Swiss Federal Supreme Court Limits
Protection of Attorney-Client Privilege
in International Tax Assistance
Proceedings

In a new landmark ruling 2C_506/2024 dated 4 May 2026 (intended for publication in the official collection), the Swiss Federal Supreme Court addressed for the first time the scope of attorney-client privilege where correspondence between a lawyer and a domestic tax authority is to be produced by that authority in response to a request made by a foreign state in international tax administrative assistance proceedings.

Factual background

In 2023, the Spanish tax authority submitted administrative assistance requests to the Swiss Federal Tax Administration (FTA), suspecting that a taxpayer claiming Swiss tax residency was in fact resident in Spain. Among the information requested were tax residence certificates issued by the Thurgau cantonal tax authority and correspondence exchanged between the taxpayer's lawyers and that authority. The taxpayer objected to the transmission, invoking attorney-client privilege.

Attorney-client privilege does not prevent transmission

At treaty level, the Swiss Federal Supreme Court recalls that art. 25bis
para. 3 let. c Double Taxation Convention CH-ES (modelled on art. 26 para. 3 let. c OECD MC) reserves legal professional privilege only for confidential communications between lawyers and their clients, to the extent that these are protected from disclosure under the domestic law of the requested state. Legal professional privilege does not cover information received by a tax authority from a lawyer, nor correspondence with a lawyer that is already in the authority's possession.

Under domestic law, in line with a previous ruling (SFC 151 II 873, c. 13.1), the Swiss Federal Supreme Court concluded that art. 8 para. 6 of the Tax Administrative Assistance Act only applies to documents in the lawyer's or their auxiliary's possession. As for art. 13 para. 1bis of the Administrative Procedure Act, this does not cover correspondence exchanged between a lawyer and an authority. The Swiss Federal Supreme Court further reminds that, according to case law, a lawyer who voluntarily transmits confidential information to a third party (such as an authority or insurer) in principle loses the protection afforded by professional privilege.

This ruling provides an important clarification for practitioners. Taxpayers and their legal counsel must be aware that correspondence exchanged with tax authorities does not benefit from attorney-client privilege and may be transmitted to a requesting state in international tax administrative assistance proceedings. This case law calls for careful consideration of communication strategies with tax authorities and the choice of information shared in this context. This ruling also appears to build on previous decisions that have gradually eroded the scope of attorney-client privilege in the context of international tax assistance.