@Alma0 Hi Alma, First of all thank you for your kindness and to take the time to answer to my doubts.
I have to say that I went personally to talk to OAE in Barcelona 2 weeks ago, who provides informations about PEUAT.
They actually told me that there's no "room licenses", licence numbers apply to entire flats only, and there are none right now. No laws have been deployed for the "room renting" in shared flats where people live in, and they still are in a "grey situation". You can do it as long as you pay your taxes and, as you say, your landlord is aware.
But, whatever is the way you do it, you cannot receive any fine from the government as there is no legislation on the matter. They recommend to keep an eye on the news because they are working on a proper legislation regarding room renting inside a shared flat where you live, but there's nothing at the moment.
So I'm not sure, where did you get this information about the fines from?