The prerequisite requirement for purchasing umbrella auto insurance is that you already have a standard auto insurance policy in good standing and active. An umbrella insurance plan is liability coverage that comes into play once the liability coverage of your base policy has been exhausted. It effectively adds an additional amount, in $1,000,000 increments, to the base policy. Also, the umbrella will extend liability limits of your existing home owner’s policy. Driving record and credit score could affect the cost of the umbrella, but because they are so infrequently purchased, most carriers sell the policies cheaply.