California Code, Streets and Highways Code - SHC § 2954

The return postcards shall be filed in the clerk's office and after the expiration of the 30-day period specified in the Notice of Proposed Assessment all demands for the investigation made to the legislative body shall be computed and it shall by order entered on its minutes determine whether or not the owners of 15 per cent of the area of the proposed assessment district have demanded such investigation.  Such determination shall be final and conclusive in the absence of actual fraud.