A variance may be granted when it is not contrary to the public interest, and, owing to special conditions, a literal enforcement of the provisions of this Chapter 170 will result in unnecessary hardship, provided that the spirit of this chapter shall be observed and substantial justice done, as follows: When a property owner can show that his property was acquired in good faith and where, by reason of the exceptional narrowness, shallowness, size or shape of a specific piece of property at the time of the effective date of this Chapter 170, or where, by reason of exceptional topographic conditions or other extraordinary situation or condition of the piece of property, or of the condition, situation, or development of property immediately adjacent thereto, the strict application of the terms of this Chapter 170 would effectively prohibit or unreasonably restrict the utilization of the property, or where the BZA is satisfied, upon the evidence heard by it, that the granting of the variance will alleviate a clearly demonstrable hardship, as distinguished from a special privilege or convenience sought by the applicant, a variance may be granted, provided that all variances shall be in harmony with the intended spirit and purpose of this Chapter 170, and shall be the minimum necessary to alleviate the demonstrated hardship.