The board of supervisors may hear additional evidence and say sustain, modify or overrule any order brought before it on appeal pursuant to Section 17.54.670, and may make such findings and decisions as are not inconsistent with state law and county ordinances; provided that, if no motion relative to the order appealed attains a majority vote of the board of supervisors within thirty (30) days from the date of the hearing by said board thereon, said order shall stand sustained and be final. (Prior gen. code § 8-102.4)