How to Join FENSA or CERTASS
Written by James Walker · Last reviewed 16 August 2026
James Walker has 2 degrees in construction (BSc (Hons) Building Surveying · MSc Environmental Design of Buildings) and an extensive background in issue resolution, resolving over £4 billion of consumer issues.
The short answer
- Both are competent person schemes for replacement windows and doors.
- They are legally equivalent, either lets you self-certify and lodge the certificate.
- Registration requires technical assessment, an insurance-backed guarantee, and Part L and Part F compliance.
- FENSA is better known to consumers and conveyancers; CERTASS has identical legal effect.
- Neither covers new openings, conservatories, extensions or new builds.
What they let you do
Certify your own replacement window and door installations against the Building Regulations, and lodge the certificate with the local authority on the customer's behalf.
Without it, the customer must notify building control before work starts and pay for inspection. Most will use a registered installer instead.
What registration requires
Technical assessment of installed work, thermal performance under Part L, ventilation under Part F including trickle vents, safety glazing, and means of escape where required.
An insurance-backed guarantee for your work. Both schemes require one.
Insurance, a complaints procedure, and evidence of trading.
Ongoing assessment, typically annual, with inspection of completed installations.
Choosing between them
Legally identical. The practical differences are fees, support, and recognition. FENSA is the name conveyancers and homeowners know, which has some marketing value, while CERTASS competes on cost and service.
What is not covered
New openings. Conservatories. Unheated porches. Conversions, extensions and new builds. Those go through building control regardless of your registration, and assuming otherwise leaves your customer with uncertified work.