Flood zone AO

Checked against FEMA and other official sources · Last reviewed October 8, 2026

What zone AO means

Federal regulations describe zone AO as an "area of special flood hazards having shallow water depths and/or unpredictable flow paths between (1) and (3) ft." An area of special flood hazard is land with "a 1 percent or greater chance of flooding in any given year."

The regulations also define an area of shallow flooding, which includes AO zones. It is flooding "to an average depth of 1 to 3 feet where a clearly defined channel does not exist, where the path of flooding is unpredictable, and where velocity flow may be evident." The same definition says "such flooding is characterized by ponding or sheet flow."

FEMA's glossary says AO flooding is "usually in the form of sheet flow." It also says "these areas have a 26% chance of flooding over the life of a 30‐year mortgage." FEMA defines the sheet flow hazard as flooding 1 to 3 feet deep "that occurs in areas of sloping land," and says it is shown as zone AO on the flood map. [1][2][3][7]

The depth number, not a base flood elevation

In zones such as AE and AH, the map shows a base flood elevation (BFE). FEMA defines it as "the computed elevation to which floodwater is anticipated to rise" in the base flood. Zone AO is different. FEMA says "average flood depths derived from detailed analyses are shown within these zones." FEMA's Elevation Certificate instructions put it simply: BFEs are shown for zones such as AE and AH, while "base flood depths are shown for Zones AO and AR/AO."

The federal building rule for AO measures the lowest floor from the highest adjacent grade, defined as "the highest natural elevation of the ground surface prior to construction next to the proposed walls of a structure." Some AO areas have no depth number on the map. The building rules cover that case too (see below).

Alluvial fans. FEMA defines alluvial fan flooding as flooding on the surface of an alluvial fan or similar landform that is "characterized by high-velocity flows" and "unpredictable flowpaths." FEMA says it "is depicted on a Flood Insurance Rate Map (FIRM) as Zone AO, with a flood depth and velocity." So in these areas the map can show a flow speed as well as a depth. [1][4][5][7][8][9]

Zone AO vs. zone AE and zone AH

• Risk level: all three are high-risk Special Flood Hazard Areas, and all are on the federal list of zones where flood insurance purchase is mandatory.
• What the map shows: AE and AH show a base flood elevation. AO shows an average flood depth in feet instead.
• Type of flooding: AO and AH are both shallow flooding of about 1 to 3 feet. FEMA says AO flooding is usually sheet flow, while zone AH flooding is usually "in the form of a pond."
• Building rules: in AE and AH, new homes must have the lowest floor at or above the BFE. In AO, the lowest floor must be raised above the highest adjacent grade by at least the depth number, or at least 2 feet if the map gives no depth number.

For the deeper-water zone with a BFE, see flood zone AE. [1][3][4][5][10]

Flood insurance in zone AO

FloodSmart lists AO among the high-risk zones and says that "in communities that participate in the NFIP, flood insurance is mandatory if mortgages are government-backed." Under federal law, that covers federally regulated lenders, federal agency lenders, and loans bought by Fannie Mae or Freddie Mac. The required coverage is at least the loan balance or the maximum NFIP coverage for the building type, whichever is less. Lenders must also accept a private policy that meets the requirements.

Your zone doesn't set your NFIP premium by itself. FEMA says its pricing approach "does not use flood zones to determine flood risk." FloodSmart says zones and elevation relative to the BFE "are no longer the only factors" in premiums. See do I need flood insurance?. [3][6][11][12]

Building in zone AO

NFIP communities must enforce these federal minimums in AO zones for new construction and "substantial improvements," meaning work costing 50% or more of the building's market value before the work starts:

• Homes: the lowest floor, including any basement, must be "elevated above the highest adjacent grade at least as high as the depth number specified in feet on the community's FIRM." If the map gives no depth number, the minimum is "at least two feet."
• Non-residential buildings: they must either be elevated the same way, or be completely floodproofed, together with their utility and sanitary facilities, to that level. A registered professional engineer or architect must certify a floodproofing design.
• Enclosed areas below the lowest floor: fully enclosed areas, other than a basement, that are used only for parking, building access or storage must be designed to let floodwater flow in and out. Unless an engineer or architect certifies the design, there must be at least two openings with a total net area of at least one square inch for every square foot of enclosed area, and the bottom of each opening no higher than one foot above grade.
• Drainage on slopes: communities must require "adequate drainage paths around structures on slopes, to guide floodwaters around and away from proposed structures." This rule applies in AO and AH zones.
• General standards: permits are required, and buildings must be anchored against flotation, collapse and lateral movement, and built with flood-resistant materials and methods.

These are federal floors. FloodSmart advises talking to your floodplain manager, "as local regulations may be more stringent." [5][6][7]

Finding your map and proving your elevation

Look up your official map at FEMA's Map Service Center, which FEMA calls "the official public source for flood map." See what flood zone am I in?. The depth number, if there is one, appears inside the AO area on the map.

An elevation certificate has a section for zone AO (Section E, no survey required) that records how high the top of your bottom floor sits above or below the highest adjacent grade. That section is for showing you meet local floodplain rules. For AO, an owner, the owner's representative, or a local official may fill it in. That shortcut does not apply if the elevations will support a LOMA or similar request, or if you need a ground elevation for flood insurance rating. FEMA's instructions say to fill in the surveyed elevation section (Section C) instead. For a LOMA, certified elevations from a surveyor, engineer or architect are needed.

If you believe your building or lot was mapped into the AO area by mistake, FEMA's process is a Letter of Map Amendment (LOMA). FEMA describes it as a determination of whether a building, lot or part of a property "is inadvertently included" in the Special Flood Hazard Area. Federal rules list AO among the zones this process covers. [1][4][8][13]

Questions people ask