Spain’s income tax campaign opens every April and closes at the end of June. During that window, millions of taxpayers receive a pre-filled draft from the Tax Agency, check whether the result is a refund or a payment, and click confirm without reading anything else. It is understandable: the document is long, the language is opaque, and the assumption that the system must have already calculated everything correctly is tempting.
The problem is structural: the Tax Agency only knows what third parties have reported about you. It knows your salary because your employer declared it. It knows your bank interest because your bank reported it. But it does not know whether you donated to an NGO, whether your child has a recognised disability, whether you rent in a region that offers its own tax deduction, or whether you recorded investment losses that could offset gains from another year. All of that is your responsibility to include. Confirming the draft without reviewing it means, in many cases, paying more than you owe.
The draft is not the final return
The draft generated by the Tax Agency is built by cross-referencing data from third parties: your employer declares your gross salary and the tax withheld, your bank reports interest from deposits and current accounts, your fund manager reports redemptions, and your broker declares securities transactions.
However, some income does not enter that automatic cross-reference. If you were paid for occasional work without registering as self-employed, if you rented out a parking space or storage unit, if you received money as a sporadic collaborator for a company, or if you earned gains through foreign platforms not operating in Spain, those amounts will not appear on their own. You are the one who must add them, and the responsibility for failing to do so falls on you.
The same applies in reverse. The Tax Agency cannot automatically apply deductions it does not know about. It does not know whether you contributed to an individual pension plan, whether you donated to an NGO, whether you are a trade union member, or whether your autonomous community offers a birth deduction you have not claimed. In all these cases, the draft presents a result that is technically correct given the available data — but incomplete given what you could provide.
Reviewing the draft is not an optional task reserved for taxpayers with complex situations. It is the first step for everyone.
The mistakes that cost the most
Among the common mistakes in Spanish income tax returns, some repeat themselves year after year with direct financial impact.
Not updating family circumstances. If during the tax year you had a child, if a family member was recognised as a dependant, if someone in your household obtained a disability certificate, or if you changed marital status, the draft may not reflect those changes. Each of these circumstances modifies the applicable deductions and, in some cases, determines whether individual or joint filing is more advantageous.
Not declaring all income sources. Many taxpayers believe the Tax Agency already has all the information. It does not. Sporadic freelance work, rental income from movable or immovable property without a registered activity, prizes from competitions and draws above 300 euros, or income from foreign digital platforms are all examples of income that is frequently omitted by mistake or simple unawareness.
Forgetting capital losses. If you sold funds or shares at a loss, those losses can offset gains made in the same year. If there are not enough gains to absorb them, the balance can be carried forward to the following four tax years. If you do not declare the loss in the year it occurred, that tax benefit is gone for good.
Errors in employment income. The withholding certificates issued by employers sometimes contain incorrect amounts, incorrectly valued benefits in kind, or severance payments whose special tax treatment has not been properly applied. Comparing the certificate against your actual payslips is a step many skip — and one that can reveal meaningful discrepancies.
Incorrectly reporting rental income from second properties. If you own a property that is not your primary residence and is not rented out, the Tax Agency imputes a fictitious income of between 1.1% and 2% of the cadastral value. If that property does not appear in the draft or is incorrectly imputed, it can trigger a subsequent regularisation with surcharges and late-payment interest.
The deductions most commonly forgotten
This is where the most money is left on the table — and where the difference between reviewing and not reviewing becomes most visible.
Contributions to individual pension plans. These reduce the general taxable base directly, capped at the lower of two amounts: 30% of net earned income, or 1,500 euros per year for personal contributions. If there is also an employer contribution to the plan, the combined limit may be higher. If you contributed during the year, the amount should appear; if it does not, you must add it manually from the reductions section.
Donations to entities covered by the Patronage Law. Donations to NGOs or foundations with that recognition carry an 80% deduction on the first 250 euros donated in the year, and 40% on amounts above that. If you have donated to the same entity for more than two consecutive years, the rate rises to 45%. The recipient organisation is required to send you a donation certificate; if you have not received one, request it before filing.
Trade union and professional association fees. If you are affiliated with a trade union, membership fees are deductible as an employment expense. The same applies to fees paid to professional bodies where membership is mandatory, up to a limit of 500 euros per year.
Legal costs for employment disputes. If you had a labour dispute during the tax year and hired a lawyer or solicitor, up to 300 euros are deductible as an employment expense.
Maternity deduction and deductions for dependants with disabilities. If you have children under three years old and you work or receive contributory benefits, you can apply a deduction of up to 1,200 euros per year per child. If you also have an ascendant or descendant with a recognised disability in your care, additional deductions apply — ones that many taxpayers never claim. Some may already have been received as advance payments during the year; if not, they are recovered in the return.
Autonomous community deductions. This is the most variable and most overlooked chapter in the system. Each autonomous community manages its own portion of income tax and may establish additional deductions: for the birth or adoption of children, for renting a primary residence, for caring for elderly relatives at home, for purchasing school textbooks, for non-subsidised nursery fees, for rehabilitating a primary residence… The amounts can be significant and depend entirely on where you were registered on 31 December of the tax year being declared.
The Ministry of Finance publishes a practical IRPF manual each year listing all current deductions. Consulting it on the Tax Agency website before filing takes half an hour and can be worth several hundred euros.
Individual or joint: when each option makes sense
Married couples have the right to choose each year between filing individually or jointly. There is no universally better option: it depends on the income structure of each spouse.
As a general rule, joint filing tends to be more advantageous when one of the two has very low or no income. In those cases, the joint reduction of 3,400 euros that applies to the family unit can exceed the benefit of filing two separate individual returns. If there are dependent minor children or people with disabilities in the household, the advantage of joint filing may increase further.
When both spouses have similar incomes or both work with moderate to high salaries, individual filing tends to be more favourable. The reason is the progressive structure of income tax: pooling two incomes into a single taxable base can push a larger share of the combined income into the highest marginal bracket, raising the total bill.
The Tax Agency provides a simulator within the Renta Web application that calculates the result under both options before you file. Using it takes no more than five minutes and can make a difference of several hundred euros in the final result. The choice only applies to the current tax year; it can be changed the following year with no restrictions.
When it makes sense to hire a professional
Filing the income tax return is straightforward when your personal situation is. A single employer, no active investments, no self-employed income, no additional properties, and no significant family changes during the year — that profile is one where the draft is usually very close to correct, and where any taxpayer can manage without external help.
Complexity grows quickly. A second employer in the same year, capital income across several different products, the sale of a property, sporadic freelance income, an inheritance received during the year, or accounts and investments held abroad are all circumstances that multiply the points where errors can occur or where deductions can be claimed.
A tax adviser or manager typically charges between 50 and 200 euros for a return of moderate complexity. If they locate deductions you would not have applied, correct errors that would have triggered a subsequent regularisation, or calculate correctly whether individual or joint filing is better, the cost is recovered easily. What matters is not whether to delegate as a matter of principle, but whether the cost of doing it yourself — in time, in risk of error, and in deductions you might miss — exceeds the price of having it done correctly.
What has no justification under any circumstances is confirming the draft without having reviewed it. The Tax Agency calculates what it can with the information it has. The rest depends on you.
