Subject to the provisions of § 43-3, if the person who shall cause a building or structure to be erected or repaired owns less than a fee simple estate in the land, then only his interest therein shall be subject to liens created under this chapter. When the vendee under a contract for the sale of real estate causes a building or structure to be erected or repaired on the land which is the subject of the contract and the owner has actual knowledge of such erection or repairs, the interest of the owner in the land shall be subject to liens created under this chapter; and for the purposes of § 43-21, the interest of such an owner in the land, to the extent of the unpaid purchase price, shall be deemed to be a recorded purchase money deed of trust lien created at the time the contract of sale was fully executed. As used in this section, “a contract for the sale of real estate” shall not include a lease of real estate containing an option to purchase the leased real estate or an option to purchase real estate unless the option is enforceable against the optionee.