After the board of county commissioners shall have established building or set-back lines on such major highways, no new building or structure shall be erected within such building or set-back lines outside the corporate limits of any city or town, and no permit for such building shall be issued by the county engineer who is hereby designated as the officer to administer and enforce such building or set-back line regulations. The county board of adjustment, hereinbefore created, shall hear appeals of any property owner aggrieved by such building line regulations. Said board of adjustment shall have the power to modify or vary the building or set-back line regulations in specific cases, in order that unwarranted hardship, which constitutes an unreasonable deprivation of uses as distinguished from a mere grant of privilege, may be avoided, the intended purpose of the regulations strictly observed, and the public welfare and public safety protected.