Upon the date fixed in the ordinance of annexation, the area annexed shall become part of the city or town. All property within the annexed territory 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 the area is annexed, approved by the voters, contracted, or incurred before, or existing at, the date of annexation. If the annexation petition so provided, all property in the annexed area is subject to and is a part of the comprehensive plan as prepared and filed as provided for in RCW
35.13.177 and
35.13.178.