Mont. Code Ann. § 76-1-501

The board of county commissioners shall by resolution establish the jurisdictional area of the county planning board. The jurisdictional area shall include the area which is both outside the incorporated limits of any city in the county as well as outside the jurisdictional area of an existing city-county planning board established pursuant to 76-1-504 through 76-1-507. Should any city or town become represented on the county planning board pursuant to 76-1-111, the jurisdictional area of the county planning board shall be extended to include that city or town.