2009 California Public Resources Code - Section 9911-9912 :: Article 7. Actions And Proceedings

9911.  All acts, proceedings, conclusions, and findings of fact,
including the levy of an assessment, by the directors of a district
concerning an improvement district therein shall be conclusive except
in an action or proceeding instituted within six months after the
acts, proceedings, conclusions, or findings were had or made.

9912.  An action to determine the validity of an assessment or of
any warrants may be brought pursuant to Chapter 9 (commencing with
Section 860) of Title 10 of Part 2 of the Code of Civil Procedure.