Mandatory occurrence reporting is the Building Safety Act duty to report certain safety occurrences on higher-risk buildings to the Building Safety Regulator. During construction the principal designer and principal contractor must set up and operate a system for capturing and reviewing these reports.
Workers and subcontractors report occurrences into that system rather than directly to the regulator. When a reportable occurrence is identified, a notice goes to the regulator promptly, with a fuller report following within a short statutory deadline.
A working reporting system depends on capture: people need a low-friction way to flag a concern, and a record that it was raised, reviewed and acted on. That capture-and-trail expectation runs through the whole Act.
Low-friction capture is the whole game, and a message in the site WhatsApp group is about as low-friction as it gets; Construction Metric covers the wider regime on its Building Safety Act page.