What are building regulation rules for changing widows (FENSA/Certass)?

SevenDay guide on what to understand for Building Regulations, FENSA and CERTASS when changing your windows and doors

What You Need to Know When Changing Windows and Doors

If you’re replacing windows or doors, the work has to comply with Building Regulations — and you’ll need paperwork to prove it. This SevenDay guide should be used to explain what the regulations require, what FENSA and CERTASS registration actually mean, and the situations where certification isn’t needed at all (including the common question about doors with less than 50% glass).

Why Building Regulations Apply to Replacement Windows

Since 1st April 2002, replacement windows and doors in England and Wales have fallen within the scope of the Building Regulations. Before that date, the regulations controlled glazing in new buildings, but the change brought existing dwellings into scope, largely to reduce energy loss across the much larger stock of existing homes.

When you replace a window, the installation must meet the relevant requirements covering thermal insulation (energy efficiency), ventilation, safety glazing, and means of escape in case of fire. In practice that means the new units have to hit current U-value or Window Energy Rating standards, provide background ventilation (trickle vents), use toughened or laminated glass in critical locations, and preserve any required fire-escape opening.

Importantly, this applies to all replacement windows regardless of the property’s age or style. There’s no exemption for older houses — though listed buildings and properties in conservation areas have their own additional consents to navigate.

What FENSA and CERTASS Actually Are

FENSA (the Fenestration Self-Assessment Scheme) and CERTASS are both government-authorised Competent Person Schemes. Membership allows a registered installer to self-certify that their work complies with the Building Regulations, without involving the local authority Building Control department on every job. To gain this authority the installer will have had to undergo scrutiny and competence prior to registration and there will be random ad-hoc post install inspections periodically to ensure they are installing to standards required.

This is the key thing to understand:

FENSA and CERTASS are not the regulations themselves — they’re a route to demonstrating compliance with them. FENSA is the most widely recognised scheme, but it isn’t the only valid path; CERTASS (and others such as Stroma) issue Building Regulations Compliance Certificates that are equally valid in law. What matters legally is that the installation complies and that you can prove it.

When a registered installer completes a job, they self-certify the work and the certificate is issued, typically within a few weeks, and the installation is registered with the local council automatically.

The Two Routes to Compliance

There are really only two ways to satisfy the regulations on a replacement window or qualifying door:

• Use an installer registered with a Competent Person Scheme (FENSA, CERTASS, etc.), who self-certifies and issues a compliance certificate. This is the simpler and more common route.
• Or notify your local authority Building Control before the work, who will inspect and issue their own compliance certificate. This is the route for installers who aren’t scheme-registered.

Either certificate is acceptable. If the installer isn’t registered with a recognised scheme, the work must instead go through local council Building Control.

Why the Certificate Matters

The paperwork comes into its own when the property is sold. A buyer’s solicitor will ask for proof that any replacement windows installed since 2002 comply with Building Regulations, and no certificate means no proof of compliance. That can delay a sale, knock buyer confidence, or trigger a price renegotiation.

If a certificate was never issued or has gone missing, there are fallback options: a retrospective compliance certificate from the local authority following an inspection, or indemnity insurance as a last resort (though insurance only covers against future enforcement — it doesn’t actually prove compliance).

When Certification Is NOT Required

This is where a lot of confusion arises. Several situations fall outside FENSA/CERTASS notification:

• Doors with less than 50% glazing. This is the big one for the trade. Certification is only mandatory for an external door where the glazed area exceeds 50%. If the door is less than 50% glass, or it’s a solid door, that item doesn’t need to be registered. A solid composite front door, or one with just a small glazed panel or vision strip, won’t generate a FENSA certificate. (Worth noting: with CERTASS, installers may still log all doors through the scheme, but those under 50% glazing aren’t notified to Local Authority Building Control. And a registered installer can usually still register a sub-50% door voluntarily if a customer specifically wants the certificate.)
• Side panels and top lights count separately. A point worth making to customers: side panels and top lights next to a door are treated as separate windows and registered in their own right, rather than being lumped in with the door’s glazed percentage. So a solid door flanked by glazed side panels may still generate certification for the panels.
• Glass-only repairs. Simply replacing a broken or failed glass unit — without changing the frame — is not notifiable. The same goes for repairs such as replacing hinges or handles. It’s the replacement of the whole window or door unit that triggers the requirement.
• New builds. Doors and windows in a newly constructed home are covered by the Building Regulations approval for the whole build, so they don’t need a separate FENSA certificate.
• Conservatories, porches and extensions. These are treated differently under the regulations. A door leading into a porch, where another external door separates the house from the porch, doesn’t need a certificate, and conservatories and orangeries go through their own process rather than FENSA.

Where New Openings or Structural Changes Are Involved

One critical distinction for the trade: the self-certification route only covers like-for-like replacement into an existing opening. If the work involves creating a new opening, altering the size of an existing one, or making structural changes, that must go through Building Control rather than being self-certified. Widening an aperture, converting a window into a door, or forming a brand-new doorway all fall outside the scope of a FENSA or CERTASS self-certificate and need separate Building Control sign-off.

The Bottom Line

Replacing windows or doors with more than 50% glazing into an existing opening? You need compliance — easiest via a FENSA or CERTASS registered installer. Fitting a solid or mostly-solid door, replacing just the glass, or carrying out a repair? Notification generally isn’t required, though the work must still be done to a safe and compliant standard. And anything that changes the structure or creates a new opening steps outside self-certification altogether and needs Building Control.

If you’re ever unsure whether a job is notifiable, it’s worth a quick conversation with your local SevenDay team or installer before you order — the depot teams deal with these questions every week.

The SevenDay depot network are the go-to place for many Certass and FENSA registered installers therefore we can happily link you up if required for a registered installer to complete your project.