Powers of Board of appeals. The Board of Appeals shall have the power to hear and decide appeals where it is alleged there is an error in any order, requirement, decision or determination made in the enforcement of this chapter, and to authorize upon appeal in specific cases such variance from the requirements of this chapter as will not be contrary to the public interest and where, owing to special conditions, a literal enforcement of any provision of this chapter would result in practical difficulty or unnecessary hardship, so that the spirit of this chapter shall be observed, public safety and welfare secured and substantial justice done.