Guide
Replacing windows: building regulations compliance
How to comply with building regulations when replacing windows. Covers the two compliance routes (FENSA or CERTASS self-certification versus building control application), Part L thermal requirements, Part K safety glazing in critical locations, listed building and conservation area considerations, and retrospective regularisation.
Why window replacement is controlled
Replacing windows in England is controlled building work under the Building Regulations 2010. This means you cannot simply swap old windows for new ones without demonstrating compliance. The regulations ensure replacement windows meet minimum standards for thermal performance (to reduce energy waste) and safety glazing (to protect people from injury).
This applies whether you are replacing windows in a dwelling (house, flat, or HMO) or a non-domestic building (office, shop, warehouse, or other commercial premises). It applies even if you are replacing like-for-like with the same size and type of window.
There are two routes to demonstrate compliance, and the choice between them is important because each has different cost, paperwork, and certification implications.
Route 1: Use a FENSA or CERTASS registered installer
The simplest route for most window replacements is to use an installer registered with a competent person scheme - either FENSA (Fenestration Self-Assessment Scheme) or CERTASS. These are government-authorised schemes that allow registered installers to self-certify that their work complies with the Building Regulations.
When you use a FENSA or CERTASS installer:
- You do not need to make a separate building control application
- The installer is responsible for ensuring the windows meet thermal and safety standards
- The scheme operator notifies your local authority within 30 days of completion
- You receive a certificate confirming compliance - this is needed when you sell the property
Route 2: Apply to local authority building control
If you choose an installer who is not registered with FENSA or CERTASS, or if you are installing the windows yourself, you must apply to your local authority building control before the work starts. A building control officer will inspect the installation to verify compliance. This route involves an application fee (typically GBP 200 to GBP 400) and requires coordination with the building control officer's availability.
Thermal performance requirements
Replacement windows must meet the minimum thermal performance set out in Approved Document L (conservation of fuel and power). The U-value limits differ between dwellings and non-domestic buildings.
An alternative to meeting the U-value directly is achieving a Window Energy Rating (WER) of Band B or above. WER ratings account for solar heat gain and air leakage as well as thermal transmittance, giving a more complete picture of window performance. Most modern double-glazed units achieve Band B or above as standard.
Safety glazing in critical locations
Wherever windows are in critical locations - areas where people could walk into or fall against the glass - safety glazing is mandatory. This requirement comes from Approved Document K (protection from falling, collision, and impact), which absorbed the former Part N glazing safety requirements.
Safety glazing must be toughened glass, laminated glass, or annealed glass with safety film applied. The glazing must meet the classification requirements of BS EN 12600 (or the older BS 6206). Your installer should confirm that safety glazing is used in all critical locations.
Listed buildings and conservation areas
If your property is a listed building, you will usually need listed building consent from your local planning authority before replacing windows. This is in addition to Building Regulations compliance. Listed building consent may impose restrictions on the window design, materials, glazing bar pattern, and opening mechanism to preserve the building's historic character.
If your property is in a conservation area, or is subject to an Article 4 direction, you may need planning permission if the replacement windows differ in appearance from the originals. Like-for-like replacements in the same material and style generally do not require planning permission, but check with your local planning authority if you are unsure.
What if you do not have a certificate
If windows were replaced without a FENSA/CERTASS certificate or building control sign-off, the work is technically non-compliant. This commonly becomes apparent when selling a property, as conveyancing solicitors will ask for evidence of compliance for any replacement windows.
You can apply for retrospective regularisation from your local authority. A building control officer will inspect the windows and, if they meet current standards, issue a regularisation certificate. The fee is typically around 150% of a standard building control application. If the windows do not meet standards, you may need to replace them again or carry out remedial work.
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1. Decide your compliance route
Choose between using a FENSA or CERTASS registered installer (simpler, no building control application needed) or applying to local authority building control (required if using a non-registered installer or doing the work yourself).
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2. Check for planning restrictions
If your property is listed, in a conservation area, or subject to an Article 4 direction, contact your local planning authority before ordering windows. You may need listed building consent or planning permission.
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3. Ensure windows meet thermal and safety standards
Confirm that replacement windows meet the relevant U-value (1.4 for dwellings, 1.6 for non-domestic) or achieve WER Band B or above. Verify that safety glazing is specified for all critical locations.
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4. Obtain your compliance certificate
After installation, confirm you receive either a FENSA/CERTASS certificate or a building control completion certificate. File this with your property records - you will need it if you sell the property.
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5. Apply for regularisation if needed
If previous window replacements were carried out without certification, apply for retrospective regularisation from your local authority building control before selling the property.