AssessmentWhich value is used — and which debts may be deducted
Two questions decide the actual burden, and both are usually skipped in overview articles.
First, the value. What is assessed is not the market price an agent quotes, but the highest of the officially prescribed reference values — the cadastral value, the value established on acquisition, or the purchase price. If you do not know your property's cadastral value, you do not know your taxable base. It appears in the cadastral extract and equally on the bill for the IBI.
Second, the debts. Deductible are liabilities connected with the assets located in Spain. This is exactly where the Tribunal Supremo drew the line: a mortgage taken out to acquire or renovate the property reduces the net wealth — a mortgage raised later on the same property for another purpose does not. That is not a detail but the difference between effect and no effect, and it is why the order matters: financing at purchase is something different from raising capital years later.
Different considerations therefore apply to mortgaging an already paid-off property — see releasing capital from Spanish property. This is not tax advice; the application in your case belongs with a Spanish tax adviser.