The Spanish Supreme Court, in its judgment no. 350/2026 of 20 March 2026 (cassation appeal 422/2024), confirms that a tax self-assessment does not constitute an administrative act in the proper sense, but an action of the taxpayer subject to a procedural regime of its own. The consequence is direct: where the self-assessment is considered contrary to law —typically, by application of a rule whose unconstitutionality or incompatibility with EU law is declared subsequently— the review channel available is the rectification of article 120.3 of the General Tax Act (LGT), exercised within the ordinary four-year statute of limitations. The extraordinary route of nullity ex tunc of article 217 of the LGT does not apply to self-assessments and, therefore, does not open a second procedural chance once that period has elapsed.
The practical consequence, for defensive purposes, requires time discipline. Where a judgment of the Constitutional Court or of the Court of Justice of the European Union declares the unconstitutionality or incompatibility of a rule under which tax has been self-assessed, it is advisable to react within the four-year statute of limitations. The available options are, in essence, two: to seek rectification of the self-assessment under article 120.3 of the LGT and, where applicable, to articulate in parallel a State liability claim against the legislator, a route the legal order keeps open in delimited situations and with its own time limits. To wait beyond the statute of limitations amounts, in most cases, to renouncing recovery.
Full analysis in → A self-assessment may not be the subject of the special review procedure for acts that are null and void