Guest registration is mandatory, even for a single unit
Since Royal Decree 933/2021, every vacation rental apartment in Spain must collect its guests' details and report them to the Ministry of the Interior through SES.HOSPEDAJES — or to the Mossos d'Esquadra in Catalonia and the Ertzaintza in the Basque Country. The obligation applies even if you manage a single apartment and even when the booking comes through Airbnb or Booking.
The report must be filed within 24 hours of arrival, and failing to do so can carry fines of up to €30,000 in serious cases. Online check-in exists precisely to meet this requirement without paperwork: the guest provides their details before arriving and the report files itself.

