Marta Quiroga is a family doctor in Vigo and lets an apartment in Sanxenxo through platforms during the summer. In her 2024 income tax return she declared 6,200 € of income from that letting, which is what she had noted down in a spreadsheet. In September 2026, while preparing this year's accounts, she downloads the platform's annual report and sees that the total for 2024 was 9,800 €: she had left out the June bookings, which she was paid through another account. Her expenses had already been declared correctly, so the difference of 3,600 € is additional net income. Her first reaction is to leave everything as it is: "If I file a supplementary return, I'm telling them where to look."
It is an understandable line of reasoning, but it starts from a mistaken premise. Hacienda, as the Spanish tax authorities are commonly known, does not need Marta to tell it where to look. The figure is already in its hands.
The figure reaches them by another route before it reaches them from you
Since the transposition of the directive known as DAC7, platforms that act as intermediaries for lettings report to the Agencia Tributaria, the Spanish tax agency, every year what each host has been paid, identified by their tax number and by the property. In Spain they do so through Modelo 238. That information adds to what was already arriving from banks, notaries, payers and the Catastro, the property register.
What the system does is simple: it compares what third parties report with what the taxpayer declares. In Marta's case, cross-checking the 2024 Modelo 238 against her 2024 income tax return will throw up a difference of 3,600 €. No official needs to suspect her. The mismatch appears on its own, and the letter or the request can arrive at any time as long as the year is not time-barred.
That is why the useful question is not whether the supplementary return draws attention, but what happens in each scenario when that attention comes.
Marta's two sets of figures, side by side
For the calculation we will assume that her marginal rate, adding the state and regional parts, is 37 %, and a late-payment interest rate of 4 % a year. Both figures are only for the example: the marginal rate depends on the rest of her income, and the interest rate is set each year by the Budget Act.
Additional tax: 3,600 × 37 % = 1,332 €.
If she files the supplementary return on 15 October 2026. The deadline for the 2024 income tax return ended on 30 June 2025, so the delay is more than twelve months.
- Surcharge under article 27.2: 15 % of 1,332 € = 199.80 €.
- 25 % reduction under article 27.5 if she pays the tax when she files and pays the surcharge on time: it comes down to 149.85 €.
- Interest only from the day after the twelve months, 1 July 2026, until filing: 107 days, around 15.62 €.
- Total: 1,332 + 149.85 + 15.62 = 1,497.47 €.
If she waits and the tax agency corrects the return in March 2027.
- Tax: 1,332 €.
- Late-payment interest from 1 July 2025, about twenty months: around 88.80 €.
- Penalty under article 191. With a base below 3,000 €, the infringement is minor and the fine is 50 %: 666 €.
- If she agrees with the assessment, the penalty falls by 30 % (466.20 €), and if she also pays on time without appealing, by a further 40 %: 279.72 €.
- Total in the best case: 1,700.52 €. If she appeals and loses: 2,086.80 €.
| Voluntary supplementary return | Correction by the tax agency | |
|---|---|---|
| Total cost | 1,497.47 € | Between 1,700.52 € and 2,086.80 € |
| Nature of what is paid | Surcharge, not a penalty | Penalty |
| Requires fault to be proved | No | Yes, and the tax agency must give reasons |
| Stays on record | No | Yes, for four years |
The difference in euros is not huge in a small case like this one. The underlying difference is the last row.
The weight of a penalty in the years that follow
Article 187.1.a) of the General Tax Act (the Ley General Tributaria) grades penalties for repeated offending: if in the previous four years you have been penalised, by a final decision, for an infringement of the same nature, the minimum penalty for the next one, if it is one of those that are graded, rises by five, fifteen or twenty-five points depending on how serious the first one was. The infringements in articles 191, 192 and 193 are all regarded as being of the same nature.
For someone who goes on letting every summer, a final penalty in 2027 weighs on any mismatch for 2025, 2026 or 2027 that comes to light later. The surcharge, on the other hand, leaves no such trace: article 179.3 says expressly that anyone who corrects their position voluntarily incurs no liability for the infringement, without prejudice to the surcharge.
Filing it is an action by the taxpayer aimed at self-assessing the tax, and article 68.1.c) gives it the effect of interrupting the limitation period: the four-year period for checking the 2024 income tax return starts again from that day. It is not an argument for keeping quiet, because without a supplementary return the year stays open anyway until the summer of 2029 and the mismatch is already visible. But it is worth filing it properly and with the paperwork kept, because during that time any other aspect of the return can be reviewed.
Correcting one year and leaving the other as it is
Going through her statements, Marta discovers that in 2023 she did the same thing: she was paid the June bookings into the other account and did not declare them either. What really draws attention is not filing a supplementary return, but filing the one for 2024 and leaving 2023 untouched. That partial correction shows that she knew about the error and chose not to fix it in the earlier year, and it is an argument the tax agency can use to justify a finding of fault if it reviews 2023.
The practical rule is to review every year that is not time-barred before filing the first one. The order, and the cost of doing it for several years, is in putting several years right at once.
If you have a similar case, you can send us the details through the voluntary correction form and we will work out the cost of each scenario before you decide anything.
When it is worth stopping before you file
There are two situations in which the supplementary return is not filed without thinking first.
The first is that something has already been notified. Article 27.1 treats as a prior request any administrative action taken with the formal knowledge of the taxpayer and aimed at correcting that debt. If one already exists, filing now does not turn the penalty into a surcharge, and the sensible course is to respond within the procedure already open. Exactly where that boundary lies is dealt with in if I go first, can I still be penalised?
The second is that the amount is large. When the tax evaded for one tax and one year exceeds 120,000 €, article 305 of the Criminal Code comes into play, and the correction must be complete, with acknowledgement and payment, before the start of an investigation is notified. It is a different operation and is prepared differently; we explain it in when it stops being a fine.
Outside those two situations, Marta's case is the usual one: the figure is already in the tax agency's hands, and the only variable she controls is whether she pays a surcharge or a penalty.
What to prepare before filing it
- The platform's annual report for each year, which must match what is declared.
- Statements for every account into which bookings were paid, not just the main one.
- The invoices for expenses already declared, in case the later review extends to them.
- A short note with the filing date and the reason for the correction, which will help if someone asks in two years' time.
Nothing guarantees that the tax agency will not later review the corrected return. What can be ensured is that, if it does, it will find a return consistent with the third-party data.
How we approach a full correction, year by year and with the cost worked out before filing, is explained in Salama Tax's voluntary correction service.