Not my speciality but I’ll try to give some guidelines
There is a basic and fundamental difference between the ordinary property rent, ruled under the General Law of Urban Renting and this professional business of tourism apartments rented on a daily basis.
The first activity is a private issue, you can freely rent your house in a long term (of course always fulfilling the Law mentioned above) or in a seasonal term. Neither the Generalitat nor the Town Hall cannot interfere here.
But when we are talking about tourism apartments we are talking about a professional business, the housing or hotel business… and this is strongly regulated all over Spain not only in Catalonia as Tourism is one of the main industries here (we do receive nearly 45 million foreign visitors a year) This activities must be registered as if you provide this service (it does not matter if you only own one dwelling to rent…) you rent on a daily/weekly basis, you advertise it, you are considered a businessman.
Regional and local authorities have full competences to regulate this activities.
Don’t know if in the UK it’s the same but I guess the ‘bed & breakfast’ service is surely also regulated there.
We do have here in Asturias a lot of Tourism apartments facilities (mostly in rural areas, the so called green-tourism) they are also strongly regulated by regional and local authorities. They have to fulfill strong quality standards and there is always a maximum ‘facilities per area rate’ just to avoid the undesirable over-exploitation.
My advice… It’s safer/easier to have a long term tenant (if the right guaranties are given) than running your dwelling as a professional business… less controls, less taxes, less maintenance expenses.
But anyway, if you are renting your house to friends acquaintances and relatives no problems should arise…