The total of all extensions, expansions, and/or alterations shall not exceed an additional 50% of the floor area of those buildings, structures, or lands devoted to the nonconforming use, or of the floor area of the nonconforming building or structure, as they existed on the date on which the use of such buildings, structures, or land, or the buildings or structures themselves, first became nonconformities. The applicant shall furnish conclusive evidence as to the extent of the nonconformity when it was created;