A declaración complementaria corrects a self-assessment you already filed when the correction means paying more, or getting a smaller refund, but today it survives only in the forms and periods where the autoliquidación rectificativa is not yet in place. In the main forms it is no longer used: in VAT since the third quarter of 2024, in personal income tax since the 2024 campaign, and in corporate tax for 2024, the rectificativa is the single way to correct a return, both when you owe more and when the correction is in your favour.
The change came with Real Decreto 117/2024, which replaces the old dual system of complementaria plus rectification request with a single figure. Where the complementaria still applies, it is filed on the same form and for the same period you are correcting, stating what was already paid at the time; if it also arrives late and without any formal notice, the article 27 surcharge of the General Tax Law is added on top.
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