When a serious injury runs past one policy's UM or UIM limits, the next question is whether other coverage in the household can apply to the same crash. That's stacking — and in Arizona, the honest answer is: sometimes, and the policy paperwork decides.
What stacking means in practice
Stacking comes in two forms. Combining the limits on multiple vehicles insured under one policy is one; combining coverage across separate policies in the same household — a spouse's policy, a parent's policy for a resident family member — is the other. When stacking applies, the coverage available for a single claim can multiply, which is why it matters most in exactly the crashes where injuries are largest.
What Arizona law says
A.R.S. § 20-259.01 is the statute that governs UM/UIM coverage in Arizona, and it permits a person covered by more than one policy to collect from multiple policies — unless the insurer has limited stacking in the specific manner the statute prescribes. Arizona courts have upheld anti-stacking provisions when the policy language complies with the statute's requirements, and have refused to enforce provisions that fall short. The law in this area continues to be refined, including recent Arizona Supreme Court decisions on how anti-stacking provisions apply to jointly purchased policies.
Why this is a documents question first
None of that can be answered in the abstract. It takes the declarations pages and policy language from every potentially applicable policy in the household — which is why "bring every policy" is standard advice for a consultation after a serious crash. Whether limits stack can change the available coverage several times over, and the answer is sitting in paperwork most people already have.
What to bring to a review
The declarations page for every auto policy in the household, and the full policy documents if available. A review covers what UM/UIM limits exist, whose coverage may extend to the injured person, and whether the policy language leaves stacking available. Saguaro Injury Law reviews UM/UIM coverage on every case, in English and Spanish — free consultations any hour: (602) 217-0000, and the UM/UIM Coverage Check on this site is a quick first pass.