that adequate ventilation is provided by other means
- Purge ventilation – habitable rooms require the ability to purge ventilate, typically through openable windows with a minimum free area of 1/20th of the floor area
- Airtightness tension – as buildings become more airtight under Part L, the ventilation strategy needs to compensate; window specification must align with the broader mechanical or natural ventilation approach
Part F was updated alongside Part L in June 2022, with an increased emphasis on indoor air quality and the avoidance of condensation in energy-efficient homes. On projects where mechanical ventilation with heat recovery (MVHR) is specified, the background ventilation requirement through windows may be reduced – but this needs to be confirmed through the ventilation design, not assumed.
Part Q: Security
Approved Document Q applies to new dwellings only and requires that windows accessible at ground level or within 2 metres of an accessible roof or floor are tested to PAS 24 or an equivalent enhanced security standard.
The key requirements include:
- Windows must be sufficiently robust to resist opportunist physical attack
- Where windows contain glass and non-key locking hardware, laminated glass meeting BS EN 356:2000 Class P1A is required
- Hardware must be appropriate for the level of security required
Part Q does not apply to replacement windows in existing homes.
Do building regulations apply to listed buildings and conservation areas?
Yes, but with an important qualification. Listed buildings are exempt from the energy efficiency requirements of Part L where compliance would unacceptably alter the character of the building.
However, this exemption is not automatic. It requires a considered judgement, typically in consultation with the local authority conservation officer and building control officer. The reasonable provision standard still applies, meaning some improvement in performance is usually expected where it is technically feasible and sympathetic to the building’s significance.
For conservation areas, permitted development rights may also restrict the type of replacement windows that can be installed without planning consent – a separate consideration from building regulations compliance.
Certificates of compliance and building control sign-off
For replacement windows installed outside of a full building regulations application, compliance is typically demonstrated through a FENSA certificate or equivalent certificate of compliance issued by the installer’s competent person scheme. This records that the installation meets the requirements of Part L and, where applicable, Part F.
Where a full building regulations application is made – as is usual on new build and larger refurbishment projects – the building control officer will assess compliance as part of the inspection and sign-off process. NBS specifications and technical documentation from the window manufacturer will form part of the evidence base.
TRC Contracts provides full NBS specifications and technical drawings to support the specification and building control process, including free downloads of CAD blocks, DWG files and BIM data.
Key takeaways
- Part L sets minimum U-value requirements for both replacement and new-build windows – currently 1.4 W/m²K for replacement windows in dwellings
- Part F requires background and purge ventilation to be considered alongside window specification, particularly on airtight new-build schemes
- Part Q applies to new dwellings only and requires accessible windows to meet PAS 24 security testing
- Listed buildings may be exempt from Part L energy efficiency requirements, but this requires a case-by-case assessment with the local authority
- Compliance documentation – whether a FENSA certificate or a building control sign-off – is a legal requirement for most window installations
Talk to TRC Contracts about compliant window specification
TRC Contracts works with architects and specifiers across England on projects ranging from listed building restoration to new residential and commercial developments. Our technical team can support specification from early-stage design through to building control sign-off, with NBS documentation, acoustic and thermal performance data, and project-specific guidance.
Contact TRC Contracts to discuss your project, or download technical drawings and NBS specifications from the TRC website.
