The Autonomous Communities which have specific regulations for touristic apartments  ( apartamentos turísticos) and holiday houses ( viviendas vacacionales) are: Cataluña , Comunidad Valenciana , Islas Baleares , Murcia , País Vasco , y el Principado de Asturias .  The rest of the Autonomous Communities just have general rules contained in their respective Tourism Acts.

A Royal Decree (by National Parliament) of 1982 (RD 2877/1982 of 15th in October of 1982 (National Official Bulletin of 9th in November), clarified the differences between touristic apartments and holiday houses, it is still in force:

According to that RD:

For touristic apartments, it is legally required the existence of  proper furnishing, installations, services and equipments for immediate use and in order to their exploitation an administrative license is needed ( the marketing company needs to obtain it before the initium of  touristic exploitation). (Supreme Court Decission of 17th of  September 1993 ). The group of touristic apartments of every touristic development is all marketed by the same company. They must be sold as touristic apartments.

Holiday houses are units of residential houses, intended and sold for personal residential use but that, of course, by title of ownership rights can be rented out. These are regulated in provision 17 of same RD of 1982 and defined as individual units of apartments, bungalows, villas, houses and similar and, in general, any dwelling which, regardless its furnishing conditions, equipment, installations and services, are offered for renting with touristic or holiday purposes.  

The only requirement for its opening and running is, according to provision 18 of the 1982RD, the communication to the Touristic Services of the Autonomous Community regarding its destination to touristic traffic.  So, just a legalised duplicated of that communication (it is not a license, just a communication and you are always allowed to rent), is needed for a legal renting of your apartment. Just some Autonomous Communities ask, by force of Administrative Regulations, that these holiday houses need to meet detailed requirements: in Baleares, Murcia, Asturias and Pais Vasco)

In Andalucia, where I am lucky to live, provision 45 of the General Tourism Act of 15th in December of 1999, specify the conditions for the use of holiday houses (both rural and urban) just say that they need to be furnished and have the necessary belongings for immediate use and a reference to future administrative regulations for the specification of minimum requirements to be met by these.

Anyhow, my best advice is for you to get legal advice at every case.