California Code, Health and Safety Code - HSC § 8113.1

(a) Except as provided in subdivisions (b) and (c), there shall be no less than 18 inches of dirt or turf on top of all vaults or caskets as measured at the time of burial.

(b) Cremated remains placed in an urn or urn vault and covered with at least three-quarters of an inch of concrete, brass, granite, marble, or metal plate, affixed to the urn or urn vault shall be exempt from the requirement of subdivision (a).

(c) In the case of consensual double burials, the casket or vault that is on top shall be covered with at least 12 inches of dirt or turf as measured at the time of burial.

(d) In a case of extreme hardship, upon request of the next of kin or other person responsible for making the burial arrangements for the deceased, a burial of less than 18, but not less than 12 inches may be provided.