Any building or structure existing at the date of the adoption of the ordinance codified in this title which is nonconforming either in use, design or arrangement shall not be enlarged, extended, reconstructed or structurally altered unless such enlargement, extension, reconstruction or alteration is in compliance with the regulations set forth in this title for the district where such building or structure is located; provided, however, that any such nonconforming building or structure may be maintained, repaired or portions thereof replaced so long as such maintenance, repairs or replacements do not exceed twenty-five percent of the building's appraised valuation as shown on the latest assessment roll of the city. (Ord. 3-83 § 6.3, 1983)