The review board shall by majority action, within three months, determine whether the property proposed to be annexed is of such character that such annexation would be in the public interest and for the public welfare, and in the best interest of the city, county, and other political subdivisions affected. The governing officials of the city, county, and other political subdivisions of the state shall assist the review board insofar as their offices can, and all relevant information and records shall be furnished by such offices to the review board. In making their determination the review board shall be guided, but not limited, by their findings with respect to the following factors:

(1) The immediate and prospective populations of the area to be annexed;

(2) The assessed valuation of the area to be annexed, and its relationship to population;

(3) The history of and prospects for construction of improvements in the area to be annexed;

(4) The needs and possibilities for geographical expansion of the city;

(5) The present and anticipated need for governmental services in the area proposed to be annexed, including but not limited to water supply, sewage and garbage disposal, zoning, streets and alleys, curbs, sidewalks, police and fire protection, playgrounds, parks, and other municipal services, and transportation and drainage;

(6) The relative capabilities of the city, county, and other political subdivisions to provide governmental services when the need arises;

(7) The existence of special districts except school districts within the area proposed to be annexed, and the impact of annexation upon such districts;

(8) The elimination of isolated unincorporated areas existing without adequate economical governmental services;

(9) The immediate and potential revenues that would be derived by the city as a result of annexation, and their relation to the cost of providing service to the area.

Whether the review board determines for or against annexation, its reasons therefor, along with its findings on the specified factors and other material considerations shall:

(1) In the case of a petition signed by registered voters calling for an election on annexation, be filed with the board of county commissioners;

(2) In the case of a resolution of a city or town initiating annexation proceedings pursuant to RCW 35.13.015, be filed with the board of county commissioners.

Such findings need not include specific data on every point listed, but shall indicate that all factors were considered.

A favorable determination by the review board is an essential condition precedent to the annexation of territory to a city or town under either the resolution method pursuant to RCW 35.13.015, or under the twenty percent annexation petition under the election method.