In the quarterly Social Security declaration there is one option many independent workers overlook: you can fix your relevant income up to 25% below or above the value calculated from the declaration.
At first glance it looks like a simple way to “pay less”. It is more specific than that. This option changes your monthly contributory base (BIC), and the BIC is the amount on which your contributions are calculated. In some cases, it can also affect the base used for sickness benefit, parental allowance, and pension calculations.
If you are still learning the basic formula, start with the general guide to Social Security for freelancers. This article focuses only on the 25% option.
What happens in the quarterly declaration
In the quarterly declaration, you declare the previous quarter’s income — income counts on the date the invoice becomes due: as a rule, the date the service was provided; for an advance, the date you receive it (art. 36.º of the CIVA). It is not the date the client pays an already-issued invoice. For service providers, Social Security usually calculates the relevant income like this:
relevant income = 70% of declared services
Then it turns that amount into a monthly base:
monthly BIC = relevant income ÷ 3
And the monthly contribution is:
contribution = monthly BIC × 21.4%
At the moment of filing, you can choose to fix the relevant income at a value up to 25% higher or lower, in 5% steps: -5%, -10%, -15%, -20%, -25%, or the same steps upwards.
You can test the effect of each percentage on your own numbers in the contribution calculator — pick the variation and see the monthly amount straight away.
What the -25% option does not do
It does not change the real income. If you provided €9,000 of services in the quarter, you declare €9,000.
It also does not reduce income tax, VAT, withholding tax, invoices, or receipts. It is an option inside Social Security to adjust the contributory base used for the following months.
Thinking of it as “declaring less” is risky. The correct logic is: declare the true income and choose a contributory base within the legal margin.
Example with real numbers
Imagine you provided €9,000 in services during the quarter.
Without adjustment:
| Calculation | Value |
|---|---|
| Services provided | €9,000 |
| Relevant income | €6,300 |
| Monthly BIC | €2,100 |
| Monthly contribution | €449.40 |
With the -25% option:
| Calculation | Value |
|---|---|
| Services provided | €9,000 |
| Adjusted relevant income | €4,725 |
| Monthly BIC | €1,575 |
| Monthly contribution | €337.05 |
In this example, you pay €112.35 less per month for the following three months. The immediate saving over the quarter is €337.05.
When -25% can make sense
It can make sense when you had a strong quarter but already know the following months will be weaker. Social Security uses the previous quarter to calculate the next contributions, so the -25% option smooths that mismatch.
It can also help during tight cash-flow periods, as long as you understand the consequence: you are contributing on a lower base.
The opposite option also exists. +25% can make sense for someone who wants to strengthen their contributory base and can afford the higher monthly contribution.
The impact on sickness, parental leave, and pension
This is where the decision stops being only about “paying less now”.
For benefits such as sickness benefit and parental allowance, Social Security uses a reference remuneration calculated from the remunerations registered in your contributory record. As a rule, it looks at the six oldest months in the last eight before the work impediment.
If those months had a lower BIC because you chose -25%, the reference remuneration may also be lower.
Using the example above:
| Situation | Monthly BIC |
|---|---|
| Without adjustment | €2,100 |
With -25% | €1,575 |
| Monthly registered difference | €525 |
If all six months used in the calculation are at this level:
| Situation | Daily reference remuneration |
|---|---|
| Without adjustment | €70.00/day |
With -25% | €52.50/day |
For sickness benefit paid at 55%, that would be approximately:
| Situation | Approximate daily benefit |
|---|---|
| Without adjustment | €38.50/day |
With -25% | €28.88/day |
For pensions, the effect is slower and cumulative. The pension formula uses the total remunerations registered over your career, within legal limits. One isolated quarter barely moves the needle. Using a lower base systematically for years can reduce future reference remuneration.
What if I issued an invoice in December but got paid in January?
Income counts in the quarter the service was provided, not of the payment. The official Social Security FAQ points to the IRS rules (art. 3.º of the CIRS): income is deemed obtained when issuing the invoice becomes obligatory — as a rule, on the date the service was provided (the invoice can legally follow up to 5 working days later, but the income stays in the service month). Advances are different: the invoice is due immediately on receipt, so an advance paid on 31 March counts in Q1 even if the work happens in April.
In other words: a service provided in December belongs in the October–December quarter, declared in January — even if the invoice goes out on 2 January or the client only pays in January. Declaring that income again in the quarter of the payment would duplicate it.
A note on document flow: if the client pays later, the correct sequence is a fatura on the service date and a recibo when you get paid; the fatura-recibo is for when service and payment coincide. If that distinction is still unclear, read the guide on recibo verde vs fatura.
Limits and cautions
The 25% option does not apply to every situation in the same way.
Minimum and maximum limits still exist: the €20/month minimum contribution remains, and the monthly base has an IAS-linked maximum. There are also special rules for people who combine employment with freelance work and contribute only on the remaining relevant income.
If you are under organised accounting, the rule also changes: you can only use this option if you are under the quarterly relevant-income assessment regime.
In summary: for a typical independent worker in the simplified regime, the -25% option is a legal tool for managing the contributory base. But it is not neutral. Paying less now can mean a lower contributory base registered for those months.
✅ In summary
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The -25% option adjusts the contributory base, not the income declared. You declare the real income and choose a lower BIC within the legal margin.
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You pay less Social Security for the following three months, but you also register a lower base for that period.
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That base can influence future benefits such as sickness benefit, parental allowance, and pension, because those calculations use remunerations registered in your contributory record.
