Idaho Code § 28-9-501: Filing office.

(a) Except as otherwise provided in subsection (b) of this section, if the local law of this state governs perfection of a security interest or agricultural lien, the office in which to file a financing statement to perfect the security interest or agricultural lien is:

(1) The office designated for the filing or recording of a record of a mortgage on the related real property, if:

(A) the collateral is as-extracted collateral or timber to be cut; or

(B) the financing statement is filed as a fixture filing and the collateral is goods that are or are to become fixtures; or

(2) The office of the secretary of state or any office duly authorized by the secretary of state, in all other cases, including a case in which the collateral is goods that are or are to become fixtures and the financing statement is not filed as a fixture filing.

(b) The office in which to file a financing statement to perfect a security interest in collateral, including fixtures, of a transmitting utility is the office of the secretary of state. The financing statement also constitutes a fixture filing as to the collateral indicated in the financing statement which is or is to become a fixture.