Three weeks ago, the Dutch Supreme Court rendered a judgment on the immunity of execution of state property (ECLI:NL:HR:2013:45). In order to execute the verdict of a Congolese court, the claimant battered a building in The Hague. This building is owned but no longer used by the Democratic Republic of the Congo (DRC) because its diplomatic agency moved to Brussels.

When a Dutch bailiff tries to execute this seizure, the Ministry of Justice announces this action incompatible with the international duties of the State. The bailiff turns to the judge, who decides that the bailiff should be able to proceed because the building is no longer used for State affairs. The Court of Appeal decides the building is still a governmental building and should be immune from execution. Then the question whether the use of the property for public affairs is decisive for the immunity of execution is laid before the Supreme Court. In order to answer this question, the Court turns to an unusual source of international law.

The Supreme Court starts with examining the existing conventions on the subject. Because there is no international convention on the subject of State immunity, the Court consults the Vienna Convention on Diplomatic Relations. This Convention is the next best thing, but it does not answer the question at hand. Therefore, the Court tries to find the answer using the oldest source of international law: customary law, containing non-written, internationally accepted rules. On the subject of immunity, customary law is quite extensive and has evolved over the years.

The immunity of states was laid down only recently in an international convention: in 2004, the General Assembly accepted the United Nations Convention on Jurisdictional Immunities of States and Their Property. However, not enough States are party to this Convention for it to enter into force. This means that, in comparison with article 38(1) Statute ICJ, it is neither an international convention because it has not entered into force, nor is it customary law because it is written down, nor is it an all-embracing survey of the existing customary law.

This does not prevent the Supreme Court from using this document. The Court considers that international customary law does not expressly require state property to be used in order to be immune to forced execution. The Court then turns to the UN Convention for support on this customary rule. In article 21, it is stated that ‘property … which is used or intended for use in the performance of the functions of the diplomatic mission of the state’ is immune to execution. Based on this line of thought, the Supreme Court concludes that the judgment of the Court of Appeal - the property is still immune to execution regardless whether it is still used by the DRC - is in line with the rules of international customary law.

The not-in-force Convention is more explicit on the matter of immunity of property by including the intention of use, whereas customary law is not expressly requiring this use. It is a small difference, but it not only broadens the view on the immunity of property, it also narrows down the room for interpretation to be given by the customary rule. The Supreme Court should not easily give non-binding Conventions such interpretative force when the matter beforehand asks for a decision based on international law. In this case, customary law should have been the only source used by the Court.