Lewisville, TX Environmental Rules: Flood Elevation Requirements (2026)

5-17 sets Lewisville's specific flood hazard reduction standards for areas where FEMA has published base flood elevation (BFE) data. New construction and substantial improvement of a residential structure must have its lowest floor, including the basement and all mechanical, electrical, heating, and plumbing equipment, elevated to two feet above the base flood elevation, a two-foot freeboard margin above the minimum flood level shown on the FIRM. A registered professional engineer, architect, or land surveyor must certify to the floodplain administrator that this standard is met.

Nonresidential structures, including commercial and industrial buildings, face the same two-foot freeboard unless the owner instead floodproofs the structure so that everything below the base flood level is watertight, with walls substantially impermeable to water and components engineered to resist hydrostatic and hydrodynamic loads; a professional engineer or architect must certify the floodproofing design, and the floodplain administrator keeps a record of the certified elevation. Fully enclosed areas below the elevated lowest floor, such as parking or storage space, must have at least two openings on separate walls providing one square inch of net open area per square foot of enclosed space, with the bottom of each opening no higher than one foot above grade, so floodwaters can enter and exit automatically.

5-19. Recreational vehicles are exempt from the elevation standard only if they stay fewer than 180 consecutive days, remain licensed and ready for highway use, or otherwise meet the manufactured-home elevation and anchoring rules.