Marta Llorente is a self-employed illustrator (an autónoma) in Castellón. In September 2026, while sorting out paperwork to apply for a mortgage, she found two gaps. The Modelo 303, the quarterly VAT return, for the third quarter of 2025, with 2,600 € to pay, was left as a draft and never sent. And in her 2024 IRPF return (Spanish personal income tax) she forgot a payment from a German client; the supplementary return comes out at 4,800 € to pay. Nobody from Hacienda, the Spanish tax authorities, has written to her. She wants to file both on 15 September and to know beforehand how much the delay is going to cost her.
What awaits her is not a fine but a surcharge. Article 27 of the Ley General Tributaria (LGT, the General Tax Law) defines it as an ancillary charge for filing late "without a prior request" from the tax authorities, and calculates it with a fixed rule.
The rule: one point for each full month
Section 2 of the article sets the surcharge at 1 % plus an additional 1 % "for each full month of delay" after the end of the filing period. It is applied to the amount payable on the self-assessment, not including penalties or interest.
The expression that decides everything is "full month". Months are counted from date to date starting on the day after the filing period ends. If the period ended on 20 October, the first full month is completed on 20 November; filing on 19 November keeps the surcharge at 1 %, and filing on the 22nd raises it to 2 %. Three days' difference is worth one point on the whole amount of tax.
| Delay from the end of the period | Surcharge | Late-payment interest |
|---|---|---|
| Less than one full month | 1 % | No |
| One full month, short of two | 2 % | No |
| Five full months, short of six | 6 % | No |
| Ten full months, short of eleven | 11 % | No |
| Eleven full months, up to twelve | 12 % | No |
| More than twelve months | 15 % | Yes, from the day after the twelve months are completed |
The jump from 12 % to 15 % is the only irregular step. From there the percentage no longer rises, but interest starts to run.
After a year: 15 % and a meter that keeps running
When the self-assessment is filed after twelve months, the surcharge stays at 15 % and the law adds late-payment interest "for the period elapsed from the day following the end of the 12 months" up to the filing date. No interest is charged for the first year of delay; it is charged for whatever goes beyond it.
That is why a delay of eighteen months does not cost 18 %: it costs 15 % plus six months of interest on the tax. The late-payment interest rate is set each year by the Budget Act (Ley de Presupuestos), and the calculation is proportional to the days: tax × annual rate × days / 365.
Marta's two calculations, step by step
The 303 for the third quarter of 2025. The period ended on 20 October 2025. From 21 October to 15 September 2026, ten full months have passed (the eleventh would be completed on 20 September).
- Surcharge: 1 % + 10 points = 11 %.
- 2,600 × 11 % = 286 €.
- With the 25 % reduction, if its conditions are met: 286 × 0.75 = 214.50 €.
- Interest: none, because the year has not been exceeded.
If Marta waits until 21 September, the surcharge goes up to 12 %: 312 €, or 234 € reduced. Five days of waiting would cost her almost twenty euros.
The supplementary 2024 income tax return. The filing campaign ended on 30 June 2025. By 15 September 2026 more than twelve months have passed.
- Surcharge: 15 %.
- 4,800 × 15 % = 720 €.
- With the 25 % reduction: 540 €.
- Interest: on 4,800 €, from 1 July 2026 to 15 September, that is, 77 days at the late-payment rate in force.
Adding both together, the delay costs Marta about 754.50 € plus 77 days of interest on the income tax return.
The 25 % reduction and what makes you lose it
Section 5 of article 27 reduces the surcharge by 25 % if two things are met:
- That the surcharge is paid in full within the period opened by the letter in which Hacienda assesses it (the one in article 62.2 of the same law).
- That the tax on the late self-assessment is paid when it is filed, or within the terms of a deferral or instalment arrangement granted against a bank guarantee or a surety insurance certificate and requested at the time of filing.
If either payment fails, the law says the amount of the reduction "shall be demanded with no further requirement than notification to the person concerned". You do not have to do anything to lose it; it is enough not to pay on time. A deferral without that guarantee, even if it is granted, does not keep the reduction.
When you file late you only pay the tax. The surcharge is not self-assessed: Hacienda calculates it and notifies you weeks or months later, with its own payment deadline. Anyone who does not know this thinks they have escaped it and lets that deadline pass, which loses them the 25 % and, if they still do not pay, sends the surcharge to enforced collection.
Filing without paying: the second surcharge on top
There is a trap in section 3. If you file the late self-assessment but neither pay the tax nor ask for a deferral, instalments or set-off at the same time, the tax goes straight into the enforcement period. On top of the article 27 surcharge, those of article 28 may follow: 5 % if you pay before the enforcement order (providencia de apremio), 10 % if you pay within the period that order opens, or 20 % with interest in all other cases.
With Marta's income tax return, filing without paying would mean the 15 % from article 27 and, on top of that, at least the 5 % of the enforcement period: 720 € plus a minimum of 240 €, and with no 25 % reduction. If cash is short, the deferral request has to go in together with the return; the page on deferring a debt from a regularisation explains how to fit it in.
The self-assessment has to stand alone
Section 4 adds a formal requirement that is often forgotten: the late return must expressly identify the period it refers to and contain only the data for that period. Putting the forgotten 2024 payment into the 2025 income tax return, "to keep things simple", is not filing late: it is filing both years wrongly. In that case the surcharge regime does not apply and the door to a penalty remains open.
That is why Marta files a supplementary return for 2024, not an inflated 2025 return.
Surcharge or penalty: the boundary of the prior request
Everything above only works if there has been no prior request. Article 27.1 defines it broadly: any administrative action, formally brought to the taxpayer's knowledge, aimed at checking, regularising or assessing that debt. A notified limited check or a request for information on that same item closes the surcharge route.
In exchange, article 179.3 excludes liability for an offence for anyone who regularises voluntarily, without prejudice to the surcharge. Filing before the letter swaps a penalty procedure for a known percentage. The difference between the two is explained in more detail in surcharge or penalty and in if I come forward first, can I still be penalised?.
If you have several periods outstanding, you can send us the list of forms, amounts and dates through the regularisation form, and we will order the filings so that none of them moves up a bracket because of a single day.
The only case with no surcharge
Since Law 11/2021, article 27.2 includes a case with no surcharge. If Hacienda regularises one year and you correct other periods of the same tax on your own initiative for identical facts, no surcharge is charged if four conditions are met:
- Filing within the six months following notification of the assessment.
- Acknowledging and paying in full whatever results.
- Not requesting rectification of Hacienda's assessment or appealing against it.
- No penalty arising from that regularisation.
Failing any one of them means the surcharge is demanded with no further formality. It is narrow, but it makes sense when a check of 2023 affects a criterion that you also applied in 2024 and 2025.
Before you file
Check which bracket you are in with the late filing surcharge calculator: with the date the period ended and the amount of tax, it tells you today's percentage and next month's. The full method is in the guide on calculating the article 27 surcharge. We cannot guarantee the outcome of a later check of the content of the return: the surcharge covers the delay, not the accuracy of the figures.
The page on voluntary regularisation sets out how correcting several years is planned, what documents we ask for and in what order it is best to file when some periods drag others along with them.