ADU and Garage Conversion Insurance Coverage Gaps in California

California has spent the last decade aggressively encouraging homeowners to build accessory dwelling units. The legislature has passed bill after bill — AB 68, AB 881, SB 13, AB 1033, and more — streamlining permitting, reducing fees, eliminating parking requirements, and even allowing ADUs to be sold separately from the primary residence. Local governments that once blocked ADU construction have been forced to approve them. The result is tens of thousands of new livable units across the state.

What the legislature has notdone is address how these units interact with homeowner’s insurance. The standard HO-3 policy — the form that covers the vast majority of California homeowners — was designed for a single-family home with a detached garage, maybe a shed. It was not designed for a property with a second livable dwelling that has its own kitchen, bathroom, electrical panel, and tenant. The coverage gaps are real, they are significant, and most homeowners do not discover them until after a loss.

How the Standard HO-3 Treats ADUs

Under the standard ISO HO-3 policy form, coverage is divided into several categories. The two that matter most for ADU owners are Coverage A (Dwelling) and Coverage B (Other Structures).

Coverage A covers the dwelling on the “residence premises” — the main house. Coverage B covers “other structures on the ‘residence premises’ set apart from the dwelling by clear space or connected to the dwelling by only a fence, utility line, or similar connection.” By default, Coverage B is set at 10% of the Coverage A limit.

That 10% figure was designed to cover a detached garage, a garden shed, or a fence — simple structures with no plumbing, no HVAC, and no kitchen. A detached ADU is none of those things. It is a fully habitable dwelling unit. But under the standard HO-3, it is still classified as an “other structure” and subject to that 10% limit.

Attached ADUs vs. Detached ADUs: Different Coverage Treatment

The distinction between attached and detached ADUs matters enormously for insurance purposes. The HO-3 policy draws a clear line between Coverage A and Coverage B based on whether a structure is “set apart from the dwelling by clear space.”

Attached ADUs

An ADU that shares a wall with the main house — such as a converted portion of the garage, a basement conversion, or an addition built onto the side of the house — is generally considered part of the dwelling under Coverage A. This is typically better from a coverage standpoint, because Coverage A limits are much higher than Coverage B. However, the homeowner needs to ensure that Coverage A is high enough to account for the added square footage, finishes, and systems. If you converted your garage into a 500-square-foot apartment and never updated your Coverage A limit, the dwelling is now underinsured.

Detached ADUs

A freestanding ADU in the backyard — the most common type built under California’s recent legislation — falls under Coverage B. This is where the 10% limit becomes a serious problem. A detached ADU that cost $200,000 to build will not be adequately covered by a $50,000 Coverage B limit. The homeowner must either increase the Coverage B limit (if the carrier allows it) or schedule the ADU as a separate structure with its own coverage amount.

Some carriers will increase Coverage B by endorsement. Others will not, especially if the ADU is rented to a third party, because that changes the risk profile of the property from a single-family owner-occupied residence to something more closely resembling a landlord-tenant arrangement.

Garage Conversions: The Permit Problem

Garage conversions are one of the most common ADU types in California, and they raise a question that makes both homeowners and insurance adjusters uncomfortable: was the conversion done with proper permits?

If the garage was converted to a living space with full permits — meaning the electrical, plumbing, framing, and HVAC work were all inspected and approved by the local building department — the insurance treatment is relatively straightforward. The converted space is part of the dwelling (if attached) and should be reflected in Coverage A. The homeowner needs to make sure the policy limit accounts for the higher replacement cost of a habitable space versus an unfinished garage.

If the conversion was done withoutpermits — and in California, a very large number of garage conversions fall into this category — the coverage picture becomes more complicated. Many homeowners assume that an unpermitted conversion means no coverage. That is not necessarily true, but it is not simple either.

California Insurance Code §533 and the Illegality Defense

California Insurance Code §533 states that “an insurer is not liable for a loss caused by the willful act of the insured.” Some carriers have tried to use this provision to deny claims on unpermitted structures, arguing that building without a permit is an illegal act and therefore excluded.

This argument is much weaker than carriers sometimes suggest. The illegality defense under §533 is narrow. California courts have generally held that §533 applies to intentional, willful acts that causethe loss — not to building code violations that merely exist at the time of loss. Building a garage conversion without a permit is a code violation, but the lack of a permit does not cause the fire, the water damage, or the windstorm that destroys the structure. The cause of loss and the permit status are separate issues. That said, unpermitted work can still create coverage complications, particularly around code upgrade coverage and rebuild requirements.

Coverage Gaps: What the Policy Was Not Designed For

The fundamental problem with insuring an ADU under a standard HO-3 is that the policy was written for a single-family home. ADUs introduce components and exposures that the standard form does not adequately address:

• Separate kitchen and bathroom. A detached ADU with a full kitchen and bathroom has plumbing, gas lines, and appliances that significantly increase both the replacement cost and the risk of water or fire damage. Coverage B was not designed to account for this.
• Independent HVAC system.Many ADUs have their own heating and cooling systems — mini-splits, wall heaters, or separate ducted systems. This adds to the replacement cost and introduces additional mechanical components that can fail or cause damage.
• Separate electrical panel. A properly built ADU has its own electrical sub-panel or even a separate service. The cost to replace electrical systems in a habitable unit is substantially higher than the cost to replace the wiring in a simple garage.
• Finished interior.Drywall, insulation, flooring, interior doors, trim, paint, lighting fixtures — these are costs that do not exist in an unfinished detached garage but are significant in an ADU.
• ADA/accessibility features. Some ADUs are built with accessibility features for elderly relatives or tenants with disabilities. These features add cost and may require specialized replacement.

ALE and Fair Rental Value for ADUs

When a loss makes the main dwelling uninhabitable, the HO-3 policy provides Additional Living Expense (ALE) coverage under Coverage D. ALE pays the difference between the insured’s normal living expenses and the increased cost of maintaining their standard of living while displaced.

But what about the ADU? If the ADU is rented, the relevant coverage is Fair Rental Value (FRV), which reimburses the landlord for lost rental income while the rental unit is uninhabitable. Here is where ADU owners often discover a gap: the standard HO-3 may provide FRV coverage for a portion of the dwelling that is rented — for example, a room within the main house — but the coverage for a detached structure rented to a third party may be limited or nonexistent without a specific endorsement.

If your ADU generates $2,000 per month in rental income and it takes 12 months to rebuild after a fire, you are looking at $24,000 in lost rental income. If your policy does not include FRV coverage for the ADU — or if the FRV limit is inadequate — that $24,000 comes out of your pocket.

Tenant’s Personal Property: Not Your Problem, but Your Headache

If someone is living in your ADU — whether a family member or a paying tenant — their personal belongings are notcovered by your homeowner’s policy. The HO-3 covers the named insured’s personal property under Coverage C, not the property of tenants.

The tenant needs their own renter’s insurance policy (HO-4) to cover their personal belongings and their own liability. This is true regardless of whether the ADU is permitted or unpermitted, attached or detached, rented at market rate or occupied by a relative for free.

As a practical matter, if a fire destroys your ADU and the tenant loses everything they own because they had no renter’s insurance, it may technically not be your financial problem — but it will certainly be your emotional and relational problem. Requiring tenants to carry renter’s insurance should be a non-negotiable term of any ADU lease.

Total Loss and the Ordinance or Law Problem

When a total loss destroys the main dwelling and the ADU, the rebuild process triggers a set of complications that partial losses do not. The homeowner must rebuild in compliance with currentbuilding codes and zoning ordinances — not the codes that were in effect when the original structures were built.

For ADUs, this can actually be beneficial in some cases. California’s recent ADU legislation has made the rules morepermissive. Current ordinances may allow a larger ADU than what was originally built, may permit ADUs in locations that were previously restricted, or may waive setback requirements that the original ADU had to comply with. However, the increased cost of building to current code — energy efficiency requirements, seismic standards, fire-resistant materials — will almost certainly exceed what the original ADU cost to build.

This is where ordinance or law coverage becomes critical. Standard HO-3 policies typically exclude the increased cost of rebuilding to comply with current codes. You need a separate endorsement — commonly called “Ordinance or Law” coverage — to cover this gap. For properties with ADUs, this endorsement is not optional.

Unpermitted ADUs and California’s Amnesty Programs

California has recognized that a large number of existing ADUs were built without permits. Several jurisdictions have created amnesty or legalization programs that allow homeowners to bring unpermitted ADUs into compliance, often with reduced fees and relaxed code requirements. SB 897 (2022) and AB 1033 (2023) further streamlined the process.

How does permit legalization interact with insurance? In several important ways:

• Disclosure to the carrier. If you legalize an unpermitted ADU, your carrier needs to know. The property now has a permitted, habitable secondary dwelling. Failing to disclose this change can be grounds for rescission or denial of a future claim based on material misrepresentation.
• Coverage adjustment. A legalized ADU should be reflected in your Coverage B limit (if detached) or Coverage A limit (if attached). The whole point of legalization is that the unit is now an officially recognized habitable structure, and the insurance should match.
• Rebuild after loss. If a legalized ADU is destroyed, the homeowner can rebuild it with a standard building permit. If an unpermitted ADU is destroyed, the homeowner may face obstacles: the building department may not allow reconstruction of a structure it never approved in the first place, or may require the new construction to meet standards the original never did. This can create a situation where the replacement cost far exceeds what the carrier has agreed to pay.

What Happens When the Carrier Discovers the ADU After a Loss

Many homeowners build ADUs without notifying their insurance carrier. The carrier’s adjuster discovers the ADU during the claims inspection, and the carrier argues the homeowner failed to disclose a material change in the risk. Under California Insurance Code §338, a policy can be rescinded for concealment or misrepresentation of a material fact. Carriers regularly take the position that an unreported habitable structure — especially one being rented — is material. Even without rescission, the carrier may deny coverage for the ADU itself, limit the claim to the inadequate Coverage B amount, or apply a coinsurance penalty.

Practical Steps to Close the Coverage Gap

1. Notify your carrier.Before building an ADU — or as soon as possible after one is completed — notify your insurance carrier in writing. Tell them the type of ADU (attached or detached), the square footage, whether it will be rented, and the estimated replacement cost.
2. Increase Coverage B. If you have a detached ADU, the default 10% Coverage B limit is almost certainly inadequate. Ask your carrier to increase it to reflect the actual replacement cost of the ADU. Some carriers will do this by endorsement; others may require a separate policy.
3. Add Fair Rental Value coverage. If the ADU is rented, make sure your policy includes FRV coverage for the ADU specifically. Get the limit in writing and make sure it reflects at least 12 months of rental income.
4. Add Ordinance or Law coverage. This endorsement covers the increased cost of rebuilding to current codes. It is important for any property, but essential for properties with ADUs, where the rebuild may need to comply with current energy, seismic, and accessibility standards.
5. Require tenant renter’s insurance.Include a lease provision requiring the ADU tenant to maintain an HO-4 renter’s policy with at least $100,000 in liability coverage and adequate personal property limits. Ask to be named as an “interested party” on the tenant’s policy so you receive notice if they cancel.
6. Consider a separate policy. Some carriers will not adequately insure a detached ADU under the main HO-3 policy. In those cases, a separate dwelling fire policy (DP-3) for the ADU may be the better solution. This is more common when the ADU is rented at market rate and functions as a true rental unit.
7. Get the ADU permitted.If you have an unpermitted ADU, take advantage of California’s legalization programs. A permitted ADU is easier to insure, easier to rebuild after a loss, and eliminates the risk that the carrier raises a permit defense when you file a claim.

The Bottom Line

California wants you to build ADUs. The state has made it easier, faster, and cheaper to add a second dwelling unit to your property. But the insurance industry has not kept pace with the legislation. Standard HO-3 policies treat a $200,000 detached ADU with a kitchen, bathroom, and tenant the same way they treat a garden shed — as a “Coverage B other structure” subject to a limit that was never designed for a habitable dwelling.

If you own an ADU or are planning to build one, treat the insurance conversation as seriously as the construction. Talk to your carrier before the project starts, get coverage confirmations in writing, and do not assume the default policy limits are adequate. The gap between what your policy covers and what it would actually cost to rebuild your ADU after a loss can be enormous — and you will not find out about that gap at a convenient time.