Stacking is the default; only clear policy language stops it
Arizona's rule is in A.R.S. 20-259.01. The Arizona Supreme Court held in Franklin v. CSAA General Insurance Co. (2023) that a multi-vehicle policy provides separate UIM coverage for each vehicle, and in State Farm v. Balzan (2026) that stacking is permitted unless the policy clearly prohibits it.
Read how stacked and unstacked coverage differ in Arizona, check what subsection (H) and the Supreme Court say, or see why Arizona treats UM and UIM as separate coverages.