Audit sits at the sharp end of accountancy's risk spectrum: the work exists to be relied on by third parties, and when fraud or misstatement surfaces, the auditor's PI policy is where claimants look.
Treat the regulatory minimum as a floor, not a target – audit exposure scales with the entities you sign off, and buying limits against your largest engagement is the sober approach. Present your audit book transparently at renewal (client sectors, fee concentrations, quality-review history), keep retroactive cover continuous when switching insurers, and add regulatory-investigation cover: in audit, the investigation often arrives before any claim does.