Must the cancellation be sent by registered mail?
The question comes up constantly: is registered mail mandatory to cancel? Legally, what matters is the proof that the insurer received your cancellation in time. Registered mail is not required by law, but it is the safest way to establish that proof.
What matters: proof of receipt within the deadline
In Swiss law, the burden of proof lies with the party who cancels. If the insurer disputes having received your letter in time, it is up to you to demonstrate receipt. Registered mail provides a posting receipt and delivery tracking that constitute this proof.
An ordinary letter can be perfectly valid in law, but in the event of a disagreement you have no way to prove the sending or the date of receipt.
The date of receipt, not the date of sending
The notice period is calculated from receipt by the insurer. So post your registered letter early enough before renewal to cover the delivery time. For a renewal on 31 December with a three-month notice period, your letter must be received by the end of September at the latest.