Developing In A Floodplain

What Types of Development Are Most Often Affected by Floodplain Regulations?

Here is a quick list of structure-related floodplain development regulations that are most likely to affect you:

  • New primary structures – New primary structures, such as dwellings and commercial structures, are not allowed to be constructed in the floodplain unless no other option exists on the property.  When allowed within a floodplain, these structures must be sited on land that is the least susceptible to flood hazard and must be elevated so that the first habitable floor is at least 18 inches above the Base Flood Elevation.
  • Dwellings – New dwellings, substantially improved dwellings, and substantially damaged dwellings must have the first habitable floor elevated 18 inches above Base Flood Elevation.
  • Accessory structures – New accessory structures, substantially improved accessory structures, and substantially damaged accessory structures may also be required to have the first habitable floor elevated 18 inches above Base Flood Elevation.
  • Additions to, or replacement of, existing dwellings or other primary structures – The existing footprint of a primary structure, such as a dwelling, may be increased or shifted by a maximum of 10% when properly elevated on a foundation wall.  Larger expansions or shifts will require the additions (and possibly the entire structure) to be elevated 18 inches above Base Flood Elevation on posts, piers, or piles.
  • Electrical, mechanical, heating, ventilation, plumbing and comparable equipment – When providing service to a structure, installation and replacement of these types of equipment typically must be elevated 12 inches above Base Flood Elevation or must be water tight during flood conditions; some exceptions to these requirements are available for structures built prior to 1986.
  • Crawlspaces – Crawlspaces in a floodplain must be adequately vented to allow for automatic flow of flood water through the crawlspace during flood events.  Flood vents must be installed below the Base Flood Elevation and not more than 12 inches above the adjacent grade.  Flood vents may be screened but must remain uncovered at all times.

Remember:  A floodplain development permit is required for all development in the floodplain, even if the work does not require a regular building permit or land use approval.

Planning a Project in the Floodplain?

Remember to get a Floodplain Development Permit before you begin your project.  A Floodplain Development Permit is required for all development in the floodplain – even if the work does not require a regular building permit or land use approval.  All new development within the floodplain must comply with current floodplain development standards, which may include elevating a structure or the utilities serving the structure to the Benton County design flood elevation.  New construction and additions to existing structures require that a licensed surveyor complete three elevation certificates:

  • Pre-construction elevation certificate for the building site to verify flood elevation and existing ground elevation. 
    • This certificate is submitted with the application for a permit. 
    • Note:  Prior to applying for a permit it is helpful to submit this certificate to the Floodplain Manager and discuss the proposed project.
  • Mid-construction elevation certificate to verify floor elevation. 
    • For site-built dwellings and accessory structures, this certificate is submitted after placement of the lowest floor and prior to vertical wall construction.  Approval of the certificate is required prior to approval of the underfloor inspection. 
    • For manufactured dwellings, this certificate is submitted after the dwelling has been placed on the property. Approval of the certificate is required prior to approval of the set-up inspection.
  • Post-construction elevation certificate for the completed structure. 
    • This certificate is prepared after all of the utilities have been placed and all finished grading is completed. 
    • The certificate must be submitted and approved prior to final inspection approval for the structure.

Other permits may require submission of an elevation certificate completed by an Oregon-licensed surveyor as well.  

A Floodplain Development Permit is also required for all work to repair a structure that has been damaged by any type of event.