Nonconforming structures or structures containing nonconforming uses may be enlarged, extended, reconstructed, or altered by a maximum of 25% of the aggregate gross floor area of the structure sought to be enlarged as it existed on May 13, 1971, or up to the maximum lot coverage allowed in the zoning district in which the nonconforming structure is located, as provided in Appendix B,[1] whichever is less. Such extension or enlargement of a nonconforming building or other structure must be by special permit from the Zoning Board of Appeals pursuant to § 165-116, Special permits, of this chapter and subject to site plan approval from the Planning Board.