You have a badly hurt client, a tortfeasor whose liability limits are gone in the first week, and a household that carries three or four auto policies with the same carrier. The reflex is obvious: stack the underinsured-motorist coverage across every policy the family bought and turn a thin recovery into a real one. In Arizona, that reflex just got harder to indulge. On July 6, 2026, the Arizona Supreme Court decided State Farm Mutual Automobile Insurance Co. v. Balzan (Opinion No. CV-24-0140-PR), and the holding is one every PI practitioner handling first-party coverage needs to internalize before drafting the next UIM demand.
This is a single-case walkthrough written for people who already know what a reducing clause and an offset do. The point is not the doctrine in the abstract. The point is what changes in your file when the household's policies all came from the same insurer.
The claim and the two competing theories
The dispute grew out of a 2019 crash that left the claimant with damages well beyond the at-fault driver's coverage. The household held several auto policies with the same carrier. The claimant sought to stack the underinsured motorist limits across those family policies to reach additional UIM money after the liability tender.
The plaintiff theory was the familiar one. Arizona has long been a stacking-friendly jurisdiction: absent a clear contractual bar, an insured who paid separate premiums on separate policies can aggregate the UIM limits. Each policy is a distinct promise the family paid for, the argument runs, so each should pay. Where a carrier has not spoken clearly, the tie goes to coverage.
The carrier answered with a two-part defense that turned out to be decisive. First, the joint purchasers of the multiple policies were, for purposes of Arizona's UM/UIM statute, a single "insured," not a set of independent insureds each entitled to a separate stack. Second, the policies contained a clear written anti-stacking clause, and Arizona law lets an insurer forbid inter-policy stacking as long as it does so unambiguously in the contract.
What the court actually held
The Supreme Court sided with the carrier. It held that joint purchasers of multiple policies from the same insurer constitute a single insured under the statute, and that on those facts the insurer may enforce a clear written anti-stacking clause to bar inter-policy stacking of underinsured-motorist benefits across the multiple household policies jointly purchased from that same insurer.
The reasoning is worth reading closely rather than summarizing to death. Arizona still permits stacking as the default. The statute does not compel a single recovery, and it does not prohibit one either. What it does is leave room for the contract to speak. When the same insureds jointly buy several policies from one company and that company writes an unambiguous provision barring the aggregation of UIM limits across those policies, the anti-stacking clause controls. The joint-purchaser framing matters because it forecloses the workaround of treating each family member as a separate insured with an independent entitlement.
The default still favors stacking. What defeated it here was the combination of one carrier, jointly purchased policies, and language clear enough to survive the ambiguity-against-the-drafter rule.
How this changes case-building
Treat Balzan as a checklist trigger, not a coverage obituary. The decision narrows a specific path; it does not close first-party recovery. Work the file in this order.
Read every household policy's stacking and single-insured language
Pull all of them, not just the policy on the involved vehicle. You are hunting for two things: the anti-stacking clause and the definition of "insured." Weak, buried, or internally contradictory anti-stacking language is still your friend, because Arizona construes ambiguity against the carrier. A clause that merely references "other insurance" is not the same as one that plainly forbids aggregating UIM limits across separately numbered policies. Quote the exact words in your demand and make the carrier defend them.
Distinguish separately purchased policies
The holding is built on joint purchase from a single insurer. That is the seam. If a policy was bought independently, by a different named insured, or from a different carrier, the single-insured framing is far harder to apply. Map who purchased what, when, from whom, and with whose premium dollars. A policy the injured party's employer, roommate, or separately residing relative bought on different paper is not obviously swept into the same single-insured bucket.
Watch the reducing and offset clauses
Even where stacking survives, reducing clauses and offsets can quietly gut the number. Confirm whether UIM limits are reduced by the liability payment already received and whether the carrier is crediting other benefits against the UIM exposure. Sequence your liability settlement and your UIM claim with those clauses in mind, and preserve consent-to-settle obligations so you do not forfeit the UIM claim while chasing the tortfeasor.
Structure the UIM demand accordingly
If the anti-stacking language is airtight and the policies were jointly bought from one insurer, do not anchor your demand to a stacked figure you cannot support; it costs credibility. Build the demand on the single available limit, then press damages hard. Where the clause is shaky or the policies are genuinely separate, plead the stacking theory explicitly and put the burden on the carrier to show its language is clear enough to meet Balzan.
The multi-state caution
Do not export Arizona's answer to another jurisdiction on autopilot. Stacking rules diverge sharply by state. Some jurisdictions mandate stacking and will not let a carrier contract out of it. Others follow Arizona's model and permit a clear written waiver or anti-stacking clause. Still others require specific statutory rejection forms before a limitation counts. If your client's policies, residence, or the crash cross state lines, the choice-of-law question can decide the coverage question before you ever reach the clause. Confirm which state's law governs each policy before you value the claim.
The larger lesson from Balzan is procedural discipline. UIM value in a same-carrier household now turns on document work you do at intake: gathering every declarations page, reading the anti-stacking and single-insured provisions word for word, and separating jointly purchased policies from independently purchased ones. For more coverage-driven decisions worth folding into your intake process, keep an eye on developments in case law and settlements, coordinate the first-party analysis with the underlying auto accident workup, and reconcile any recovery against your liens and settlement ledger before you sign off on numbers.