The Tax Office (Hacienda) is carrying out a mass survey across the province to detect undeclared property improvements such as swimming pools and extensions.
As these things are found, your property value will be automatically updated and your annual IBI will be increased, as well as other taxes based on property worth.
The survey, which is also going on in other parts of Spain, will be completed by 2016. Whether they will attempt to count illegal homes at the same time is unknown. Making all those homes legal and putting them onto the IBI would probably generate a lot more cash than this penny pinching update…
Anyway, the survey is currently concentrating on coastal urbanisations and large urbanisations in rural areas.