Royal Oak, MI Environmental Rules: Flood Elevation Requirements (2026)

Section 351-6 of the Flood Damage Prevention chapter applies once base flood elevation data has been provided under § 351-4C(2), and sets the City's standard for the "base flood," defined in § 351-2 as the flood with a one-percent chance of being equaled or exceeded in any given year. Subsection A requires that new construction and substantial improvement of any residential structure have the lowest floor, including any basement, elevated to or above the base flood elevation; there is no floodproofing alternative for residential buildings, only elevation.

Subsection B gives commercial, industrial, and other nonresidential structures a second compliance path: either elevate the lowest floor to the base flood elevation the same as a residential structure, or floodproof the structure and its utility and sanitary facilities so that, below the base flood level, the building is watertight with walls substantially impermeable to water, and has structural components able to resist hydrostatic and hydrodynamic loads and buoyancy. Whichever floodproofing path a nonresidential project takes, § 351-6B(3) requires certification by a registered professional engineer or architect that the standards are satisfied, and that certification must be filed with the Director of Community Development under § 351-4C(3)(b).

"Substantial improvement" is defined in § 351-2 as any repair, reconstruction, or improvement whose cost equals or exceeds 50% of the structure's market value, so a major renovation of an existing home in a mapped flood hazard area triggers the same elevation requirement as new construction.