📅 Updated April 2026 · ⏱ Full guide in progress · ✍️ Findfitter Team
Three UK schemes certify replacement window installers: FENSA, Certass, and Trustmark. They sound interchangeable — they’re not. Here’s the short version while our full guide is being finalised.
What they have in common
All three are Competent Persons Schemes authorised by the UK government. An installer registered with any of them is legally allowed to self-certify that their work meets building regulations — saving you the cost and delay of a local council inspection. All three provide insurance-backed warranties so your install is protected even if the installer goes out of business.
FENSA
The largest scheme. Operated by the Glass and Glazing Federation. Most UK installers are FENSA registered. If you’re buying replacement windows or doors, FENSA is the default — you want your installer on this register.
Certass
The second-largest scheme — similar scope to FENSA. Equally valid. Some smaller installers prefer Certass due to lower membership fees.
Trustmark
Broader government-endorsed quality scheme covering multiple trades (not just glazing). If a window installer is Trustmark registered, they’ve been vetted on workmanship, customer service, and trading practices — but not necessarily on building regulation compliance for windows specifically. Best: an installer with both Trustmark and FENSA or Certass.
Verifying an installer’s registration
- FENSA: fensa.org.uk
- Certass: certass.co.uk
- Trustmark: trustmark.org.uk
Any fitter claiming certification that doesn’t appear on these public databases is lying. Walk away.
Our full side-by-side comparison guide (coverage, fees, dispute resolution, pros/cons of each) is being finalised. In the meantime, you can get quotes from pre-vetted installers who carry at least one of these certifications.
Get quotes from FENSA-registered installers
Enter your postcode — we will match you with 4 vetted UK installers for a free, no-obligation comparison.
Sources & references
This article draws on data and guidance from the following authoritative UK sources:
- FENSA — Find an installer — primary UK certification for window/door installers
- Certass — installer database — alternative UK installer certification
- Trustmark — find a tradesman — government-endorsed quality scheme
- Glass and Glazing Federation (GGF) — UK industry body for glazing professionals
- Companies House — free company lookup — verifying installer registration, directors, filed accounts
- Citizens Advice — rogue traders — consumer rights and complaint routes
"Was worried about pushy salespeople. Found a FENSA fitter 3 miles away who was honest about pricing."
"Straightforward and no pressure. Picked the FENSA-registered installer with the 10-year warranty."
"Quotes arrived within 6 hours. Gas bill in winter dropped by a third."