Protection of Personal Data issue
Something else to consider when you rent our your property, as you need to meet the requirement to provide the Guardia with the personal ID data of your guests within 24 hrs after their arrival.
As the collection of personal data (complete names and passport info) to pass on to the Guardia clearly doesn´t fall under the exceptions as mentioned in article 2.2.a) of the Ley Organica de Protección de Datos, i.e. clearly is not for exclusive personal or domestic use, all homeowners that rent out their property are required to register with the Agencia Española de Protección de Datos ´dar de alta un fichero / register a file´. Info available in English, but automatically generated by Google, so not perfect.
You can do this online, through their website, but in our experience it´s not a simple matter and best ask for the help of a gestor or an agency specialised in ´protección de datos´.
Update 2019: it is no longer required to register with the Data Protection Agency, but you are obliged by law to follow the rules and regulations of current Data Protection law, so the advice above, to contact a specialist, still stands.