Exposures from neighboring parcels must be accounted for when assessing the defensibility of a property. The fully involved parcel (including structure, vehicle, and vegetation) seen above illustrates the very high fire exposures that can be generated during WUI fires. Fully involved fuels with flame lengths greater than 6 m (20 ft), as in the image above, would be difficult to contain even with several firefighting apparatus and cannot be contained by defensive actions by residents.

In this scenario, the structure separation distance (SSD) was 13 m (43 ft) from the burning home shown in the image to the neighboring structure. The structure to property line distance was 8 m (26 ft). Defensible space may be difficult to implement in moderate and high-density communities where significant fire exposures can originate from neighboring parcels and structures are spaced even closer than in this example.