Spain has mandatory disclosure rules requiring the reporting of cross-border arrangements that may involve CRS avoidance or aggressive tax planning, as established in the Spanish General Tax Law, additional provisions 23rd an 24th  (LGT: https://www.boe.es/buscar/act.php?id=BOE-A-2003-23186) and its implementing regulation, article 49 ter (RGAT:  https://www.boe.es/buscar/act.php?id=BOE-A-2007-15984)

These provisions transpose into domestic law both Council Directive 2011/16/EU on administrative cooperation in taxation (DAC6) and the OECD model rules for the mandatory disclosure of CRS avoidance arrangements and opaque offshore structures.

Additional Provision 23rd LGT establishes the obligation for intermediaries (such as tax advisers, lawyers, or financial institutions) and, in some cases, the relevant taxpayers themselves, to disclose information to the Spanish Tax Administration regarding:

(a) Cross-border arrangements containing any of the hallmarks listed in Annex IV of Directive 2011/16/EU.
(b) Updates on marketable cross-border arrangements.
(c) The use in Spain of such cross-border arrangements.
(d) CRS avoidance mechanisms and opaque offshore structures covered by the OECD model rules and the Multilateral Competent Authority Agreement.

The provision clarifies that arrangements based on tax regimes expressly approved by the European Commission are excluded. Certain intermediaries may be exempt from reporting if bound by legal professional privilege, although they must notify the relevant taxpayer. The law also establishes a sanctioning regime: failure to report, late reporting, or providing incomplete/false information constitutes a serious tax infringement, punishable with fixed monetary penalties (generally €2,000 per item, with minimums of €4,000 and maximums linked to fees earned or tax benefits obtained).

Additional Provision 24th LGT introduces specific obligations of communication among intermediaries and taxpayers. For example, intermediaries who are exempt from reporting due to professional secrecy must inform their clients, and intermediaries or taxpayers who have filed a disclosure must notify the others involved, thereby relieving them of their reporting duty. Non-compliance with these communication duties is sanctioned with a fixed penalty of €600, which may increase if the lack of communication results in a failure to disclose the arrangement.

Article 49 ter RGAT provides regulatory detail on these obligations. It confirms that intermediaries and, in some cases, the relevant taxpayers, must report to the Spanish Tax Administration cross-border mechanisms involving CRS avoidance or opaque structures whenever the prescribed hallmarks are present. It defines when an arrangement is considered cross-border, identifies the criteria linking an intermediary to Spain (residence, permanent establishment, incorporation, or place of management), and sets the reporting deadline at thirty days from the triggering event. The regulation also specifies that reporting must cover all jurisdictions where the arrangement is made available, and that the hallmark related to CRS avoidance applies even if certain conditions are not strictly met.

In summary, Spain has a comprehensive mandatory disclosure regime that requires the reporting of cross-border tax planning arrangements, including CRS avoidance structures, with clearly defined obligations for intermediaries and taxpayers, as well as a specific penalty framework for non-compliance.