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Except as provided in Section 5.5, below, the reconstruction, extension or structural change of a nonconforming structure in such a manner as to decrease an existing nonconformity, shall not require the issuance of a variance. Nonconforming single and two family residential structures may be reconstructed, extended, altered, or structurally changed upon a determination by the Building Commissioner that such proposed reconstruction, extension, alteration, or change does not decrease the nonconforming nature of said structure by more than 50% gross floor area.