Nonconforming structures. Nonconforming structures may be altered, reconstructed, or enlarged, provided that such alteration, reconstruction or enlargement does not increase the extent of the nonconformity existing on the effective date of this chapter. In the case of a nonconforming structure which is used by a nonconforming use, such alteration, extension, or enlargement shall also meet the requirements of Subsection 3C of this section.