Thanks Liam. My concern is not really about the option of renting out the room or not: it is my place, and I believe I am free to do with it whatever I feel right. Still, I would like to know whether there are specific rules, in Rotterdam, that regulate long term stays: for instance, sombody advised me that a contract with the guest is compulsory, although there is already an agreement that we both 'sign' through AirBnB. Others say in certain towns the lon-term renting is not allowed. I would just like to know whether there are laws about this, or not.
Moreover, in this specific case, the issue is not between host and guest, but between neighbors, which may have repercussions on guests, who obviously have no responsibility in this whole thing.