The size of, ownership of and number of single-family dwellings on a parent tract as it existed on June 4, 1978, shall be used in determining the number of lots, or single-family detached dwellings, or other principal nonagricultural buildings which may be erected on a parent tract in accordance with the provisions of Subsection B(1), as defined through a chain of title document search of the parent tract in existence on or before June 4, 1978. In the event that a tract which was not classified as part of the A-Agricultural District on June 4, 1978, is hereafter classified as part of the A-Agricultural District, the size of, ownership of and number of single-family dwellings on such tract shall be determined as of the effective date of the change in the zoning classification.