The umbrella insurer prices its policy on the assumption that the underlying policies absorb the first part of every loss. So it requires the insured to buy, and keep, specific underlying limits.
Typical personal umbrella requirements, using one insurer's published rule as an example: auto bodily injury limits around $250,000 per person and $500,000 per accident (or $300,000/$300,000) with $100,000 property damage, and about $300,000 of homeowners liability. State minimum auto limits are too low for most umbrella underwriters. Each insurer sets its own requirements, so the umbrella's schedule controls.
If the insured lets an underlying limit drop, the umbrella still pays only above the required limit. Example: the umbrella requires $250,000 of auto bodily injury per person, but the insured carries $100,000. A covered $500,000 judgment is paid $100,000 by the auto policy and $250,000 by the umbrella, and the insured pays the $150,000 gap personally.
Two California rules show why the underlying layer matters:
For commercial umbrella and excess policies, §676.6(c) adds cancellation grounds after the policy has been in effect 60 days, including cancellation or nonrenewal of an underlying policy that is not replaced without lapse, a material change in an underlying policy, and a reduced financial rating of an underlying insurer. The notice must still comply with §677.2.
Under §11580.1(a), none of the mandatory auto provisions of subdivision (b) apply to a policy that requires underlying insurance equal to or greater than the Vehicle Code §16056(a) limits ($30,000/$60,000/$15,000 for policies issued or renewed on or after January 1, 2025). That is why an umbrella can set its own auto terms.