Joliet, IL Environmental Rules: Flood Elevation Requirements (2026)

The article defines the flood protection elevation (FPE) in Sec. 8-502 as the base flood, also called the one hundred year frequency flood, plus one foot of freeboard added for a margin of safety against calculation uncertainty, localized conditions and wave or debris effects. Sec. 8-509(5) requires that all buildings within the SFHA be protected from flood damage below the FPE, and applies that duty to new construction, substantial improvements that add more than 20 percent to the first-floor area, buildings with substantial flood damage, new manufactured home placements, and travel trailers sited for more than 180 days.

Sec. 8-509(6) sets out one compliance path: a building placed on permanent fill must have its lowest floor, including the basement, at or above the flood protection elevation, with the fill extending at least ten feet beyond the foundation before sloping below the FPE, placed in lifts no greater than one foot before compaction, and protected against erosion and scour. Sec. 8-509(7) offers an elevation-on-open-foundation alternative: the building can instead sit on crawl space, stilts, piles or walls that stay open to floodwaters, provided the openings are no higher than one foot above grade, number at least two, and total at least one square inch of net opening for every square foot of enclosed area below the FPE.