Upon the date fixed in the ordinance of annexation, the area annexed shall become a part of the city or town. Upon the date fixed in the ordinances of annexation and adoption of the comprehensive plan, the area annexed shall become a part of the city or town and property in the annexed area shall be subject to and a part of the comprehensive plan, as prepared and filed as provided for in RCW 35.13.177 and 35.13.178. All property within the territory hereafter annexed shall, if the proposition approved by the people so provides after June 12, 1957, be assessed and taxed at the same rate and on the same basis as the property of such annexing city 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.