UIM Stacking State Rules 2026: Underinsured Motorist Coverage Stack-on by State

NOT LEGAL ADVICE. UIM and stacking rules are highly state-specific and policy-specific. Read your policy and consult a licensed attorney for case-specific guidance.

Underinsured Motorist (UIM) coverage protects you when an at-fault driver has insurance, but their policy limit is insufficient to cover your damages. "Stacking" combines multiple UIM policies or coverage layers to increase available compensation. State rules on stacking vary widely.

1. UIM basics

UIM coverage applies when:

Another driver is at fault
That driver has insurance, but limits are below your damages
You have UIM coverage on your own policy

UIM is separate from UM (uninsured motorist), which applies when at-fault driver has no insurance. Many states allow combined UM/UIM coverage.

2. Two types of stacking

Type	Description
Intra-policy stacking	Combining limits across multiple vehicles on the SAME policy. Example: 3 cars on one policy at $50K each = $150K stacked.
Inter-policy stacking	Combining limits across DIFFERENT policies (your policy + family member's policy + employer's policy etc.).

3. State allowing stacking (intra-policy)

States that generally permit intra-policy stacking (combining multiple vehicles on same policy):

Pennsylvania (default; can be waived in writing)
New Jersey (limited; subject to anti-stacking provisions)
Maryland (allowed in many circumstances)
Massachusetts
Connecticut
Vermont
West Virginia
Several others (always verify state-specific case law)

4. States with anti-stacking statutes

States with statutory anti-stacking provisions or strong case law against stacking:

Florida (statutory anti-stacking, but exceptions exist)
Texas
Wisconsin (post-2009 statute change)
Washington
Several others

5. Stack pricing impact

Policy structure	Premium impact (typical)
Stacked UIM ($100K × 3 cars = $300K)	+15-30% premium vs unstacked
Unstacked UIM ($100K total)	baseline
Waived UIM	-5-10% premium savings

6. UIM payment math example

Plaintiff has $200,000 in damages. At-fault driver has $50,000 BI policy. Plaintiff has $100,000 UIM stacked ×3 cars = $300,000 stacked UIM.

At-fault BI pays: $50,000
UIM coverage available: $300,000 (stacked) - $50,000 (offset for BI received) = $250,000
UIM owed: $200,000 - $50,000 = $150,000 (capped by remaining UIM)
Total recovery: $50,000 + $150,000 = $200,000 (full damages)

Without stacking ($100,000 UIM unstacked): Plaintiff would recover $50,000 + $50,000 (UIM after $50K offset) = $100,000, leaving $100,000 uncompensated.

7. Key gotchas

Setoff vs offset: Some states reduce UIM by at-fault BI received; others don't.
Household exclusions: Some policies exclude household members from collecting UIM on a vehicle they own.
Owned vehicle exclusion: If plaintiff was driving an owned but uninsured vehicle, UIM may not apply.
Resident relative coverage: May extend UIM to family members not named on policy, depending on state.
Selection/rejection forms: Many states require written waiver to reduce UIM below state minimum or BI policy limits.

Sources: State insurance code, NAIC model rules, state court interpretations, ABA insurance section reports.

Last reviewed by Mustafa Bilgic on 2026-05-08.