Taxes
The 7 most common mistakes in the 2025 income tax return that cost money
The 2025 tax campaign has already closed, but knowing the most common mistakes prepares you for next year's. From not declaring foreign income to forgetting regional deductions.
Source: Spanish Tax Agency (AEAT), statistics from the 2024 tax campaign.
Key takeaways
40% of the supplementary returns filed by the Tax Agency come from avoidable mistakes that the taxpayer could have corrected before filing.
- Not including income from foreign accounts or investments is the most serious mistake: it can lead to a penalty of up to 150% of the undeclared amount.
- Accepting the draft without reviewing it is the most frequent mistake: the Tax Agency does not automatically include all deductions you are entitled to.
- Forgetting regional deductions can mean losing between 100€ and 500€ depending on the autonomous community and personal situation.
Mistakes 1 and 2: blindly accepting the draft and forgetting regional deductions
The Tax Agency draft is a starting point, not a final declaration. The Tax Agency does not have complete information about all deductions you may be entitled to: donations, nursery expenses, rental of main residence (in regions that still maintain it), union dues, or the maternity deduction. Reviewing and completing the draft can mean recovering hundreds of euros.
Regional deductions vary enormously by autonomous community. Madrid, for example, has deductions for birth of children, schooling expenses, donations to cultural foundations, and even for being a large family. Check your autonomous community's website or ask a tax advisor before filing.
Quick tips
- Use the individual vs. joint filing comparator offered by the Renta Web program: in families with very different incomes, joint filing can save up to 3,400€.
Mistakes 3, 4 and 5: not declaring extra income, online sales, and cryptocurrencies
Any income obtained as a freelancer, from renting your home on Airbnb, selling on Wallapop or Vinted above certain amounts, or from cryptocurrency gains must be declared. The Tax Agency receives information from platforms and banks. Thinking 'they are small amounts' is a mistake: the Tax Agency's data cross-referencing is increasingly sophisticated.
Cryptocurrency gains and losses are taxed as capital gains (between 19% and 28% depending on the amount). If you sold at a loss, you must also declare it: you can offset those losses against other capital gains of the year or the following 4 years.
Quick tips
- Always keep a record of all your cryptocurrency transactions (purchase price, sale price, date, exchange) to correctly calculate gains or losses.
Mistakes 6 and 7: not offsetting losses and not requesting time to pay if you cannot
If you sold stocks, funds, or any asset at a loss in 2025, those losses can offset gains from other investments or even 25% of capital income (dividends, interest). Many taxpayers do not apply this offsetting due to lack of knowledge and end up overpaying.
If the result of your tax return requires payment and you do not have immediate liquidity, do not fail to file: the surcharge for late filing is greater than the late payment interest of an installment plan. Request payment in two installments (60% in June and 40% in November) without additional surcharge.
Quick tips
- If you file a tax return with an amount to pay and request installment payment before the deadline, the remaining 40% does not accrue any additional interest.
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