Cal. Civ. Code § 1

Section 1. If a forcible entry into land or tenements has been made, if a peaceable entry has been made and the possession is unlawfully held by force, if the lessee of land or tenements or a person holding under him holds possession without right after the determination of a lease by its own limitation or by notice to quit or otherwise, or if a mortgage of land has been foreclosed by a sale under a power therein contained or otherwise, or if a person has acquired title to land or tenements by purchase, and the seller or any person holding under him refuses to surrender possession thereof to the buyer, or if a tax title has been foreclosed by decree of the land court, or if a purchaser, under a written agreement to purchase, is in possession of land or tenements beyond the date of the agreement without taking title to said land as called for by said agreement, the person entitled to the land or tenements may recover possession thereof under this chapter. A person in whose favor the land court has entered a decree for confirmation and registration of his title to land may in like manner recover possession thereof, except where the person in possession or any person under whom he claims has erected buildings or improvements on the land, and the land has been actually held and possessed by him or those under whom he claims for six years next before the date of said decree or was held at the date of said decree under a title which he had reason to believe good.