What are the legal consequences of an outgoing tenant not returning the apartment keys to the owner?

A tenant who has given notice to his landlord must give up the premises.

To do this, he is obliged to return the keys to the owner or owner's agent.

Legally, an apartment is only regarded as being vacated from the date of return of the keys.

If the keys to the apartment are not given up, the tenant has not fulfilled his obligation to give up the premises, even if he has physically vacated them.

In this event, the tenant runs the risk of being ordered to pay occupation rent covering the period from the date stated in the notice to the actual date that the keys are given up.