The regulations shall not apply in respect to the continued use of any building or land which use was in conformity with the use regulations then existing, and shall not prevent the restoration of such building damaged by fire, explosion, act of God, or public enemy; nor prevent the continuance of the use of such building or part thereof as such use existed at the time of damage; nor prevent the expansion, enlargement or structural alteration thereto; provided, however, that the integrity of the original structure is not substantially reduced or eliminated. Whenever there is to be a change in general use classification of any building or land subsequent to the effective date of these regulations the same shall only be accomplished in accordance with the provisions of these regulations and in a district in which it is permitted. All existing structures or buildings, the use of which is in accordance with the use regulations of the district in which they are located, are hereby declared to be in conformity with the dimensional requirements of the assigned district herein upon the date of adoption of these regulations. Provided further, all single-family dwelling buildings, existing as legal conforming uses, upon the adoption of these regulations and the use of which is not in accordance with the use regulations of the assigned district in which they are located, may be enlarged, structurally altered, and accessory structures erected, and are hereby declared to be in conformity with the dimensional requirements herein upon the date of adoption of these regulations. Mobile home parks may continue for such purposes, including the replacement of individual mobile home units. In no event shall the number of mobile home units exceed the total number of placement sites within the boundary of a mobile home park; provided, that required and applicable approved development plans are of record with the appropriate governmental jurisdiction in respect to the location and number of planned placement sites. (Code 1995 § 48-1.03.)