Filing late on your own costs 1% a month

You file your tax return on 3 October, four months after the deadline closed, and without the tax office having sent you a single letter. Plenty of people never file precisely because of that: they assume a fine is coming. A fine is not coming. A surcharge is, and the surcharge has a published rate.

The arithmetic sits in article 27 of Spain's General Tax Act, rewritten by Law 11/2021:

1. A flat 1%, plus 1% for each full month of delay. Four months late: 5%. 2. From twelve months on, a flat 15% plus late-payment interest from that thirteenth month. 3. On whatever comes out, a 25% reduction if you have paid the tax itself and settle the surcharge within the period you are given.

What changes the price is who moves first. If you wait for the tax office to demand it, article 27 no longer applies: article 191 does, and the minimum penalty for underpayment is 50% of the tax due.

So if the deadline has slipped past you, do not let another month run by waiting for a good moment: every full month you cross adds a whole point. File the return, pay what it says and wait for the surcharge letter, which arrives on its own with the 25% box already ticked.