Uninsured and underinsured motorist coverage in Massachusetts explained.

Part 3 vs Part 12: uninsured and underinsured motorist coverage in Massachusetts

Massachusetts requires uninsured motorist coverage. It does not require underinsured motorist coverage. Both live in the standard Massachusetts Automobile Insurance Policy, in different parts.

Part 3, Bodily Injury Caused by Uninsured Auto, is compulsory. It protects you, anyone you let drive your car, and household members and passengers who are not covered under another similar Massachusetts policy, against losses caused by an uninsured or unidentified hit-and-run driver. Part 12, underinsured motorist coverage, is optional. It comes into play when the at-fault driver has insurance but not enough to cover your losses.

Both start at the same basic limit: 25/50, meaning $25,000 per person and $50,000 per accident. Both are capped at your bodily injury liability limits. The difference is the trigger. Part 3 answers when there is no coverage on the other side. Part 12 answers when there is coverage, but it runs out before your damages do. For a fuller map of how these fit into the whole policy, see our guide on how car insurance works in Massachusetts.

Why Part 3 is compulsory and what it must cover

Uninsured motorist coverage is one of the four compulsory coverages you must buy to register a car in Massachusetts. The statute behind it is M.G.L. c.175 §113L(1): no policy may be issued or delivered for a vehicle registered here unless it provides uninsured motorist coverage. That coverage must include hit-and-run vehicles and cases where the at-fault vehicle’s liability insurer is insolvent. If the other driver carries no insurance, flees the scene, or has a carrier that goes under, Part 3 responds.

CAR Rule 35 sets the mechanics. Basic limits are $25,000 per person and $50,000 per accident. The coverage is excess over Personal Injury Protection, which means PIP pays first up to its limit, then Part 3 picks up. The limits may not exceed your bodily injury liability limits. PIP still handles the first layer of medical and wage-loss costs; for the detail on that, see our guide on PIP in Massachusetts.

How underinsured (Part 12) actually pays: the offset

Underinsured motorist coverage is where drivers get surprised. It does not hand you your full limit. Under M.G.L. c.175 §113L(2), Part 12 applies when the at-fault vehicle’s bodily injury limit is less than your underinsured limit and insufficient to satisfy your damages. Recovery is limited to the extent your coverage exceeds the at-fault driver’s bodily injury liability limits.

That is the offset. Whatever you collect from the at-fault party’s bodily injury coverage is deducted from your underinsured coverage. Here is the worked example from the Division of Insurance: with a $50,000 per-person underinsured limit, if you collect $25,000 from the at-fault insurer, the most you could then collect under Part 12 is $25,000. The two numbers together, not your limit alone, define what is available.

The practical lesson is that a low underinsured limit matching typical at-fault limits gives you almost nothing. If your limit and the other driver’s limit are the same, the offset can wipe out your recovery entirely. Buying a meaningfully higher limit is what creates real room above the offset.

How much you can buy and the mandatory offer

Insurers and agents must offer you the option to buy Bodily Injury Caused by Uninsured Auto at 25/50. That is a floor, not a ceiling. Under M.G.L. c.175 §113H(6), uninsured limits are available up to your bodily injury liability limits.

Raising those limits usually means buying optional Part 5 bodily injury coverage, which can lift your liability to $40,000 for one person and $90,000 for all people hurt in an accident, or 40/90. Since uninsured limits cannot exceed Part 5, your uninsured protection rises with it. For underinsured coverage, the CAR mandatory-offer schedule goes further: Protection Against Underinsured Motorists is offered at limits up to $500,000 each person and $500,000 each accident, again capped at your bodily injury limits. The Division of Insurance premium-comparison examples use a profile carrying 100/300 for bodily injury, uninsured, and underinsured coverage together, a common step up from the basic limits. To see where Part 12 sits among the other elective parts, read our guide to optional Massachusetts auto coverages.

The July 1, 2025 minimum increase: 20/40/5 is gone

If you are working from an older quote or an old assumption, correct it. As of July 1, 2025, the mandatory minimum amounts increased. For Parts 1, 3, 5, and 12, the basic limits moved from 20/40 to 25/50, and property damage under Part 4 rose from $5,000 to $30,000. The compulsory floor is now 25/50/30, up from the old 20/40/5.

The change traces to House Bill 5111, signed December 23, 2024, and to amendments of M.G.L. c.90 §34A that apply to policies issued or renewed on or after July 1, 2025. PIP benefits remain up to $8,000 per person. Any coverage figure from before that date is obsolete. When a policy renews after July 1, 2025, the new floor applies automatically, but the floor is only a starting point.

Statutory rules that shape a UM/UIM claim

Three statutory rules decide how much you can actually recover. First, anti-stacking. Under §113L(5), regardless of the number of vehicles, policies, or premiums, limits from two or more vehicles or policies may not be added together, combined, or stacked. Owning three cars does not triple your uninsured protection.

Second, the own-vehicle exclusion. If you are injured while occupying your own highway-registered vehicle that does not carry the coverage required by c.90 §34A, you cannot recover uninsured motorist coverage from any policy. Registering and insuring a car correctly is a precondition to this protection working at all, which matters especially for anyone new to the state; see our guide on insuring a car after moving to Massachusetts.

Third, rideshare. Under §228, transportation network drivers must maintain c.90 §34A and §113L coverage during periods the vehicle is operated but not providing network services. Uninsured coverage is required to the extent §113L requires it.

How to set your limits and what to do next

Start with your bodily injury liability limit, because it caps everything above. Uninsured and underinsured limits cannot exceed it, so raising liability is what unlocks higher UM and UIM. Then set underinsured coverage high enough that the offset still leaves you real protection; matching your limit to the average driver’s minimum defeats the purpose. If you carry an umbrella policy, your underlying auto limits usually need to meet the umbrella’s threshold, so these decisions connect; our guide on umbrella insurance in Massachusetts covers that link.

We shop these limits across our carriers and place the coverage that fits how you drive and what you have to protect. If you want us to price uninsured and underinsured options against your current policy, start with our personal auto page.