There is an obligation to declare any income you derive in Spain to the Spanish Taxman. In your case, the gross benefit is taxed at a 25% rate, obviously, if you declare it. I have to admit there are many property owners who never declare these amounts to the taxman, since more often than not, these tenancy agreements are for holiday occupation and the tenants are foreigners who do not request proper invoices. It is more difficult to evade the tax obligation if the occupant starts demanding invoices and declares the monthly rental as a cost, to offset it against his income.
I suggest you engage a financial advisor to look after your affairs connected with the rental agreement.
