To authorize, upon appeal in specific cases, such
variance from the terms of this code as will not be contrary to the
public interest where, owing to special conditions, a literal enforcement
will result in practical difficulty or unnecessary hardship, so that
the spirit of the code shall be observed, public safety and welfare
secured and substantial justice done; provided, however, that no such
action shall have the effect of establishing in any district a use
or uses not permitted in such district.