Hi AnnieR,
yes you are right, the IBI tax is calculated on the basis of the Cadastral value, which is fixed by the catastro. This value changes depending - among other things - on the built area. The maximum deadline to get refunds is 4 years after the tax is paid.
Concerning the measures in Catastro and in the Escritura, in principle the Escritura should prevail unless the changes in the property have taken place after the escritura was done or the Deed measures are wrong (this is not unusual either, considering that old deeds contain old descriptions and also that the developers sometimes commit mistakes when describing the properties). it is also possible that the catastro measures refer to an older building which no longer exists. Finally, it we are talking about an urbanisation or apartment complex, it is possible that the cadastro has added the proportional area in communal property as well...
The measures in Catastro are obtained not only on the basis of the Deeds but also through "revisiones" or "ponencias", which are administrative process whereby the technicians selected by catastro visit the towns in order to make a new catastro. Such process is complex and takes usually 6 months, afterwards there are also 6 months to appeal, etc. The catastro takes into account then many data from many different sources, such us aerial photographs and obtain the measures from those photos. The result is the updated catastro with new ownership situations and new descriptions of properties, which might well include differences in the areas.
Having said that, it is not unusual that the abuse on the aearial photographs gives rise to different measures as in the reality. I have had a case where a property had 3 floors, the area of the first two was the same but the area of the 3rd was lower, and the catastro has simply taken that the three floors are do just have the same area. of course, the catastro technicians have not been there, but no one cared to go to the Catastro when the update was made in order to complain.
Consequently, I would suggest the following:
1.- Arrange an appointment with the Cadastro services, the contact number is 902373635, you have to identify yourself and provide for the cadastral reference of your property.
2.-Provided you are not satisfied with their explanations, have the actual area checked by a technician. This will tell you who's wrong, the deed or the catastro. Please note that private drawings are not currently accepted, it has to be done by someone with Spanish degree in a technical issue.
3.- Change the catastro by submitting an application/appeal with a copy of your Deed and if possible a certificate and/or plans from the technician who visits the property.
4.- In the application you should estate that the measures of the property were like that since you became the owner, and therefore the change SHOULD HAVE EFFECT SINCE THE PURCHASE DATE. With this you will have the possibility to fight for the proportional refund of the unduly paid moneys.