Idaho Code § 67-6505
The boards of county commissioners of two (2) or more adjoining counties, alone or together with the council of one (1) or more cities therein, or the board of county commissioners of a county together with the council of one (1) or more cities within the county, or the councils of two (2) or more adjoining cities, are empowered to cooperate in the establishment of a joint planning, zoning, or planning and zoning commission, hereafter referred to as a joint commission, and may provide for participation by invitation of other public agencies deemed necessary to exercise the powers conferred in this chapter. The number of members of a joint commission, the method of appointment, and the allocation of costs for activities to be borne by the participating governing boards shall be agreed upon by the governing boards and agencies involved. A joint commission is further authorized and empowered to perform any of the duties for any local member's governing board when the duties have been authorized by that member government.
Idaho Code § 67-6505