Notwithstanding the provisions of §§ 270-203 through 270-214, a valid nonconforming farm use or nonconforming farm structure, in a Medium Density Residential Zone, may be enlarged, increased, altered, or extended upon receipt of a special approval for such action from the Board of Appeals instead of a variance. The criteria for granting such special approval shall be as set forth in § 270-200 rather than the normal criteria for granting a use variance. This section shall apply only when the principal use of the property is as a farm.