Hello Christian and Rogerio,
We also live in Catalunya, and we are renting out a whole flat, but as it is part of a duplex (its complicated, but we have 2 flats, one above the other, with separate entrances, registered as one.), the Ajuntamiento said we DIDN'T have to register it - registration was only for WHOLE apartments. I called the generalitat in Girona, and this was confirmed. ( I am waiting for it in writing of course).
(There is an option when registering with AIR BNB, which says something like "We are exempt from registration because we are only renting out a room/part of a property" )
If I hear more, I'll let you know.
Patricia