Skip to page body Home About Us What's New City Government City Services Community Visitors Business How Do I...

Floodplain Management

Corte Madera Creek

A permit is required prior to any construction conducted in a floodplain designated by Federal Emergency Management Agency (FEMA) as “AO, AE or Floodway” (see descriptions below). Properties located within these designated “Special Flood Hazard Areas" (AO, AE and/or Floodway) with mortgages from federally regulated or insured lenders are required to have flood insurance.  These areas are also subject to development restrictions and specific regulations related to construction (See Chapter 18 of the Mill Valley Municipal Code, entitled “Floodplain Management”).

The City of Mill Valley has been a part of the National Flood Insurance Program (NFIP) and has had floodplain regulations (Chapter 18 Municipal Code) in effect since 1979. The City’s participation with FEMA in regulating development in the floodplain is required by law, and can affect the availability of, and rates for, flood insurance.

FEMA Floodplain Designations with the City.
In Mill Valley, there are the following Special Flood Hazard Areas 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 in a Floodway.
  • 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 Municipal Code and substantial improvement discussion below). 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. Mandatory flood insurance purchase requirements apply and the property is subject to special development regulations and restrictions (see Chapter 18 Municipal Code and substantial improvement discussion below). 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.

Is my property in a FEMA Floodplain/Special Flood Hazard Area? Residents can view the Special Flood Hazard Areas and proposed base flood elevation by viewing our local map application page, MarinMap. Zoom to your specific parcel by selecting orange "I Want To" box and search by address or parcel number. Then select "show layer list" to highlight the different FEMA information, including base flood elevations and floodplain designations.

FEMA Flood Map Updates. Periodically, FEMA updates the Flood Insurance Rate Maps (FIRM) based on new hydrological studies and information. The City of Mill Valley is impacted by two FEMA studies due to its topography of hillsides, creeks, and proximity to Richardson Bay. These two studies are the Riverine Flood Study and the San Francisco Coastal “Bay Study”. In 2014, FEMA maps were updated based on a Riverine Flood Study which established a regulatory “floodway” within the City (see definition below). In March 2016, FEMA Flood Maps will be updated once again based on the Bay Study. The 2016 Map update does not make many substantial changes or modifications to the Special Hazard Areas, but does establish additional based flood elevations throughout the FIRMs.

Will my Floodplain Designation change with the new March 2016 Map Update? Viewour local map application page, MarinMap. Zoom to your specific parcel by selecting orange "I Want To" box and search by address or parcel number. Then select "show layer list" to highlight the different FEMA information, including 2014 FEMA Map and 2016 FEMA MAP.

Construction and/or Improvements in the Floodplain. Most construction designated within the FEMA AO, AE and/or Floodway Zone requires review and approval by the City of Mill Valley. Understanding the scope of work associated with your proposed renovation project is important in these FEMA designated areas. Smaller projects may qualify and be permitted based on a review of the scope of work, ensuring that the scale of improvements made to a property within a 2 year period are under the “substantial improvements” threshold (see details below). Larger projects may require additional planning approvals and/or may require larger scale improvements (such as raising the house) to comply with the City’s Floodplain Management Regulations (Chapter 18 Municipal Code).

  • A substantial improvement is defined in the Municipal Code as, “any repair, remodel, rehabilitation, addition, or improvement the cost of which equals or exceeds 50% of the “depreciated market” value of the existing structure either, before the improvement is started, or if the structure has been damaged and is being restored, before the damage occurred.”
  • A Substantial Improvement worksheet must be accompanied by a current appraisal for the structure (from within one year), the Cost Evaluation worksheet, an itemized breakdown of the valuation of the project. The Substantial Improvement worksheet must be signed by both the property owner and the appraiser certifying that the information provided is accurate. The Planning and Building Department will then review the forms to ensure they have been filled out correctly and determine that the project is under substantial improvement threshold.
  • Over the counter permit applications to the Building Department will not have to complete the worksheet, unless it is the second over the counter building permit that has been submitted within a two year time period. Projects on structures within the floodplain are cumulative within two years.
  • See below for more information on the substantial improvement process.

What if my parcel is in a Special Hazard Area Floodplain, but the structure(s) are not or my structure is raised?

FEMA allows for certain properties that have the structure(s) out of the floodplain and/or the structures are raised above the base flood elevation to apply for a Letter of Map Amendment. This process allows property owners to opt out of Flood Insurance Requirements.

FEMA Websites: Bay Coastal Study; general flood insurance information; National Flood Insurance Program; Flood Hazard Mapping Program; Letter of Map Amendment Application Process

Additional Information

Base Flood Elevation. Base Flood Elevation is the elevation shown on Flood Insurance Rate Maps for the AE Flood Zone, and indicates the water surface elevation resulting from a flood that has the 1% or greater chance of being equaled or exceeded in a given year. Base flood elevations can be view and identified using www.MarinMap.org (explained above) or by contacting the Department of Public Works.

Letter of Map Change (LOMC). If a property owner has evidence that their property has been inadvertently mapped in a Special Flood Hazard Area (SFHA), or flood hazard data needs to be revised due to man-made changes, a request to FEMA for a Letter of Map Change (LOMC) may be submitted. Click here to download a flyer with more information.

Letter of Map Amendment (LOMA). If your structure is above the Base Flood Elevation, it may be possible to apply to FEMA for a Letter of Map Amendment. This process allows property owners to opt out of Flood Insurance requirements. Letter of Map Amendment Application Process.

Flood Insurance & Elevation Certificates. Homes and buildings in high-risk flood areas with mortgages from federally regulated or insured lenders are required to have flood insurance. If you are required to carry flood insurance and need to find a provider, FEMA operates a agent locator service that can be accessed online or by calling 1-888-379-9531.

As part of securing flood insurance, some property owners may be required to get an Elevation Certificate. An Elevation Certificate verifies the elevation data of a structure on a given property relative to the ground level. The Elevation Certificate is used by insurance agents and companies in the rating of flood insurance policies. If you need to have an elevation certificate completed, you will need to contact a licensed land surveyor.

FEMA Flood Maps. FEMA Flood maps identify the boundaries of the 100-year floodplain and divide the floodplain into various “zones” based on the estimated depth of flood waters. A 100-year flood is the statistical probability, based on the review of historic rainfall and flood data, that there is a one percent chance in any given year that a flood of this magnitude will occur.

Floodway. A regulatory floodway means the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation by more than one foot. Structures located within Regulatory Floodways have additional development restriction beyond those imposed by being located within a floodplain. Note: All properties within the regulatory floodway are also in a floodplain and must follow the requirements of both designations.

Construction in a Floodplain and Substantial Improvement Worksheet

What is the definition of a substantial improvement? Substantial improvement means any repair, remodel, rehabilitation, addition, or improvement, the cost of which equals or exceeds 50 percent of the "depreciated market" value of the existing structure either, before the improvement is started, or if the structure has been damaged and is being restored, before the damage occurred.

What is the definition of the depreciated market value of a structure? Depreciated value means the value of the structure only and does not include the land or any other accessory buildings or site improvements. It is the replacement cost of the existing structure less any depreciation that results from age or condition. For the purposes of the City’s floodplain management regulations the “market value” of a structure shall mean the same as “depreciated market value”.

How do I determine the valuation of my project? To establish the value of any proposed improvements, a contractor shall submit an itemized breakdown of estimated material/labor costs and the Cost Evaluation form (attached to this packet). The contractor shall use the attached Material Receipts/Estimates handout to help them determine the valuation of the proposed project. These submitted documents and the project valuation will then be reviewed and approved by the Building Official in comparison to market value rates within Marin County and the City of Mill Valley.

What if the scope of work increases after my building permit is pulled? The valuation of the project is cumulative. Therefore, if during construction changes are made to the permit; those valuations must be added to the total valuation submitted for the project. If during construction the project increases to over 50% of the depreciated market value of the structure, it must conform to the City’s current floodplain management regulations. Project valuations are cumulative for two years, meaning that all open permits for a structure are totaled together. A permit must be finaled for two years before a new project can begin to avoid it having a cumulative effect on valuation.

Do I need to fill out this form for a permit that is done over the counter? Substantial improvements are typically not triggered by over-the-counter types of permits, so the first over-the-counter permit submitted for a property will not require these documents. However, since the valuation is cumulative for two years, if you pull an additional over-the-counter permit within that time frame you will need to fill out the form and include the total valuation of all the permits pulled in that two year period of time.

What if my project is a Substantial Improvement? If the proposed repair, remodel, rehabilitation, addition, or improvement, equals or exceeds 50 percent of the depreciated market value of the existing structure it is considered a substantial improvement and you will be required to conform to Chapter 18 of the Mill Valley Municipal Code. Depending on the zoning and floodplain designation you may be required to flood proof the structure (commercial zones) or elevate the finished floor of the structure above the base flood elevation. If the proposed project is a substantial improvement, please refer to Chapter 18 of the Mill Valley Municipal Code, and discuss your scope of work with the Planning Department.

Where can I get more information?

Chapter 18.04 of the Mill Valley Municipal Code

Additionally, for information on the Floodplain, Regulatory Floodway, or the Map Updates for properties within the City of Mill Valley you can contact the Planning Department at (415) 388-4033.

Last updated: 3/10/2016 6:03:39 PM