This is from the Sacramento County website, but applies to the whole state:
Proposition 193, effective March 27, 1996, was a constitutional amendment approved by the voters of California which excluded from reassessment transfers of real property from grandparents to grandchildren, providing that all the parents of the grandchildren who qualify as children of the grandparents are deceased as of the date of transfer. Proposition 193 is also codified by section 63.1 of the Revenue and Taxation Code.
(http://www.assessor.saccounty.net/GeneralInformation/ChangesinOwnership/SAC_ASR_DF_CIO_Excl_Avoid_Reas)