Approve the issuance of a permit wherever it is provided in this chapter that the approval of the Board of Zoning Appeals is required; vary or modify the application of any of the regulations of this chapter relating to the use, construction or alteration of buildings or the use of land, so that the spirit of this chapter shall be observed, public safety and welfare secured and substantial justice done when, in the opinion of the Board of Zoning Appeals, there are practical difficulties or unnecessary hardships in the way of carrying out the strict letter of this chapter.