Substantial Damage & Improvement

If your home or business is located within the floodway fringe, density floodway, or floodway as designated on the adopted 2016 Flood Insurance Rate Maps (FIRMs), you will want to be aware of the “50% rule.” For help in understanding how the County applies this rule, please contact the St. Charles County Planning and Zoning Division at 636-949-7335 or email the Planning and Zoning Division.

Structures located in these flood-prone areas of unincorporated St. Charles County which are “substantially damaged” or “substantially improved” must be brought into compliance with the County’s floodplain development regulations, including elevating the lowest floor of the structure at least 1.0 foot above the base flood elevation.

Substantial Damage

When the cost to repair any damaged structure to its previous condition would equal or exceed 50% of the market value of the structure (and not the land) before the damage occurred.

Substantial Improvement

When the cost of any reconstruction, rehabilitation, addition, or other improvement of a structure would equal or exceed 50% of the market value of the structure before "start of construction". This term includes structures which have incurred "substantial damage" regardless of the actual repair work performed. The term does not, however, include:

  • Any project for improvement of a structure to correct existing violations of state or local health, sanitary, or safety code specifications that have been identified by the local Code Enforcement Official and which are the minimum necessary to assure safe living conditions;
  • Any alteration of a "historic structure," provided that the alteration will not preclude the structure's continued designation as a "historic structure"; or
  • An improvement that does not require a building permit under applicable codes enacted in Title V, Ordinances of St. Charles County, Missouri.

Identifying Classification

St. Charles County, following National Flood Insurance Program (NFIP) requirements, has the responsibility to determine "substantial-damage" and "substantial improvement" and has implemented the following procedures to do so: St. Charles County will calculate Market Value by using the tax assessment value of your structure (excluding the land). If you disagree with this estimate of Market Value, you may hire a state licensed appraiser and submit a comparable property appraisal for the market value of the structure.

  • You must obtain and submit to St. Charles County a detailed and complete cost estimate for the addition, remodeling, reconstruction, or repair of all the damages sustained by your home or business, prepared and signed by a licensed general contractor. St. Charles County will evaluate the costs of improvements or repairs and determine if they are fair and reasonable.
  • If your home or business is determined to have "substantial damage" or is proposed to be "substantially improved", then an elevation certificate must be submitted to determine the lowest floor elevation.
  • If the lowest floor is below the base flood elevation, then the building must be elevated at least 1.0 foot above base flood elevation. Likewise, all utility and sanitary facilities must be elevated or floodproofed up to the regulatory flood protection elevation. Single electric service for recreational uses must be elevated a minimum of 10 feet above existing average grade.
  • Any enclosures located below the base flood elevation must have flood venting which automatically opens to allow flood waters to enter the enclosed space.
  • Building plans must be prepared and submitted that show how the building is to be elevated. One set of your sealed building plans must be submitted to the St. Charles County Building Code Enforcement Division. An elevation certificate must be submitted to the St. Charles County Planning and Zoning Division and have the seal of an engineer or land surveyor.


Activity in the floodway or the density floodway may require approval and/or permits from other agencies, such as the U.S. Army Corps of Engineers.