Cal. Rev. & Tax. Code § 71

The assessor shall determine the new base year value for the portion of any taxable real property which has been newly constructed. The base year value of the remainder of the property assessed, which did not undergo new construction, shall not be changed. New construction in progress on the lien date shall be appraised at its full value on such date and each lien date thereafter until the date of completion, at which time the entire portion of property which is newly constructed shall be reappraised at its full value.