Thousands of self-employed individuals pay more income tax than necessary by not including expenses such as supplies, internet, or training. The Tax Agency allows deductions of up to 300 euros extra if justified correctly.
The income tax campaign once again presents an opportunity that many self-employed individuals miss: recovering up to 300 euros in income tax thanks to expenses that the tax office allows to be deducted but are rarely declared. The Tax Agency estimates that, on average, self-employed workers under direct estimation declare 20% less in deductible expenses than they could legally apply. The result is a higher tax bill and a refund that falls short.
The five most commonly overlooked (and legal) expenses
The most common mistake is thinking that only direct expenses such as self-employment contributions, rent for premises, or office supplies are deductible. The reality is that any necessary outlay for the activity can be included, as long as it is under direct estimation and there is an invoice. The most forgotten expenses are household supplies, internet and phone, computer materials, association fees, and training.
If you work from home, you can deduct the proportional part of electricity, water, gas, and community fees. The key is to calculate the percentage of square meters used for the activity. For example, in a 100-square-meter home with an office of 15 square meters, 15% of those bills is deductible. Internet and phone follow the same rule, unless you have a line exclusively for the business, in which case the expense is fully deductible.
Computer materials and software — computer, printer, licenses, cloud subscriptions — are also included, even if used for personal purposes, as long as the professional part is reflected. Fees for professional associations required to practice the activity are 100% deductible, as are courses and seminars related to the sector.
A self-employed person working from home can deduct the proportional part of electricity, water, gas, and internet without the tax office questioning it... as long as they prove professional use.
Self-employment contributions, civil liability insurance, and advertising on social media also make the list of expenses that many overlook. The trick is to keep all invoices and, when the expense is shared, have a documented distribution criterion so that a possible tax office review does not invalidate the deduction.
How to calculate the deduction if you work from home
The Tax Agency does not require juggling, but a coherent calculation. The first step is to measure the square meters of the home and those actually used for the activity (office, storage, etc.). That percentage is applied each month to the bills for electricity, water, gas, heating, and community fees, as long as they are in the name of the self-employed person or someone sharing the home.
With internet and phone, if there is a line exclusively for the business, the expense is deductible at 100%. Otherwise, the same square meter percentage applies. The invoice must be kept with the other supporting documents, as an inspection may request it even several years later. Be careful with the amounts: the tax office usually does not dispute a 10-15% expense for housing, but if an outrageous figure is declared without justification, it will raise a red flag. It’s better to be prudent and stick to the reality of daily use.
The mistake that costs the refund: not justifying expenses
The biggest slip-up of self-employed individuals is not forgetting the expense, but failing to keep the invoice or not correctly reflecting the professional percentage. Without paper (physical or electronic), the deduction is lost in a review. Furthermore, in the case of supplies, if the invoice is not in the name of the self-employed person but they live with a partner or family member, they must be able to prove that the holder grants them the use of the home for the activity.
There is also a common mistake in the objective estimation (modules): in that regime, almost all expenses are already included in the indices, so they are not added separately. A self-employed person in modules who intends to deduct individualized supplies may find themselves facing a request from the tax office, and the result is usually a complementary settlement and a subsequent fine.
Therefore, the most repeated advice from consultancies is simple: keep every receipt, number them by month, and have the square meter calculation handy. Those ten minutes of organization per quarter translate into hundreds of euros a year. With the marginal rates of income tax, every 100 euros of non-deducted expense means between 19 and 47 euros more in tax, depending on the income bracket. If the forgetfulness of a year is added up — supplies, phone, computer materials, insurance — the extra cost can easily exceed 300 euros.
There are legal precedents that reinforce the taxpayer's position: several resolutions from the Central Economic-Administrative Court have reiterated that household supply expenses are deductible if professional use is proven. The next income tax campaign, starting in April 2026, is the time to review invoices and not miss out on that extra refund.
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