Aliso Viejo, CA Environmental Rules: Flood Elevation Requirements (2026)

Section 7.50.060(A)(3) sets the elevation rule for new construction and substantial improvement of a residential structure in the City of Aliso Viejo. In an AO zone the lowest floor, including basement, is elevated two feet above the highest adjacent grade to a height equal to or exceeding the depth number printed on the flood insurance rate map, or at least four feet above the highest adjacent grade where the map gives no depth number. In an A zone it is elevated two feet to or above the base flood elevation as determined by the city. In all other zones it must be at least two feet above the base flood elevation. The base flood is the flood with a one percent chance of being equaled or exceeded in any given year, also called the 100-year flood.

When the structure is finished, a registered professional engineer or surveyor certifies the lowest floor elevation, the city building inspector verifies it, and both go to the floodplain administrator, who keeps the certificates available for public inspection. Nonresidential buildings can either meet the elevation standard or be floodproofed so the structure is watertight with walls substantially impermeable to water, able to resist hydrostatic and hydrodynamic loads and buoyancy, with a certification from a registered professional engineer or architect.

Enclosed areas below the lowest floor that are used only for parking, building access or storage must equalize flood pressure automatically: either an engineer or architect certifies the design, or the wall has a minimum of two openings with a total net area of not less than one square inch for every square foot of enclosed area, with the bottom of every opening no higher than one foot above grade. Manufactured homes in Zones A1-A30, AH and AE sit on a permanent foundation with the lowest floor at least two feet above the base flood. Electrical, heating, ventilation, plumbing and air conditioning equipment must be designed or located to keep water out during flooding. Subdivision proposals must identify the flood hazard area and the elevation of the base flood, and filled pads need certified lowest floor and pad elevations.