Boulder, CO Environmental Rules: Flood Elevation Requirements (2026)

Boulder's flood protection elevation is not the base flood elevation itself: under Section 9-16-1, "General Definitions," it is set at two feet above the water surface of a one-hundred-year flood, or two feet above the AE-zone base flood elevation or AO-zone flood depth, whichever is higher, and in the five-hundred-year floodplain it drops to the lower of one foot above that flood's water surface or the one-hundred-year figure. Section 9-3-3(a)(17) applies this freeboard to construction: anyone building a new residential structure must elevate the lowest floor, including the basement, to or above the flood protection elevation, while new nonresidential and mixed-use buildings must floodproof or elevate to the same line and elevate any lodging units.

Existing structures undergoing a "substantial improvement" or "substantial modification," as defined in Section 9-16-1, face the identical elevation duty for the entire structure, not just the added portion, under Section 9-3-3(a)(16). No development in the one-hundred-year floodplain may proceed before the city manager issues a floodplain development permit under Section 9-3-6, and the fee schedule in Section 4-20-44 charges $700 for a permit covering a new residential, commercial, or mixed-use structure elevated to flood protection elevation, rising to $3,675 where floodproofing is used instead. Section 9-3-3(a)(7) separately bars storing hazardous substances at or below the flood protection elevation, and hotels, motels, dormitories, schools, daycare centers, and group homes in the Boulder Creek one-hundred-year floodplain must maintain a city-approved, twenty-four-hour-monitored flood warning system under Section 9-3-3(a)(9). Rental property in the floodplain must post a city-approved flood-hazard sign at the entrance under Section 9-3-3(a)(10).