What Is the WUI?

The wildland-urban interface (WUI) is where homes and wildland vegetation meet or intermingle. It is where wildfires become disasters, and most of San Mateo County's hillside and coastal communities are in it.

Defining the WUI

What exactly constitutes the WUI has been debated for decades. The National Wildfire Coordinating Group defines it as "the line, area, or zone where structures and other human development meet or intermingle with undeveloped wildland or vegetative fuels." The National Fire Protection Association defines it by outcome: locations where topography, fuels, weather, and wind create the potential for structures to ignite from the flames and firebrands of a wildland fire.

Two forms are commonly distinguished. In interface communities, development sits sharply against wildland, as along the edges of our open space preserves. In intermix communities, homes are scattered through the vegetation, as in much of the Skyline corridor and the coastside canyons. Intermix communities face fire on all sides; interface communities face a defined front plus the embers it launches.

Why the Definition Matters Less Than the Exposure

Definitions drive grant eligibility and code applicability, but the practical question for a homeowner is exposure, not classification. Wind-driven embers travel a mile or more, which puts neighborhoods well inside city limits within reach of a wildland fire, a lesson the 2025 Los Angeles fires taught at scale. Even urban parks and open space "islands" create interface conditions blocks from downtown. If embers can reach your roof, the WUI includes you.