A petition for an election to vote upon the annexation of a portion of a county to a contiguous city or town signed by qualified voters resident in the area equal in number to twenty percent of the votes cast at the last election may be filed in the office of the board of county commissioners. The petition shall be filed with the legislative body of the city or town to which the annexation is proposed, and such legislative body shall, by resolution entered within sixty days from the date of presentation, notify the petitioners, either by mail or by publication in the same manner notice of hearing is required by RCW 35.13.040 to be published, of its approval or rejection of the proposed action. In approving the proposed action, the legislative body may require that there also be submitted to the electorate of the territory to be annexed, a proposition that all property within the area to be annexed shall, upon annexation be assessed and taxed at the same rate and on the same basis as the property of such annexing city or town is assessed and taxed to pay for all or any portion of the then outstanding indebtedness of the city or town to which said area is annexed, approved by the voters, contracted, or incurred prior to, or existing at, the date of annexation. Only after the legislative body has completed preparation and filing of a comprehensive plan for the area to be annexed as provided for in RCW 35.13.177 and 35.13.178, the legislative body in approving the proposed action, may require that the comprehensive plan be simultaneously adopted upon approval of annexation by the electorate of the area to be annexed. The approval of the legislative body shall be a condition precedent to the filing of such petition with the board of county commissioners as hereinafter provided. The costs of conducting such election shall be a charge against the city or town concerned. The proposition or questions provided for in this section may be submitted to the voters either separately or as a single proposition.