Upon the date fixed in the ordinance of annexation the area annexed shall become part of the city or town. All property within the territory hereafter annexed shall, if the annexation petition so provided, 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 of 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. If the annexation petition so provided, all 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.