Arapahoe County, CO Environmental Rules: Flood Elevation Requirements (2026)

C, Freeboard, sets Arapahoe County's minimum elevation buffer above the mapped 100-year flood for construction inside a floodplain or special flood hazard area in unincorporated Arapahoe County: the standard does not reach Aurora, Centennial, Littleton or the county's other incorporated cities, which administer their own floodplain elevation rules. Subsection (a) requires a minimum of two feet of freeboard between the 100-year water surface elevation and the lowest floor elevation, including any basement, of every substantially improved structure within the floodplain or special flood hazard area, including critical facilities, and of structures adjacent to those areas.

Subsection (b) relaxes that to one foot of freeboard for facilities that are not structures and typically do not require a building permit, such as roadways, utility cabinets, parks and trail improvements. Subsection (c) addresses shallow flooding: in AO and AH zones, or areas adjacent to them, the lowest floor including any basement must be elevated two feet above the highest adjacent grade, or two feet above the flood depth number shown on the Arapahoe County Flood Insurance Rate Map, with a four-foot minimum where no depth is specified.

2 development-permit requirements are both satisfied. The county explains the margin is needed because floodplain modeling carries uncertainty, drainageways change over time as sediment raises the invert, and the extra buffer gives structures and facilities added protection against damage during a flood.