Aliso Viejo, CA Environmental Rules: Flood Zones (2026)

Section 7.50.040 sets the reach of the rule. It covers all areas of special flood hazard within the city limits of Aliso Viejo, and it adopts by reference the FEMA flood insurance study for Orange County, CA and Incorporated Areas dated February 18, 2004, with its flood insurance rate maps and all later amendments. The study and maps sit on file at Aliso Viejo City Hall in the department of public works. The mapped area is the minimum: the floodplain administrator can recommend additional studies to the city council to extend it. A special flood hazard area is defined as land with a one percent or greater chance of flooding in any given year, shown on the map as Zone A, AO, A1-A30, AE, A99 or AH.

Inside those zones, section 7.50.050 requires a flood hazard development permit before any construction or other development begins. The application goes on forms from the floodplain administrator, who is the director of public works, and must include a site plan with spot ground elevations at building corners and at 20-foot or smaller intervals along the foundation footprint (or one-foot contours across the building site), foundation details, and a description of any watercourse that would be altered or relocated. Foundations placed on fill must show the location and height of fill and compaction to 95 percent using the standard proctor test method.

The overlay is a stricter layer on top of other rules. Where this chapter and any other ordinance, easement, covenant or deed restriction conflict, whichever imposes the more stringent restriction prevails. In the regulatory floodway, section 7.50.060(F) prohibits fill, new construction, substantial improvement and other new development unless a registered professional engineer or architect certifies that the encroachment will not raise the base flood elevation. A substantial improvement or substantial damage is any project or loss costing 50 percent or more of the structure's market value. Disputes over a mapped boundary or an administrator decision go to the city council on appeal, and a variance under section 7.50.070 is reserved for land with physical characteristics so unusual that compliance would create an exceptional hardship.