[A]n applicant for a variance must prove:
- relief is necessary because of the unique character of the property rather than for personal considerations; and
- applying the strict letter of the ordinance would result in unnecessary hardship; and the
- imposition of such a hardship is not necessary for the preservation of the plan; and
- granting the variance will result in substantial justice to all.
Although all the requirements must be satisfied, it is generally held that "'[u]nnecessary hardship' is the principal bases on which a variance is granted."