Any lawful nonconforming structure existing on the effective date of this Chapter, or subsequent amendments, or rendered nonconforming by this Chapter, may be altered or enlarged, provided that such alteration or enlargement conforms to all of the lot area, width, impervious coverage, yard requirements, height and design standards of the zoning district in which it is located. If the new addition or enlargement cannot meet the requirements, it shall not be permitted unless, upon application, the Zoning Hearing Board grants a variance.