An extension agreement is an international treaty between the European Patent Organisation and an extension state which the President of the EPO is authorised by the Administrative Council to conclude with the Council's approval (Art. 33(4) EPC). Such an agreement serves the interests not only of applicants, by providing a simple route to patent protection in the extension states, but also those of the extension state, by enabling it to offer patent protection for its territory by extending the effects of European and Euro-PCT applications and patents (J 9/04; see also J 14/00, OJ 2002, 432).

In T 7/07 the board stated that the extension system largely corresponds to the EPC system operating in the EPC contracting states, except that it is not based on direct application of the EPC but on national law modelled on the EPC. The national law of the extension state governs the proceedings and the legal effects of the extension.

In J 9/04 the board noted that, as a bilateral ordinance, the Extension Ordinances essentially deal – exhaustively and strictly separately from the Convention – with matters pertaining to the integration of extended European applications and protective rights into the respective national law and their relationship to national applications and rights. This includes, in particular, the conferral of the same effects on extended applications and patents as on national ones, the obligation to provide the national patent offices with a translation of the claims into the respective language, the national authentic text of extended applications and patents, their prior-art effect with respect to national applications and patents, and, finally, simultaneous protection. According to the board, none of these provisions give rise to any obligations on the part of the EPO. The EPO merely undertakes vis-à-vis the national patent authorities to assist with the administrative tasks associated with the extension of European patents, namely receiving requests for extension, levying extension fees and, after deducting an amount to cover its expenses, forwarding the remaining amount to the national patent offices. There is no right of recourse to the boards of appeal in respect of extensions of patent applications and patents to the extension states. Instead, the respective national jurisdiction is responsible. For example, Art. 6(2) of the Slovenian Law provides for appeals against decisions of the Slovenian Patent Office (see also J 2/05).