Temporary offices, storage, signs and other structures of contractors shall be permitted in any district, if such uses are deemed necessary by the Director of Inspections to the development of the premises and provided a building permit has been obtained for the principal work. The structures may be erected within the limits of lots whereon buildings are in the course of erection or on adjoining vacant lots. The sanitary regulations for such structures shall be as prescribed by the Director of Inspections. Such structures shall be removed within thirty days after completion of work on the premises or if construction is not pursued diligently as determined by the Director.