As used in this section, “radiation fallout shelter” means a structure that is: constructed: outside of another building; as an underground or partially underground addition to or an alteration of an existing building; or as an underground part of a new building, but from materials more dense than the general materials used for the other parts of the building; constructed in accordance with specifications published by the United States Office of Civil and Defense Mobilization for a shelter type or design approved by that office to provide protection from radiation fallout; and located on land occupied for residential purposes by not more than 2 families. A radiation fallout shelter that is not used for any other purpose may not be assessed to the owner of the property on which the shelter is located. Any other structure used to provide protection from radiation fallout shall be assessed on the basis of the difference in the value of the improvements to the real property as actually constructed and the value if constructed of materials similar to the residence on the property.