California Code, Revenue and Taxation Code - RTC § 11341

(a) The board shall render its decision on the petition for reassessment within 45 days of the date of the hearing on the petition, and shall mail notice of its decision to the applicant.

(b) If the board determines that the amount of the assessment should be increased, the notice shall advise the applicant of the amount of additional tax that is due, and the date by which payment must be made. If the additional tax is not paid within 15 days following the date on which notice is mailed, it is delinquent, and a penalty of 10 percent of the amount of the additional tax shall be added thereto, plus interest on the amount of the additional tax at the rate of 3/4 of 1 percent per month, or fraction thereof, from the date on which the additional tax became due until the date of payment.

(c) If the board determines that the amount of the assessment should be reduced, the notice shall advise the applicant of the amount of the tax that will be refunded or credited. The board shall proceed to refund or credit the overpayment in the manner prescribed in Section 11551, and interest shall be added to the credit or refund in the amount provided in Section 11555.