That at the time set for the hearing the board of county commissioners shall first ascertain whether notice has been given of the time of hearing as is required by this act and if it shall determine that such notice has been given to make a declaration and finding of that fact and cause same to be entered upon the records and thereupon to hear all persons in favor of or opposed to granting the prayer of said petition and all other evidence it may desire to hear for the purpose of ascertaining whether such petition contains the proper number of signers possessing the qualifications prescribed by this act, and whether the statements in said petition are true; and if upon such hearing it shall be found that such petition is in conformity to the requirements of this act, then such board of county commissioners shall make a finding and decision to that effect, and shall thereupon immediately declare the territory described in the petition to be annexed to and included in the said improvement district: Provided, however, That no petition shall be granted unless the consent of the board of directors of such improvement district to such annexation of territory shall have been filed with the county clerk at the time of or prior to such hearing.