# How is the potential loss to building structure over time determined?

#### Learn how the estimated average costs for potential flood damages over the next 30 years are determined.

The potential loss to a building structure over the next 30 years is the sum total of estimated annual potential flood damage over a 30 year period. These estimates are based on a property’s flood risk projections from the First Street Foundation Flood Model and each year’s annual estimated risk of flooding resulting in damage to the structure for the selected period of time.

## What does potential loss to building structure refer to?

Potential loss to building structure is the estimated cost a property owner can expect to pay as a result of structural damage to their building from flooding over a selected period of time. These estimates are based on the projected depth, likelihood, and type of flooding the building is at risk of, as well as building characteristics, and property value.

Estimated projected depth, likelihood, and type of flooding come from the First Street Foundation’s Flood Model. The flood model considers the location of a building when determining the type and the subsequent cost of its flood risk. For example, a surge from a hurricane causes a different type of damage then flooding from heavy rain. Historical data on flood damage costs comes from the United States Army Corps of Engineers (USACE).

The approximate property value includes the value of the physical building structure as well as the value of the land the property sits on.  Estimated property values are provided by ComeHome by HouseCanary’s automated valuation model (AVM). Where AVMs are not available from ComeHome, they are estimated using the building characteristics through a peer-reviewed methodology developed by First Street Foundation.

Building characteristics refers to factors such as the number of stories of a structure, the presence of a basement, and the first floor elevation. This data comes from the National Structures Inventory (NSI) database or Lightbox, a leading provider of commercial real estate data.

## How is the potential loss to building structures over time calculated?

The potential loss to a building structure over time is the sum total of all potential flood damage, based on a property’s likelihood, depth, and source of flood risk. The potential loss to a building’s structure is calculated by adding each year's annual flood damage estimate over the selected period of time. For example, if a property’s annual flood damage is estimated to be \$100 every year, the potential loss to that building would be \$1,500 in 15 years.

## Why is the estimated loss increasing over time?

Property-specific flood projections from the First Street Foundation Flood Model are used to determine the total amount a property owner can expect to pay in flood damage repairs.

Because of changes in the climate, future flood projections may differ significantly from a property’s risk today. Changes in the environment have already increased flooding across the United States, particularly in coastal and low-lying areas. As these trends continue into the future, 1.7 million more American properties will be at risk, and the damage and cost of flooding will continue to add up.

If a property’s flood risk is projected to increase in the future, the average annual flood damage in 30 years will be higher than the estimate this year.

Future annual flood damage estimates do not consider inflation or home improvements that might increase building value, both of which would increase flood damage costs nor do they include future mitigation or adaptation efforts which may reduce flood risk.

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