Why you owe money on your Spanish tax return even though they withhold tax every month (and how to see it coming in autumn, not in April)
Every month, part of your salary gets withheld. In theory, that’s “your share for the tax office” already paid. And yet, when the annual return comes, the result says you owe more. You haven’t made a mistake, and the tax office isn’t charging you twice: the withholding is only an advance payment, based on an estimate that may not match what you actually owe. And that gap can be calculated well before the filing season starts, not after.