There’s an invisible line where human development meets flammable vegetation, and it’s where the most destruction from wildfires occurs.

It’s called the wildland-urban interface. In San Diego, developers are looking to build nearly 6,000 more homes along this frontline.

“When I heard this fire started where it started, with those weather concerns, I was concerned,” North County Fire Protection District Chief Steven Abbott said at a public meeting in Fallbrook Saturday of the Lilac Fire.

The Lilac Fire destroyed more than 180 structures, roughly half of which were homes, considered in the wildland urban interface. Nearly 7,500 residents evacuated and several were injured, including firefighters.

County leaders may soon decide whether to let developers move forward with projects that would be located in areas of extreme wildfire danger, an approval the projects need because they’re proposed in areas where far fewer new homes were envisioned in the county’s long-term growth plan, approved in 2011.

In Houston, development decisions may have contributed to the devastation caused by flooding from Hurricane Harvey.

In Southern California, the same may be true for wildfires.

A 2013 study by the U.S. Geological Survey, for instance, found that sprawl projects built far away from existing development, often called leapfrog development, have led to more houses being lost to fire.

“The first set of homes are going to be in danger because they’ll be surrounded by wildland,” said Jon Keeley, one of the study’s authors. “Leapfrog, without a doubt, leads to the highest loss of homes.”

The safest way to build new housing is to build it near existing housing, the study found.

The large development projects still waiting for approval are not just located in severe wildfire risk areas. Some are just on the other side of the I-15 from where the Lilac Fire burned. Some are in the same area as the 2014 Cocos Fire, which destroyed roughly 65 buildings, costing roughly $28.5 million to contain and incurring an estimated $29.8 million in property damage. They’re also near the 2007 Witch Fire, which amounted to roughly $1.3 billion in damages, and the 2003 Cedar Fire, which destroyed 2,820 buildings and killed 15 people.

The projects are proposed in northeastern San Diego County because it’s some of the last bare land for development in the county. But that’s exactly why they’re concerning, experts say: The risk is highest when the first developments go in, and eventually subsides when there is a much larger mass of buildings and people like in downtown San Diego, Oceanside or Escondido.

One of the projects, Lilac Hills Ranch, proposes roughly 1,700 homes on 600 acres of rolling hills in Valley Center. Harmony Grove South would add 463 homes around the corner from Valiano, another proposed development including 326 homes. Another Harmony Grove project of about 700 homes is already under construction. Newland Sierra, between Vista and Valley Center, would bring another 2,135 homes. Warner Ranch, near Pala, proposes another roughly 780 homes.

Keeley said this year’s fires in Napa and Sonoma counties, where more than 40 people died and hundreds of homes and buildings were destroyed, showed the dangers of new development.

“That area had fires very similar to the fires they had this year in 1964, but no one died in those fires,” Keeley said. “The primary thing that has changed wasn’t the fire.”

The increase in deaths and destruction was more likely due to the drastic increase in people populating Santa Rosa, he said.

Richard Halsey, the director of the California Chaparral Institute, said from the minute you build a home in the wildland-urban interface, it becomes more and more dangerous, as litter builds up, homes age, people accumulate stuff that could be flammable and let dried leaves or other brush build up in their gutters and yards. It’s not until you reach a certain density, where most of the vegetation is removed from areas, that the danger diminishes.

Halsey said he’s concerned about a project in his backyard in Escondido, Safari Highlands.

“That was in the fire corridor that the 2007 fire raged through,” he said. “It’s in an area pretty much identical to the area that burned in Santa Rosa, the neighborhoods being taken out by the Thomas Fire right now.”

The large projects all require amendments to the county’s growth blueprint because they would put more density than is currently allowed. The amendments must go through the county’s Planning Commission and Board of Supervisors.

Newland Sierra is expected to come before the board in 2018.

Decision-makers can impose requirements on the developments that could minimize their fire risk. Valiano, for instance, would need to build new roads leading out of the property to help with evacuation, according to its Fire Protection Plan, a document included in the county’s development review process. Harmony Grove Village would build a new fire station if approved.

Developers for Lilac Hills Ranch, meanwhile, have not yet proposed how they’ll ensure fire crews can reach homes in the project’s furthest reaches within required fire response timelines – though early phases of the development can begin before they figure it out.

The most common needs for North County projects, according to the San Diego County Fire Authority, mostly deal with adequate roads into and out of the projects.

During the Cocos Fire, traffic gridlocked on Country Club Drive as residents tried to evacuate, according to some accounts. The fire created the same problem for another nearby development, San Elijo Hills, near San Marcos. San Elijo Road led to all three exits out of the development and became gridlocked as thousands of residents fled.

Lilac Hills Ranch, which is roughly less than a mile from the Lilac Fire at the closest point of the two and roughly three to four miles from the center of the fire zone to the center of the development, already tried to skirt development requirements related to fire and public safety.

A 2015 Voice of San Diego investigation found that developers behind the project refused to pay for a new fire station for the more than 3,000 additional residents they would bring in to the area. The project would build 1,746 homes in a mostly rural area where current restrictions allow only 110 homes.

In fall 2015, the county’s Planning Commission recommended the Board of Supervisors approve the project, but with several changes – including funding the construction of a new fire station.

But instead of agreeing to that stipulation, Lilac Hills’ developers eventually opted to instead put the project before voters last November as Measure B. The initiative also specified ways the project could avoid certain safety investments.

For instance, the developer had asked the county for exceptions to various road standards, so they wouldn’t have to flatten hills, or widen country roads to accommodate the influx of traffic the development would bring.

In the initiative, they simply said that West Lilac Road – which was the easternmost boundary of the Lilac Fire, and one of the roads the Planning Commission said needed widening – didn’t need to be changed.

The Lilac Hills ballot measure failed, but the project and others that would require amendments to the county general plan are not dead.

Keeley said there are known avenues to make at-risk developments safer, but politicians need to demand them.

“People already know what the best thing to do in those cases is,” Keeley said. “If you ask any fire chief in the county, they’ll tell you, you got to have access roads, you’re going to need a way to get firefighters in the area. Fire chiefs in San Diego County know pretty clearly what makes development risky, and they do have a lot of suggestions how to make those developments succeed. If developers are trying to avoid that, that’s a political question.”

Halsey also said politicians should work on retrofitting existing buildings and homes to improve their fire safety – for example, by putting in automatic external sprinkler systems that could wet the entire house during wildfire risk.

“The climate is changing,” Halsey said. “If we’ve built in wildland areas, there are things we can do to strengthen them.”