The Federal Emergency Management Agency (FEMA) defines and maps the floodway and the primary floodplain (also called the 100-year floodplain or 100-year flood hazard area) as part of their flood hazard area mapping and Flood Insurance Rate Map (FIRM) program. The County's floodway and floodplain mapping is derived from FEMA mapping and policies related to it are implemented through the following combining districts: F1 Floodway and F2 Floodplain. You can find out if your parcel is within an F1 Floodway or F2 Floodplain on the County's Zoning and Land Use Map
F1 Floodway Regulations. The F1 combining district applies to properties that lie within the floodway as shown on the most recent FEMA maps.
Undeveloped Property (no existing structures)
No new permanent structures are allowed in the F1 Floodway
Property with Existing Structures
No new permanent structures, including additions, are allowed in the F1 Floodway. However, the following exceptions may be allowed:
Flood elevation projects where the building's total floor area is unchanged, subject to Policy and Procedure 8-1-12. Construction Limitations Within the F-1 Floodway, Section C. Flood Elevation Projects, and Policy and Procedure 1-4-4. Flood Elevation Policy
Repairs and improvements that do not increase the existing total floor area of each floor of existing structures, subject to Policy and Procedure 1-4-4. Flood Elevation Policy, which may require that structures be elevated to at least one foot above the base flood level.
Note that decks may be repaired or replaced, but not increased in size or scope. (i.e., an unenclosed deck without a roof can be replaced, but cannot add a roof or be enclosed, which is considered new construction, not a repair)
Other improvements that do not result in expansion of building floor area, such as:
Septic system pre-treatment devices, cleanouts, and septic riser lids
Wells including casing, annular seal, well vault, pump, pump block, or discharge pipe
F2 Floodplain Regulations. This F2 combining district applies to properties that lie within the 100-year flood hazard area as shown on the most recent FEMA maps.
Any structural development within the F2 Floodplain must be constructed in accordance with Chapter 7B of the Sonoma County Building Regulations, which may require submission of topographic data, engineering studies, or other studies to determine the effects of flooding on a proposed structure.
See also:
What if I disagree with the FEMA mapping?
When FEMA remaps a flood hazard area, they will publish preliminary maps 1 – 2 years before the official release. About 6 – 9 months after release of the preliminary maps, FEMA will publish a public notice in the local paper which starts a 90-day appeal period. After maps have been finalized and published, they can no longer be appealed to FEMA. The maps for Sonoma County are being updated to reflect the final map for the FEMA Russian River Watershed published on July 31, 2024, and cannot be appealed.
Once maps have been published, the only way to revise the maps is with a Letter of Map Revision (LOMR). These typically require development and submission of technical hydrology data, hydraulic models, and studies to demonstrate that an area is not within the flood hazard area or is above the base flood elevation (BFE).
See FEMA's Flood Maps webpage for more information.