Civil/Structural Engineering, Vashon, Washington, USA 206-463-5311

Building in a Floodplain

A wide net was cast in drawing the floodplain maps; these maps include many homes which rightly should not be included in the floodplain.  The BFE lines were drawn by computers lacking the accuracy to show the actual elevations of individual homes.  This will have short-term or long-term effects on any property owner who wants to remodel an existing building, or build a new building, within the floodplain.  New regulations include:


  • The lowest floor must be elevated a minimum of 3 above the 100-year floodplain.
  • New buildings must be elevated on pilings and columns.
  • The foundation must be anchored to prevent floatation, collapse and lateral movement.  There are also other special new foundation design requirements.  An engineer must prepare the design.
  • A FEMA elevation certificate prepared by an engineer, architect, or surveyor must document the floor elevation.


For existing homes with the first floor within the floodplain or the 3 buffer, this floor is no longer allowed to be habitable space.  It can be used for storage, access, or parking cars.  This may affect daylight basement homes in particular.  For existing homes, options are to move the home to higher ground, raise the home, or vacate the lowest story.  All may be difficult or expensive.  The first step, of course, is to determine your floor elevation relative to the BFE plus 3 buffer.