The agency shall operate each program or activity so that the program or activity, when viewed in its entirety, is readily accessible to and usable by individuals with handicaps. This section does not—

(a) Necessarily require the agency to make each of its existing facilities accessible to and usable by individuals with handicaps; or

(b) In the case of historic preservation programs, require the agency to take any action that would result in a substantial impairment of significant historic features of an historic property.