Flood zones and maps

Flood zone AH

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

What zone AH means

Federal regulations describe zone AH as "areas of special flood hazards having shallow water depths and/or unpredictable flow paths between (1) and (3) feet, and with water surface elevations determined." 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 AH 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 AH flooding is "usually in the form of a pond." It also says "these areas have a 26% chance of flooding over the life of a 30‐year mortgage." [1][2][5]

The base flood elevation in zone AH

What sets AH apart from zone AO is the base flood elevation (BFE). FEMA defines it as "the computed elevation to which floodwater is anticipated to rise during the base (1-percent-annual-chance) flood event." FEMA says that in AH zones, "base flood elevations derived from detailed analyses are shown at selected intervals within these zones."

FEMA's Elevation Certificate instructions list AH among the zones where BFEs are shown, along with zones such as AE. In zone AO, the map shows a flood depth instead. To compare your building to the BFE, you need its elevation. See elevation certificate. [1][3][6]

Zone AH vs. zone AE and zone AO

• 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: AH and AE both show a base flood elevation. AO shows an average flood depth in feet instead.
• Type of flooding: AH and AO are both shallow flooding of about 1 to 3 feet. FEMA says AH flooding is usually ponding, while AO flooding is usually "in the form of sheet flow."
• Building rules: the home elevation rule in AH is the same as in AE: lowest floor at or above the BFE. AH and AO also share a federal rule on drainage paths around buildings on slopes. That rule names only zones AH and AO, not AE.

For details on AE, see flood zone AE. [1][2][3][7][8]

Flood insurance in zone AH

FloodSmart lists AH 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?. [2][4][9][10]

Building in zone AH

NFIP communities must enforce these federal minimums in AH 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 to or above the base flood level." Limited exceptions for basements exist only where FEMA has granted one to the community.
• Non-residential buildings: they must either be elevated the same way, or be floodproofed so that below the BFE the structure is watertight. 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 AH and AO zones.
• Manufactured homes: they generally must be elevated so the lowest floor is at or above the BFE, and securely anchored. Some sites in existing manufactured home parks may instead use reinforced piers at least 36 inches high.

These are federal floors. FloodSmart advises talking to your floodplain manager, "as local regulations may be more stringent." [4][5][8]

Finding your map and challenging your zone

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?.

If you believe your building or lot was mapped into the AH 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 AH among the zones this process covers. If you use an elevation certificate to support a LOMA, FEMA's instructions say it must include certified elevations. Those come from a surveyor, engineer or architect authorized by state law. [3][6][11]

Questions people ask