California Code, Streets and Highways Code - SHC § 10403

All assessments not paid within 30 days after they become due, except all unpaid assessments for which bonds are to be issued, shall become delinquent and the recording officer shall add to each delinquent assessment 5 percent of the amount thereof. Assessments may be paid in whole or in part during the 30-day period after the same become due and payable.