The board of supervisors may revoke or modify by written order, after a public hearing held upon not less than ten days’  notice, any order permitting a variance. The board of supervisors shall serve notice of the time and place of a hearing to revoke or modify any order permitting a variance not less than ten days prior to such hearing, by personal service, or by first class mail, postage prepaid, mailed to the person to whom such variance was granted, at the address shown in the application for such variance or to such other address as may appear reasonable to the board of supervisors under the particular circumstances. (Prior gen. code 3-72.4)