Issues of sub-national referendums regulated at the national level

Constitution 

Article 67.

(1) Municipality inhabitants` assemblies shall realize a territorial self administration by local referendum, by referendum on the territory of the higher territorial unit, by municipality authorities or by higher territorial unit authorities. The manner of carrying out the local referendum or referendum on the territory of a higher territorial unit shall be laid down by a law. 

(2) The duties and limitations in realization of territorial self-administration may be imposed on a municipality and a higher territorial unit by a law and on the basis of an international treaty according to Art. 7, para. 5.

(3) The State may intervene in the activities of a municipality and a higher territorial unit only by means laid down by a law.