FEMA Says Your Lot Is Safe. The Algorithm Disagrees by Six Million Homes.
When you close on a house with a federally backed mortgage, your lender checks a map. If the map says you are inside a Special Flood Hazard Area, you buy flood insurance. If the map says you are outside, you probably do not. Nobody at the closing table explains that the map may be based on hydrology data collected when the current president was in middle school, that it was never designed to predict future flooding, or that a nonprofit in Brooklyn ran the same analysis with current climate data and found 70 percent more homes at risk than the federal government acknowledges.
Nobody explains because nobody is required to.
The Gap Nobody Closed
FEMA's Flood Insurance Rate Maps classify 8.7 million properties as sitting inside zones with substantial flood risk, the kind where a major flood event is expected roughly once every hundred years. First Street Foundation, a nonprofit that applies machine learning to geospatial and climate data, ran its own model across 142 million properties and arrived at a different number. 14.6 million. The difference is 5.9 million households whose actual risk exceeds what the federal map depicts, a population roughly the size of Wisconsin living on ground the government says is fine.
5.9 million properties carry flood risk that FEMA's maps do not show. By 2050, the AI model projects 16.2 million at substantial risk.
Three failures drive the gap, and each one widens it in a direction that matters for anyone buying or building right now. First, FEMA does not model precipitation as a standalone flood source. Rain that overwhelms a storm drain in a neighborhood miles from any river does not appear on a riverine flood map. Second, FEMA has not mapped large portions of the country at all. Third, and most consequential, FEMA's maps do not incorporate projected climate change, reflecting only past floods rather than what the next thirty years of warming will bring.
First Street's model accounts for all three. It integrates sea-level rise projections, warming sea surface temperatures, and shifting precipitation patterns to score every property on a 1-to-10 scale representing cumulative flood risk over a 30-year mortgage, the same timeframe as the debt most buyers are taking on when they sign the paperwork that cites the FEMA map. Realtor.com began publishing those scores alongside FEMA designations in 2020, though not every listing platform followed.
Who Already Knows
Here is what should bother you about the asymmetry: institutional investors, commercial lenders, and insurance carriers have purchased private flood models for years. Cape Analytics, the same computer vision firm that insurers use to grade your roof from a satellite, also maps flood exposure at the parcel level. Risk management consultants sell climate-adjusted loss projections to developers evaluating land acquisitions, the same land acquisitions that produce the subdivisions marketed to families who never see that data before they sign. A hedge fund buying distressed coastal real estate in 2024 had access to better flood intelligence than the family that bought the house next door the same week, and neither the fund nor the family's lender was required to share the difference.
"For-profit companies and commercial actors are taking this data, taking this knowledge, and using it to have an advantage to buy or sell homes," said Matthew Eby, First Street's executive director. "A democratization of this data is how we like to think about it."
First Street made its data free, and that is a meaningful contribution, but it does not fix the structural problem, which is that the federal government's own maps remain the legal standard for mortgage-linked insurance requirements, and those maps are wrong about millions of properties.
The Disclosure Desert
If you are buying a home in Texas, the seller must disclose whether the property has previously flooded, how much insurance money they received, and whether the property sits in a designated floodplain. If you are buying in Florida, the most hurricane-vulnerable state in the country, the seller is not legally required to tell you any of those things. Same coast, different rules.
An analysis by the Natural Resources Defense Council and Columbia University's Sabin Center for Climate Change Law found that 21 states have zero statutory or regulatory requirement for sellers to disclose flood history or risk during a real estate transaction. Only ten states require disclosure of actual past flood damage to structures.
21 states have no flood disclosure requirement. Only 10 require sellers to reveal past flood damage. 74% of Americans support a federal mandate. None exists.
New York used to allow sellers to pay a $500 credit to skip flood disclosure entirely, but closed that loophole in 2023. Missouri still has zero provisions, and Florida's disclosure form is voluntary. Meanwhile, a 2022 FEMA analysis found that states with stronger disclosure requirements have higher rates of residents carrying flood insurance, the predictable outcome of telling people the truth about the ground beneath their house.
Twenty-five percent of all National Flood Insurance Program claims come from properties outside FEMA's designated high-risk zones, meaning one in four payouts goes to a homeowner who was told their property was safe. That statistic alone should make anyone building or buying outside a flood zone pause long enough to check First Street's data before signing.
What Risk Rating 2.0 Fixed and What It Did Not
In October 2021, FEMA replaced its 50-year-old zone-based pricing system with Risk Rating 2.0, which calculates premiums at the individual property level using multiple flood sources, rebuild costs, and structure-specific characteristics. Overdue and directionally correct, the overhaul repriced millions of properties that had been systematically undercharged, in some cases by more than $100 per month.
What Risk Rating 2.0 did not change is the underlying map: pricing methodology improved, but the data that determines whether you are required to buy flood insurance in the first place did not, which means a property that sits outside the Special Flood Hazard Area on a FEMA map from 2003 is still exempt from mandatory flood insurance, regardless of what Risk Rating 2.0's algorithm would charge if the property were inside the zone.
This creates a peculiar regulatory structure: FEMA now prices flood risk with sophisticated, property-level precision for properties it already identifies as at-risk, while leaving millions of at-risk properties outside the system entirely because the map that gates entry has not caught up.
What to Do With This
If you are buying a home anywhere in the United States, go to floodfactor.com and enter the address before you make an offer. A Flood Factor score is free, takes thirty seconds, and covers risks that the FEMA designation on your mortgage paperwork does not. A score of 5 or higher means a cumulative 15 percent or greater chance of flooding over thirty years. That is the kind of probability most people would want to know about before committing several hundred thousand dollars to a piece of ground.
If you are building, do not spec your foundation elevation solely to FEMA's Base Flood Elevation, because First Street's data suggests that BFE is inadequate for a meaningful fraction of the country and an additional foot of freeboard above BFE costs roughly $2,500 to $5,000 during construction while retrofitting after a flood costs orders of magnitude more.
If you are in one of the 21 states with no disclosure requirement, understand that the seller's silence is not evidence of safety. It is evidence of a statute that has not been updated to reflect what AI models have known for half a decade.
Limitations
First Street's model is not infallible. Its flood risk scores rely on climate projections that carry inherent uncertainty, particularly at the property level where microtopography, drainage infrastructure, and recent grading can alter outcomes that no satellite-derived model captures. Of the 5.9 million homes in the gap, some reflect model assumptions rather than ground truth, and the cost of the additional freeboard recommended above varies substantially by region, soil conditions, and foundation type. That range reflects national averages for slab-on-grade construction and should be verified with a local engineer.
What is not uncertain is the direction. More properties face flood risk than FEMA maps show. More homebuyers lack access to that information than should be acceptable in a country that requires disclosure of lead paint but not of a flooded basement. And more of the people profiting from that information asymmetry are on the institutional side of the transaction than on yours.