A covenant for the addition of some new thing to real property, or for the direct benefit of some part of the property not then in existence or annexed thereto, when contained in a grant of an estate in such property and made by the covenantor expressly for the covenantor's assigns or to the assigns of the covenantee, runs with the land so far only as the assigns thus mentioned are concerned.
N.D.C.C. § 47-04-27