Cal. EVID § 816

When relevant to the determination of the value of property, a witness may take into account as a basis for his opinion the price and other terms and circumstances of any sale or contract to sell and purchase comparable property if the sale or contract was freely made in good faith within a reasonable time before or after the date of valuation. In order to be considered comparable, the sale or contract must have been made sufficiently near in time to the date of valuation, and the property sold must be located sufficiently near the property being valued, and must be sufficiently alike in respect to character, size, situation, usability, and improvements, to make it clear that the property sold and the property being valued are comparable in value and that the price realized for the property sold may fairly be considered as shedding light on the value of the property being valued.