“Catastro” is the institution which considers properties for maps, drawings, plans, and is the source of the valor catastral, the assessed value of property for Tax purposes.
Whilst the Registro de la Propiedad and the escritura may well confirm the ownership of a property and the conditions of the same, like in which percentage, if there are limitations or charges (like mortgage, public auctions, Tribunal disputes,etc), the Cadastre will give you a better understanding of the boundaries, the exact location of the property (usually in a visual form), size and description of the property, usually with maps, location, and drawings.
But, when you complete the building of a house (or an extension like a new room), or othe construction like a pool, garage, shed, etc, then you must inform the land registry about this new change on the property, but ALSO to Cadastre . You must provide plans and drawings to Cadaste about these new buildings, or new extended parts of your property. If you do not do so, you can have future problems because the Cadastre will never know about the existence of that new constructions.
There is a general misconception from which owners, when they complete the construction of a house, or the extension of an existing construction, or others (like garages, pools, etc), they think that Catastro will automatically register these constructions when passed by a notary. Unfortunately, this is not the case. When you complete any of these works, you have to declared them at the notary, land registry, and the Catastro.
Once the construction is duly inscribed in the Catastro, the job is finished, and soon (sometimes it takes around 1 or 2 years) you will start receiving the bills from the Council Tax.
Catastral records for many properties in Spain are out of date and inaccurate. The Catastral system has been used to formulate the value of properties and therefore to fix the taxable level on each property. For many Spaniards the avoidance of tax is a national past-time and many of them failed to update their Catastral details for fear of paying an increased level of tax.
This updating of the Catastro was always a voluntary system so anyone who failed to update their property in the Catastro (and therefore saved on the tax due) was doing nothing wrong. In many cases nowadays, you will find Spanish owners very reluctant to update their property in the Catastro and it is very difficult to persuade them otherwise.
But, now, in 2014, it is OBLIGATORY. So, now, it has been established a period of 2 years to REGULARIZE and UPDATE CATASTRO. Catastro has noted that there is a high number of properties that are not updated in the Catastro database. Usually, these properties have under-declared size of building, or parts of the property (like pool, garages, or other parts of the house), not declared in Catastro, and, consequently, PAYING LESS COUNCIL TAX THAT THEY SHOULD. Thus, there is now a process of regularization and update of properties in Catastro which ends on 2016. So, it is absolutely necessary that you ask for legal advice to check if you have to UPDATE YOUR CATASTRO RECORDS.
If your property was not updated in Catastro, you should be paying LESS COUNCIL TAX THAN YOU SHOULD. So, once you have presented the necessary documents to Catastro, and the regularization process is completed, Catastro may send you retrospective bills for the amount of Council Tax not paid for the undeclared constructions up to 4 years back.
So, as well to consult the land registry records of the property, it is always highly recommendable to consult the catastro to check how the property is inscribed and recorded.