The Supreme Court has agreed with the Treasury and toughens the requirements for taxpayers to explain to the Tax Agency any income that does not match what they declared. This comes as a result of a couple from Asturias who received more than 66,000 euros in their bank accounts from her father between 2013 and 2015. To justify it to the Treasury, they presented a loan contract for 75,000 euros signed with the father, but that document was dated 2014 and part of the money had arrived in 2013, a year before. Thus, they must pay 40,920 euros in settlement for the deposits that they could not justify.
The ruling STS 5378/2025 (available in the Portal of the Judicial Power) explains that when the AEAT inspection reviewed the couple’s income tax returns from 2013 to 2015 and found a series of income in their accounts that did not match what they had declared, so they had had an “unjustified capital gain.” That is, money that appears in the account and cannot be convincingly explained.
In these cases, the Tax Agency treats it as if it were undeclared income, adds it to your income tax return for the year in which it is discovered and applies the corresponding tax rate to you. In the highest income tax bracket, this can mean pay up to 47% of that amount (according to the retention tables published by the Tax Agency). The couple received a settlement of 40,920 euros (of which 36,855 were in fees and 4,064 in late payment interest) and, in addition to that, a penalty.
The couple tried to prove that the money was a loan from one of the couple’s parents and to do so they provided the credit contract. But the Inspection found three problems. The first is that a contract signed in 2014 cannot justify payments received in 2013. The second is that, if they added up all the deposits that they wanted to cover with that document, the total figure exceeded the 75,000 euros of the loan. The third, that the contract did not say which bank account the money was going to go into nor did it include any record of the movements, so it was impossible to link each specific payment with the paper.
What does the taxpayer now have to prove so that the Treasury does not tax that income?
The Supreme Court’s ruling resolves the common question, which is: is it enough to be able to indicate who the money comes from so that the Treasury does not treat it as undeclared income? The answer is no.
The Supreme Court establishes that three things must be proven at the same time. The first, where the money comes from: which bank account transferred it and by what means. The second, who it comes from: full name of the person who sent it. And the third, the one that fails the most in practice, is why it is sent: what specific agreement justifies that transfer, with a contract that exists before the money arrives and with bank transactions that corroborate it.
An interest-free loan between family members can be perfectly legal. But if that contract is signed after the money was already in the account, or if there is no way to link each payment to the loan, the Supreme Court says that the paper is useless. Bank traceability (that the money trail can be followed unambiguously from the account of the lender to the account of the recipient) is now essential, as explained by notary María Cristina Clemente.
The small detail that saved four of the subscriptions that the father had sent
The most revealing thing about the Asturian case is that not all deposits were treated equally. Four of the father’s payments that he had sent appeared on the bank statement with the literal reference “Transfer Aureliano”. This detail explains that upon seeing that reference in the movement, the Economic-Administrative Court was able to trace the money to the father’s account and it was clear who sent it and by what means. Those four season tickets were saved.
The rest of the income, which arrived without any written reference connecting it to the father or the loan contract, was taxed as unjustified capital gain. The ruling of the Superior Court of Justice of Asturias, which the Supreme Court confirms, explains it like this: “It is not explained that if in other income it is stated that they come from a bank transfer, in these three nothing is stated in that sense.”
What this ruling makes clear is that, when receiving money from a family member, you must ensure that the transfer includes the name of the sender and the reason. That the contract is signed and dated before the first payment, not after. And that the sum of the loan is not less than the total of the payments that you want to cover with it. Without these three elements aligned, the Supreme Court has established in STS 5378/2025 that the Treasury can tax that money as if it had never left your pocket.