Nonconforming structures. Nonconforming structures may be altered, reconstructed, or enlarged, provided that such alteration conforms with all the dimensional requirements of this chapter, except for the nonconforming requirement. Alteration shall not increase the extent of the nonconformity existing on the effective date of this chapter; however, the alteration may extend the nonconforming part of the structure along the same line, plane, or dimension as existed on the effective date of this chapter for a distance not to exceed 16 linear feet. In the case of a nonconforming structure that is used by a nonconforming use, such alteration, extension, or enlargement shall also meet the requirements of Subsection C of this section.