Such liens shall only attach to the interest in the real property of the owner of the improvement or for whose benefit the improvement was constructed; provided that in no event shall any owner’s interest be subject to a claim of lien based upon a work of improvement contracted for by a person claiming an estate under said owner, nor shall such person be deemed that owner’s agent, where the terms of estate interest of said person claiming an estate is for a term of at least thirty (30) years from the commencement of the work of improvement or where the terms of the estate interest were negotiated at arms length upon commercially reasonable terms. Such liens are preferred to any lien, mortgage, or other encumbrance upon the work of improvement on the site, which attaches subsequent to the commencement of the work of improvement. The term work of improvement or site improvement means the entire structure or scheme of improvement as a whole, except where a project consists in the construction of two (2) or more separate residential units, and then each unit shall be considered a separate work of improvement.