ADU insurance in California: what your policy must cover

A standard California homeowners policy typically caps coverage for detached structures at a percentage of the dwelling coverage limit — often far less than the replacement cost of a newly built ADU. If you don't notify your carrier and add explicit coverage before construction starts, you may be uninsured for the most expensive structure on the property. This is one of the cheapest mistakes to fix and one of the most expensive to ignore.

Three coverages you must verify

• —Dwelling extension or 'other structures' increased to the full replacement cost of your ADU — get this figure from your contractor's estimate and confirm the number with your carrier, not a rule of thumb.
• —Course-of-construction (builder's risk) coverage from groundbreak through CofO — usually written separately for 12 months.
• —Liability extension covering the rental use if the ADU will be tenanted, including loss-of-use coverage if a covered event makes it uninhabitable.

Rental occupancy: a different conversation

Once tenanted, most carriers re-rate the property. How much the premium rises depends on the insurer's underwriting inputs — construction type, protection class, occupancy, and whether the rental is long-term or short-term. Ask your agent for a landlord-policy rider quote and compare it against upsizing the homeowners limit; a rider is often the cheaper path.

Earthquake and fire-zone considerations

California Earthquake Authority (CEA) policies must be amended to include the ADU. In WUI (wildland-urban interface) zones, fire-hardening details (Class-A roof, ember-resistant vents, 5 ft non-combustible perimeter) can reduce premiums — the exact discount depends on the insurer's own underwriting guidelines. Get the credit by submitting the certificate of compliance from your installer and ask your carrier what its specific fire-hardening credit is.