A building may be erected or extended on any lot or contiguous lots held at the effective date of this chapter or any amendment thereto in single and separate ownership, which lot is not of the required minimum area or width or is of such unusual dimensions that the owner would have difficulty in providing the required open spaces for the district in which such lot is situated, provided that the building shall comply with the front, side and rear yard requirements of the district in which such lot is situated, unless otherwise authorized as a special exception by the Zoning Hearing Board, and in any case where Zoning Hearing Board review is required, the building shall comply with the yard requirements of the district to the maximum extent practicable.