Floodplain Management and Construction in a Floodplain

Construction in a Floodplain

A permit is required prior to any construction conducted in a floodplain designated by Federal Emergency Management Agency (FEMA) as “AO, AE or Floodway” (defined below), and subject to specific development restrictions and conditions.  Flood insurance is also required if the property is mortgaged from federally regulated or insured lenders. 

Understanding the scope of work associated with your proposed renovation project in AO, AE and Floodway zones is important as most construction requires the City’s review and approval prior to starting work. Property owners are required to fill out and submit a Substantial Improvement Worksheet (PDF)  to the City to determine if the proposed renovations are considered “substantial”.   The Building Official will review the scope of work and scale of improvements, which include any renovations made to a property within a 2-year period. Proposed renovations that meet the “substantial improvement” threshold may require additional planning approval and/or larger scale improvements (such as raising the structure above the base flood elevation) to comply with FEMA regulations.  

For more information: see Chapter 18 of the Mill Valley Municipal Code and the Floodplain Management Handout (PDF) for details.
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View Parcels and the FEMA Floodplain/Special Flood Hazard Areas

To determine if your property is in a floodplain and view base flood elevations, visit Marin County’s online mapping website at MarinMap. Under GIS Map Applications, click on “FEMA Flood Information.” Under FEMA LOMC: Letter of Map Change, click on either “Desktop webmap” or “Mobile webmap.” To locate a specific parcel, enter the address in the box. Once the Map populates, click the tab under “Flood Hazard Zone” to highlight the different FEMA information, including base flood elevations and floodplain designations. 

For more detailed FEMA maps and descriptions, go to FEMAs Guide to Flood Zones and Maps.
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FEMA Floodplain Designations with the City 

In Mill Valley, there are the following Special Flood Hazard Area (or “floodplain”) designations:

    Floodway. Considered the channel of watercourse. FEMA requires that the creek channel and the adjacent land areas identified within a Floodway be reserved to carry water during a flood. Any change to a building footprint and/or improvement to a site in the Floodway must illustrate and ensure that no net increases in flood elevations will occur. Mandatory flood insurance purchase requirements apply to this area. Structures located within a Floodway have additional development restrictions imposed by FEMA. Special regulations and limitations may apply to construction, improvements, renovations and/or possible expansion of building envelopes within  a Floodway. See Chapter 18 Municipal Code for details.

    AE Flood Zone. A flood insurance rate zone that corresponds to the 1% annual chance floodplain, as determined by a FEMA Flood Insurance Studies. Mandatory flood insurance purchase requirements apply and the property is subject to special development regulations and restrictions (see Chapter 18  of Municipal Code and Floodplain Management Handout (PDF)). Typically Base Flood Elevations are available for the AE Flood Zone and can be determined through a survey.

    AO Flood Zone. A flood insurance rate zone that corresponds to the 1% annual chance shallow floodplain, as determined by FEMA Flood Insurance Studies. Mandatory flood insurance purchase requirements apply and the property and special development regulations and restrictions (see Chapter 18 Municipal Code and Floodplain Management Handout (PDF)). Base Flood Elevations are not typically identified for the AO Flood Zone.

    X Flood Zone. Corresponds to the areas that are outside the 1% annual chance floodplain, as determined by FEMA Flood Insurance Studies. Flood insurance purchase requirements are optional. Construction/Improvements made in this Flood Zone are not subject to the Floodplain Management regulations.
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FEMA Websites

    Flood insurance information 

    Flood Hazard Mapping Program

Updating or Amending FEMA Maps or Floodplain Designations