Variances. To authorize upon appeal in specific cases such variance from the terms of this chapter as will not be contrary to the public interest, where, owing to special conditions, a literal enforcement of the provisions of this chapter will result in practical difficulty or unnecessary hardship, so that the spirit and purposes of this chapter shall be observed, public safety and welfare secured, and substantial justice done. For an area variance, unnecessary hardship exists when strict compliance with this chapter would unreasonably prevent the property owner from using the property for a permitted purpose or would render conformity with this chapter unnecessarily burdensome. For a use variance, unnecessary hardship exists when strict compliance with this chapter would leave the property owner with no reasonable use of the property in the absence of a variance. In every case where a variance from the terms of this chapter has been granted by the Board of Appeals, the minutes of the Board shall affirmatively show that a practical difficulty or unnecessary hardship exists and the records of the Board shall clearly show in what particular and specific respects a practical difficulty or an unnecessary hardship is created.