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Floodplain maps

Floodplain maps are one tool that communities can use to know which areas have the highest risk of flooding. The maps help people make decisions about where to live, what to build, and how to reduce flood risks. Property owners, insurance agents, and lenders can use flood maps to determine flood insurance requirements and policy costs.

King County conducts flood studies using the latest data and technology to produce more accurate floodplain maps. These maps are submitted to the Federal Emergency Management Agency (FEMA) for adoption.

Floodplain Map Information

King County iMap

The iMap application can produce custom aerial maps within King County using a variety of data layers that you can switch on and off. The most helpful floodplain layers include:

  • Parcel boundaries: the property lines of a plot of land.
  • Floodway: a federally regulated area including a river channel and nearby land that must remain open for flood overflow to protect the community around it from high water.
  • 100-year floodplain: areas of the floodplain that have a 1 in 100 chance of flooding each year.
  • 500-year floodplain: areas if the floodplain that have a 1 in 500 chance of flooding each year.
  • Area with reduced risk due to levee: an area that is protected from flooding by a levee (a wall of earth or stone).
  • Channel Migration Hazard Zones

Some additional, more detailed layers include:

Elevation Certificate

A form created by FEMA under the National Flood Insurance Program to help determine the cost of flood insurance within federally mapped floodplains. An elevation certificate will tell you a building’s location, lowest point of elevation, flood zone, and more. King County has elevation certificates for many properties within the Special Flood Hazard Area. Elevation Certificates may contain errors, omissions, or inaccuracies.

Letter of Map Change (LOMC)

Includes Letter of Map Amendment and Revisions. A letter which reflects an official change to an effective Flood Insurance Rate Map. LOMCs are issued based on a request of FEMA to revise or amend its flood map to remove a property or reflect changed flood conditions on the effective map.

Learn how to use iMap from this how-to page.

FEMA Flood Map Service Center

Visit the FEMA Flood Map Service Center to find all flood hazard mapping products created under the National Flood Insurance Program (NFIP). 

Flood map information for realtors, lenders and insurance agents

Find county definitions and rules on development standards, restrictions, and requirements as well as permitted uses in King County floodplains and other sensitive areas in the Critical Areas chapter of King County Code. Codes about floodplains can be found under sections 21A.24.230 to 21A.24.275.

Flood Insurance Requirements

The Flood Disaster Protection Act of 1973 and the National Flood Insurance Reform Act of 1994 require flood insurance for federally backed mortgages on buildings located in the 100-year floodplain.

To find out if a property is located in the FEMA mapped 100-year floodplain, enter an address or property tax lot number into King County’s Interactive Mapping Tool, iMap, and select the “Flooding info” and “FEMA 100 year floodplain” layers. If an address or tax lot number is not available, simply zoom in on a specific area or parcel.

Development in Floodplains

All development in a mapped floodplain needs a permit, and buildings must follow King County Code. King County manages development in floodplains as part of our participation in the National Flood Insurance Program. This ensures that residents in unincorporated King County can get flood insurance.

All development proposals in unincorporated King County must go through the Department of Local Services- Permitting [link] to apply for a floodplain development permit.

Floodplain development standards are part of the Critical Areas chapter of King County Code 21A.24. Some of the key requirements of the floodplain development standards are:

  • New or substantially improved homes be elevated to 3 feet above the 100-year flood elevation.
  • No development will occur in areas where the 100-year flood depths are more than 3 feet and where the speed of floodwaters are faster than 3 feet per second.
  • Development in the floodway will not cause a rise in 100-year flood elevations for any other property.
  • Any development that includes fill will provide compensatory storage to offset the loss in flood storage.
  • In coastal flood hazard zones, buildings are only allowed to be built on piers or posts. 

All buildings must have a FEMA Elevation Certificate completed by a licensed surveyor that shows compliance with the King County Code. 

Contact the King County River and Floodplain Management Section for help with:

  • Confirming if a property is within a Federal Emergency Management Agency (FEMA) mapped Special Flood Hazard Area (SFHA), or flood prone area identified by King County.
  • Finding flood insurance rating information such as FIRM zone, panel number, suffix, panel date and index date.
  • Finding FEMA elevation certificates, if available.
  • Determining what regulatory requirements may apply to a specific property.
  • Finding information on past flood damage to a property, if available. 

Call 206-477-4812 or email