Cal. CIV § 1029

Where one has made use of materials which in part belong to him and in part to another, in order to form a thing of a new description, without having destroyed any of the materials, but in such a way that they cannot be separated without inconvenience, the thing formed is common to both proprietors; in proportion, as respects the one, of the materials belonging to him, and as respects the other, of the materials belonging to him and the price of his workmanship.