Ordinary maintenance and minor repair of a nonconforming building is permitted; provided, that the aggregate cost of the work done in any period of twelve (12) months on minor alterations or replacement of interior walls, fixtures or plumbing shall not exceed twenty-five (25) percent of the assessed value of the building according to the assessment thereof by the assessor of the county for the fiscal year in which the work was done, and provided further that neither the cubical content of the building nor the number of dwelling units therein shall be increased. The provisions of this section shall not apply to dwellings in the A district that are nonconforming solely because they were not subject to site development review pursuant to Section 17.06.090 of this title. (Prior gen. code § 8-62.6)