@Luc *******
Officially and practically, the responsibility to report the stay of a foreigner rests with the host: the landlord of a property, the hotel or the service apartment. Delayed or late reporting of a foreigner can be penalized with 800 THB to 1,600 THB per person (normally the penalty does not exceed that amount, even if your landlord reports months too late).
This is from the thaiembassy.com website...