In the UK, it’s a legal requirement to comply with Building Regulations when installing new or replacement windows. Understanding which rules apply — and how they have changed — can save you from costly mistakes and compliance failures.
Building regulations for windows in the UK are a set of rules and guidelines that ensure all new and replacement windows meet certain standards for safety, energy efficiency, ventilation, and security.
These regulations are important because they help keep you and your family safe, reduce your energy bills, and make your home more comfortable.
In this updated 2025 guide to Building Regulations for windows in the UK, we’ll discuss what regulations apply to new and replacement windows, how to comply with the regulations, and why they’re important — including the latest changes you need to know about.
| Regulation | What It Covers | Key Requirement | Applies To |
|---|---|---|---|
| Part L | Energy efficiency | U-value ≤ 1.4 W/m²·K (replacement) / ≤ 1.2 W/m²·K (new) | New & replacement |
| Part B | Fire safety & escape | Opening ≥ 0.33m², ≥ 450mm wide or high | New & replacement |
| Part F | Ventilation | Trickle vents or adequate background ventilation required | New & replacement |
| Part K | Safety glazing | Safety glass required at low-level critical locations | New & replacement |
| Part O | Overheating prevention | Limit solar gain; provide adequate means to remove excess heat | New builds only |
| Part Q | Security | Windows tested to PAS 24 standard | New builds only |
The approved documents for Building Regulations in the UK provide detailed guidance on how to comply with the regulations. The following documents are relevant for new or replacement windows:
Part L of the Building Regulations sets out requirements for the energy efficiency of windows. The U-value is a measure of how much heat is lost through a material, and the WER is a measure of the overall energy efficiency of a window.
It’s important to note that the U-value requirement differs depending on the type of installation:
This distinction matters. Confusing the two is one of the most common compliance errors homeowners make. Part L was significantly updated in June 2022 as a stepping stone towards the government’s Future Homes Standard — see the section below for what’s changing.
Part B of the Building Regulations outlines the requirements for fire safety in buildings. New and replacement windows in habitable rooms must have suitably sized openings to provide emergency escape.
The openable area in such a window must be at least 0.33 m² and at least 450 mm high or wide. The bottom of the openable area must also be no more than 1,100 mm above the floor. This requirement is in place to make sure that you and your family can escape safely from your home in the event of a fire.
Emergency egress windows are required in:
Part F of the Building Regulations covers the requirements for ventilation in buildings. New and replacement windows must provide adequate ventilation. This can be achieved through background ventilators, trickle vents, or openable windows.
An important update: as of June 2022, if the original window already had background ventilation (for example, trickle vents), any replacement window must provide at least the same amount of ventilation. All new-build windows will require trickle vents as standard. Ventilation is important to remove stale air and moisture from your home, which improves indoor air quality and reduces the risk of mould growth.
Part K requires that glazing in critical locations is safe. Safety glazing is required where glass is positioned at low level — specifically below 800 mm from the floor in walls and below 1,500 mm in doors and side panels. In buildings more than four metres above ground level, window restrictors must also be installed to limit the opening to no more than 100 mm, reducing the risk of falls.
Introduced in June 2022, Part O of the Building Regulations is specifically designed to tackle overheating in new residential buildings. It requires that new homes are designed and constructed to limit unwanted solar gain in summer and to provide adequate means of removing excess heat from indoor environments.
Part O directly affects the sizing, positioning, and specification of windows in new builds. Key requirements include:
Please note: Part O applies to new residential buildings only. Replacement windows in existing homes and conservatories added to existing properties are not affected by Part O.
Approved Document Q applies to all new dwellings and requires that reasonable provision is made to resist unauthorised access. For windows, this means that all easily accessible windows — those within 2 metres of ground level or an accessible roof — must be tested to PAS 24:2012 (or equivalent enhanced security standard).
The key requirements for windows under Part Q include:
Please note: Part Q applies to new build dwellings only. Replacement windows in existing homes are not required to comply with Part Q, though meeting PAS 24 standards remains good practice.
In addition to the approved documents, there are also a number of British Standards (BS) that are relevant to windows. These standards provide technical guidance on how to design, manufacture, and install windows in compliance with the Building Regulations.
The following BS are particularly relevant to new or replacement windows:
This is one of the most common points of confusion for homeowners. Building Regulations and planning permission are entirely separate requirements.
If you replace like-for-like windows, you do not need planning permission (except in conservation areas or listed buildings). However, Building Regulations will still apply to that replacement work. If you are creating a new window opening or widening an existing one, you will need both planning permission for the structural work and Building Regulations approval for the window itself.
If your property is in a conservation area or is a listed building, additional rules apply. New and replacement windows will generally need to be double-glazed with energy-efficient low-emissivity glass to comply with Building Regulations — however, if you are in a listed building or conservation area, you should consult with your local authority before proceeding, as conservation rules may take precedence over certain standard building regulation requirements.
The exact Building Regulations that apply to windows depend on whether you are installing new windows or replacing existing windows.
All new windows must comply with the following Building Regulations:
Replacement windows must comply with the following Building Regulations:
Note: Part O and Part Q do not apply to replacement windows in existing homes.
The government’s Future Homes Standard is a major set of regulatory changes aimed at ensuring new homes produce significantly fewer carbon emissions. It forms part of the UK’s commitment to reach net-zero carbon by 2050.
While industry speculation suggested the minimum U-value for windows in new dwellings could drop as low as 0.8 W/m²·K, the notional values are currently set to remain at 1.2 W/m²·K under the Future Homes Standard. However, calculation methods are changing, which will affect how overall fabric performance is assessed for new builds.
If you are planning a new build or a significant renovation, it is worth speaking to a specialist now about future-proofing your window specification — for example, by considering triple glazing, which already meets and exceeds current Part L requirements.
Once you’ve installed new or replacement windows in your home, you must obtain a Building Regulations Certificate of Compliance, such as a FENSA certificate, from a qualified professional. This certificate confirms that your windows comply with the relevant Building Regulations. You will need this certificate when selling your property.
There are two main ways to obtain a Building Regulations Certificate of Compliance for windows:
Important: As the homeowner, you are ultimately responsible for ensuring the work complies with the Building Regulations. Always ask your installer whether they are registered with a Competent Person Scheme before work begins.
There are a number of reasons why you need a Certificate of Compliance:
Since 2002, Building Regulations have applied to all new and replacement glazing to ensure buildings are safe and energy efficient. Here’s why each area matters:
U-value is a measure of how much heat is lost through a material. The lower the U-value, the better the insulation and the less heat is lost. New and replacement windows must have a low U-value to comply with Building Regulations. Improving your windows’ thermal performance directly reduces your energy bills and carbon footprint.
Safety glazing is designed to reduce the risk of injury from broken glass. All new and replacement windows in critical locations — such as at low level in walls — must use safety glazing that complies with Part K.
Ventilation is important to remove stale air and moisture from your home. New and replacement windows must provide adequate ventilation to comply with Building Regulations. Poor ventilation leads to damp, mould, and poor indoor air quality — all of which affect your health and your property’s condition.
Fire safety is important to ensure you and your family can escape safely from your home in the event of a fire. New and replacement windows in habitable rooms must be able to open to provide emergency escape, unless the room already has another means of escape.
As UK summers become warmer, preventing overheating in new homes is increasingly important. Part O of the Building Regulations ensures that new residential buildings are designed to limit excess solar gain through windows and to provide adequate means of cooling — protecting both occupant health and comfort.
For new build properties, Part Q ensures that all easily accessible windows are sufficiently robust and fitted with appropriate hardware to resist physical attack. Windows must meet the PAS 24 security testing standard. This provides meaningful protection against opportunist burglary.
Do I need building regulations to replace windows?
Yes. Building Regulations apply when you replace the whole of a fixed frame and opening parts. They do not apply if you are simply replacing broken glass or a failed double-glazed unit.
What U-value do windows need for building regulations?
New windows require a U-value of no more than 1.2 W/m²·K. Replacement windows require a U-value of no more than 1.4 W/m²·K. Both require a Window Energy Rating (WER) of B or better.
Do I need planning permission to replace my windows?
Generally no — replacing like-for-like windows does not require planning permission. However, if you live in a conservation area, a listed building, or are changing the style or material of the windows, you should check with your local authority first.
What is a FENSA certificate and do I need one?
A FENSA certificate is a type of Building Regulations Certificate of Compliance issued when a FENSA-registered installer completes your window installation. It proves legal compliance and is needed when you sell your property. You do not need a separate Building Regulations certificate if you have a FENSA certificate.
Do new build windows need to comply with Part Q?
Yes. All easily accessible windows in new build dwellings must comply with Approved Document Q and be tested to PAS 24:2012 or equivalent. Part Q does not apply to replacement windows in existing homes.
What is Part O and does it affect my windows?
Part O came into force in June 2022 and applies to new residential buildings only. It requires that new homes are designed to limit solar gain through windows and provide adequate means of removing excess heat. It does not apply to replacement windows in existing homes.
Do I need trickle vents when replacing windows?
If your existing windows already had trickle vents, your replacement windows must provide at least the same level of background ventilation. All new build windows require trickle vents as standard under Part F.
Building Regulations for windows in the UK are important to ensure your home is safe, secure, and energy efficient. By complying with the Building Regulations, you can help to reduce your energy bills, make your home more comfortable, and meet your legal obligations.
To find out more about window and door regulations, or to get a quote for fully compliant bespoke timber windows, get in touch with our friendly, knowledgeable team today. When you need replacement windows and doors, choose Windows & Doors UK for bespoke, hardwood windows and doors — all installed in full compliance with current UK Building Regulations.
Yes, of course you're allowed to fit your own windows, but you must ensure they comply with the Building Regulations. Once they're fitted, you should seek approval from the relevant Building Control Body at your local authority. They'll check each replacement window or door, and if satisfied, will issue you a certificate of compliance.
If you aren't sure, you should consult a qualified professional. Or have a professional installer registered with the Competent Person Scheme, to carry out the work. They'll install your windows and doors without involving the local authority building control. When the work is complete, you'll receive a certificate showing the work was done by a registered installer.
