Hello,
I have been researching this a lot in and out, as it has changed with the new government.
To get your question out of the way. YES you must declare it. pay taxes ,,, report it. All of it. Also and MOST importantly, YOU MUST ASK Permission prior to running a short term rental.
My advice to you is you study the regulations of that region. Some regions have banned short term rentals, others have their own country regulations. For example how much to charge, whether The owner Must LIVE IN THE house. SO again I suggest you check the regulations, legalities and requirements of the specific region. They vary region to region. The only common factor throughout Spain is to REQUEST Permission. otherwise I hear the Fine is INSANE.
I am looking for any information on the regulation in the Canary Islands dealing with the aesthetics of the structure.