One has to pay these taxes if one has two or more homes or let dwellings in Spain.  They are payable between 01 May and 30 June of the following year.

Because a dwelling which is let is situated on Spanish territory, the income has to be declared here.  The information that's required is the income the lessor receives, the expenses which they pay (Council Tax, water and electricity bills) and whether a mortgage exists on the dwelling, because the mortgage will reduce the taxes.