The provisions of this article shall apply to legally preexisting structures, lands and uses or any combination thereof that are made nonconforming as a result of the adoption and periodic revisions to the City's Code and regulations. It is the purpose of this article to permit these nonconformities to continue until they are removed or changed to a conforming, use but not to encourage their survival. It is further the purpose of this article to acknowledge that nonconformities shall not be used as grounds for adding other structures or uses prohibited in the district or enlarging, expanding or extending nonconformities except where expressly permitted under the provisions of this article. Possession of a valid building permit for a building or structure which will be nonconforming under the terms of this chapter shall be proof of construction in progress, provided that construction is pursued diligently to completion.