§ 306. Service of notices and orders. Every notice or order issued by the department relative to a premises shall be served at least thirty days before the time for compliance therewith. It shall be sufficient service of a departmental notice or order, if it is posted in a conspicuous place upon the premises affected and a copy thereof mailed, on the same day it is posted, to the person to whom it is directed at the address filed by him in the department, and, if his address is not so filed in the department, then in such case, such notice shall be sent by registered mail to his last known address or place of residence.