Whenever real property has been divided into condominiums, as defined in Section 783 of the Civil Code, (a) each condominium owned in fee shall be separately assessed to the owner thereof, and the tax on each such condominium shall constitute a lien solely thereon; (b) each condominium not owned in fee shall be separately assessed, as if it were owned in fee, to the owner of the condominium or the owner of the fee or both (and the tax on each such condominium shall be a lien solely on the interest of the owner of the fee in the real property included in such condominium and on such condominium), if so agreed by the assessor in a writing of record; such an agreement shall be binding upon such assessor and his successors in office with respect to such project so long as it continues to be divided into condominiums in the same manner as that in effect when the agreement was made.