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50% Substantial Improvements

If your home or business is located within a 1% annual chance floodplain (formerly referred to as the 100-year floodplain) and you are planning to build a room addition or make a major structural renovation or other improvements, and you obtained a County Building Permit for your existing building before October 31, 1985, then your construction project may be subject to the 50% Substantial Improvement provisions of the National Flood Insurance Program (NFIP).

Buildings in floodplains that were issued a County Building Permit before October 31, 1985 are technically referred to as Pre-FIRM (Flood Insurance Rate Map) buildings and may or may not be exempted from flood construction requirements depending on the total cost of the proposed construction and the current appraised market value of the existing building (value of the building only; land and any other structure that is physically detached from the building may not included in the market appraisal). In general:

  • If the total project construction cost (including the cost of contractor labor, overhead and profit) is less than 50% of the current market value of the existing building, then flood protection requirements are not mandatory; it is voluntary.
  • If the total project construction cost (including the cost of contractor labor, overhead and profit) equals or exceeds 50% of the current market value of the existing building, then all applicable flood protection requirements will be applied to the proposed project as well as to the entire existing building.

In both cases, your project must obtain a Floodplain Development Permit from the Public Works Agency prior to the issuance of a Building Permit.

Buildings (habitable, non-habitable) in floodplains that were issued a County Building Permit after October 31, 1985 are referred to as Post-FIRM buildings and do not benefit from the 50% Substantial Improvement provisions. All applicable flood protection requirements will be applied to your project and a Floodplain Development Permit will need to be obtained prior to the issuance of a Building Permit.

More Information:

Substantial Improvement and Substantial Damage


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County of Ventura Instructional Handout: 50% Substantial Improvement


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800 S Victoria, Ventura, C.A. 93003
Ventura County Watershed Protection District
Watershed Protection: (805) 654-2001