California Code, Revenue and Taxation Code - RTC § 10770

(a) If the fee for an original registration is not paid within 20 days after it becomes delinquent, a penalty equal to 20 percent of the fee shall be added and collected with the fee.

(b) A penalty of 20 percent of the license fee shall be added on any application for original or renewal of year-round or annual registration made later than midnight of the date of expiration or on or after the date penalties become due.This penalty shall be computed after the vehicle license fee has been combined with the registration and weight fees as provided in Sections 9250 and 9400 of the Vehicle Code.

(c) Notwithstanding subdivision (a), any penalty that became due prior to January 1, 1978, shall be computed at the rate of penalty which was then in effect.