(3) twenty-four hours in the case of immediately hazardous violations in which case the notice shall be served by personal delivery to a person in charge of the premises or to the person last registered with the city as the owner or agent, or, by registered or certified mail, return receipt requested, to the person in charge of the premises or to the person last registered with the department as the owner or agent; provided that where a managing agent has registered with the department, such notice shall be served on the managing agent. Service of the notice shall be deemed completed five days from the date of mailing. The department may postpone the date by which a violation shall be corrected upon a showing, made within the time set for correction in the notice, that prompt action to correct the violation has been taken but that full correction cannot be completed within the time provided because of technical difficulties, inability to obtain necessary materials, funds, or labor, or inability to gain access to the dwelling unit wherein the violation occurs or such other part of the building as may be necessary to make the required repair. In the case of immediately hazardous violations such showing must be made prior to the close of business on the next full day the department is open following the period set for correction. The department may condition such postponement upon the applicant's written agreement to correct all violations placed against the premises by the department or other appropriate governmental agency and to satisfy within an appropriate period of time, all sums owing to the department for repairs made to said premises. The department may require such other conditions as are deemed necessary to insure correction of the violations within the time