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

Section 22-311(b)(1)a requires that new construction, substantial improvement, or reconstruction of any residential structure in Thornton's special flood hazard area have its lowest floor, including the basement, along with all electrical, heating, ventilation, plumbing, and air conditioning equipment and service facilities, elevated to at least one foot above the base flood elevation; this covers external equipment too, such as air conditioning compressors, meters, and master switches. Once built, the lowest-floor elevation must be certified by a registered Colorado professional engineer, architect, or land surveyor on the prescribed FEMA form, and that certification is submitted to and kept by the floodplain administrator.

Section 22-311(b)(1)b layers on a freeboard requirement for substantial improvements, renovations, reconstructions, or repairs of residential structures in the special flood hazard area: the lowest floor, including basements, must be placed with a minimum of one foot of freeboard above the base flood elevation or the model-backed approximate flood elevation. In AO and AH flood zones on the FIRM, Section 22-311(b)(1)c requires the lowest floor to sit above the highest adjacent grade at least as high as the depth number shown on the FIRM plus one foot of freeboard, or at least two feet if no depth number is given.

Nonresidential structures face a parallel standard under Section 22-311(b)(2): the lowest floor and utility equipment must be elevated the same amount, or the structure and its utilities must be floodproofed watertight to at least one foot above the base flood level with walls able to resist hydrostatic and hydrodynamic loads.