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Highest Adjacent Grade

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The purpose of this page is to define highest adjacent grade, a commonly used term in floodplain management.


The highest natural elevation of the ground surface prior to construction next to the proposed walls of a structure. In AO Zones, all new construction and substantial improvements of residential structures shall have the lowest floor including basement elevated above the highest adjacent grade at least as high as the depth number specified in feet on the community's Flood Insurance Rate Map (FIRM); or at least two feet if no depth number is specified. All new construction and substantial improvements of non-residential structures shall meet the above requirements or, together with attendant utility and sanitary facilities, be floodproofed to the same elevation.

NFIP Requirement


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Last Updated: 
04/26/2015 - 14:14
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