Your tax liability is calculated by reference to the cadastral value of your property.
The cadastral value ("valor catastral") is equivalent to the UK rateable value. The total cadastral value ("valor catastral total") for the year in question is the figure we need. This is made up of two components: the land value ("valor catastral suelo") and the property value ("valor catastral construccion"). It does not relate directly to the purchase price or market value of your property.
The cadastral value will be shown on your council tax bill or receipt (also known as IBI/suma/rates) normally as "valor catastral/base imponible".
If these documents are not to hand your local Town Hall will be able to help or your bank if you've set up a direct debit.
Alternatively, if you have previously used a fiscal rep, they may have the information. If the property is newly built, and a cadastral value has not yet been determined, your income tax liability is calculated based on a formula using half of the purchase price (escritura value) exc IVA.
Taken from Spanish Tax Forms website http://spanishtaxforms.co.uk/page/1on0t/Spanish_property.html