Message 1: Get a permit before you build.
Visit the Public Permit Counter Office at the Marin County Civic Center, Room 308. For online information, visit the Community Development Agency Planning Applications and Permits website.
Message 2: Know the substantial damage rules.
For substantially damaged guidelines see NFIP Substantial Improvement/Substantial Damage: Requirements and Definitions.
Message 3: Know the requirement for building in the floodplain.
Check out the Marin County Code 23.09 on Floodplain Management.