Additional territory adjoining a highway district and lying contiguous with and within one (1) or more counties may be added to and be included in the district, by the affirmative vote of a majority of the qualified electors of the additional territory voting on the question at an election held for that purpose, which vote shall be taken at an election on a date authorized in section 34-106, Idaho Code. Additional territory shall not be annexed to or included in the district unless annexation and inclusion shall be first approved by the commissioners of the county in which the area proposed to be annexed is located if it shall be deemed to be in the best public interest, and by the highway district commissioners of the existing district by resolution, entered on their minutes prior to the election on the question of annexation.