In just a single month, 2025 is the second most destructive fire year in California history, with more than 16,000 homes and other structures damaged or destroyed by two fires in the Los Angeles area. Most of those structures were in neighborhoods where the boundaries between human development and natural landscapes blur — the area fire officials and researchers call the wildland urban interface or WUI (woo-ee).
When a wildfire approaches one of these areas, the results can be hazardous as a fire can transition from consuming trees, shrubs and plants to devouring homes and other structures often constructed in ways that are vulnerable to burning. And it’s also where California has been building homes for decades — nearly 45% of homes built between 1990 and 2020 are located in places with lots of vegetation ready to fuel a fire.
A CalMatters analysis has found that as of 2020, nearly 14 million Californians, or 1 in 3, lived in the sprawling 7-million-acre zone that makes up the WUI. And this isn’t just a problem for Californians living in rural parts of the state: All 58 counties in California have WUIs, along with many areas across the country. The WUI grows by about 2 million acres per year nationwide, according to the U.S. Fire Administration.
Sarah McCaffrey, a social scientist who worked for the U.S. Forest Service for decades, put it another way: Structural fires and wildland fires — and the interface is where the two come together.
Just because a development is located within the WUI doesn’t mean a fire will occur there, but there may be deadly and destructive consequences if one does. While under 3% of the statewide WUI has been affected by wildfire in the last decade, thousands of homes in the zone have been destroyed, according to a CalMatters analysis.
Since 2018, Cal Fire, the state’s fire agency, has inspected all of the buildings within 100 meters of a fire perimeter and assessed the level of damage. The most destructive fires in California history have largely damaged or destroyed homes within the WUI, including the two largest fires in Los Angeles County this year – the Palisades and Eaton fires, which demolished entire neighborhoods and killed 28 people as of Jan. 24.
The Eaton Fire killed at least 17 people and destroyed more than 9,400 structures, roughly 9,200of which were within the WUI as is much of Altadena, where the San Gabriel Valley rises to meet the San Gabriel Mountains. The map below shows inspections of buildings conducted by Cal Fire following the Eaton Fire along with the WUI, illustrating just how destructive a major conflagration can be for suburban areas.
The other major fire in LA, the Palisades Fire, apparently raced down the Santa Monica Mountains towards Pacific Palisades, a coastal neighborhood in Los Angeles that is nearly entirely within the WUI. The hurricane-force winds stoked and spread the flames and destroyed at least 6,800structures while nearly 4,100wereundamaged.
The deadliest and most destructive fire in California history, the Camp Fire, destroyed the town of Paradise in Butte County in 2018 as the massive blaze burned both the natural and built environment. The fire damaged or destroyed nearly 20,000 structures and killed 85 people.
Having a home in the WUI doesn’t guarantee that it’s going to burn down if a wildfire engulfs the neighborhood. Many factors contribute to that possibility, such as the year when the home was built, nearby fuel management practices that aim to reduce nearby flammable objects — and what some folks call “weather” and the rest of us call “luck.” That’s why there’s photos of undamaged homes next to their razed neighbors after so many destructive California fires.
Despite all this, some experts say the WUI isn’t the only, or perhaps even the best, way to measure fire hazard or risk because it wasn’t designed for that. It can collapse nuance to narrowly focus our attention.
“There’s nothing about fire risk in the WUI map,” said McCaffrey. “There are some papers that show that roughly a third of houses lost in fires are not in the WUI right? But we target all our attention to the WUI.”
An example of how the boundary between the wild and urban landscape doesn’t necessarily translate to high fire risk is the 2017 Tubbs Fire in Sonoma County. The Coffey Park neighborhood of Santa Rosa was nearly leveled even though it’s not technically in the WUI, just next to it. The fire destroyed more than 5,000 structures and killed 22 people.
“There’s actually a legal or technical forestry definition of the wildland urban interface and it has to do with the number of structures per acre and the amount of vegetation,” said Judson Boomhower, an assistant professor in the economics department of UC San Diego. “I think sometimes it’s more useful to talk about building in high wildfire hazard places.”
The ways to reduce the risk of fire spreading from house to house depend on the nuances of each neighborhood, particularly how densely packed the buildings are.
“If houses are, you know, 80 or 100 feet apart, then all our focus on defensible space and zone zero and all that stuff probably makes a lot more sense,” said McCaffrey. “Because then, you know, it’s a good chance my house could survive if I’ve done all this, even if my neighbor’s house catches on fire.”
On the other hand, some of the homes in Pacific Palisades look so close together that even ensuring that there is nothing flammable within five feet might not have prevented the spread of the fire, said McCaffrey. A law signed by Gov. Gavin Newsom in September 2021 required the State Board of Forestry and Fire Protection and the State Fire Marshal to make suggestions for creating an ember-resistant zone within five feet of a structure in a high fire severity zone.
State intervention has helped to reduce destruction in these more wildfire-prone areas. Research from 2021 concluded that California homes built after 2008 were nearly half as likely to be destroyed compared to those built in 1990 if a wildfire burned through the neighborhood, largely because of improved building code regulations.
California faces a chronic housing shortage and megafires boosted by climate change, but with millions of residents already living in the wildland urban interface, state policymakers will face questions of how to protect people and where to rebuild after disaster strikes.
