Defensible Space refers to the area surrounding a building, which is analyzed to provide risk assessment data for properties. The area surrounding a building is divided into zones as explained below. The API measures coverage across different zones around structures and identifies potential risk objects, such as vegetation and yard debris, that could affect a building’s vulnerability.
Defensible Space results are used as inputs to calculate the following peril scores:
- Hurricane Vulnerability Score
- Wildfire Vulnerability Score
- Wind Vulnerability Score
The data is calculated in real time and introduces some latency; therefore, as a default, it is excluded from the AI Features API response. Detailed information is provided in the sections below.
Quick info
Defensible Space zones
Defensible Space is calculated in three concentric zones around each building:
- Zone 1: 0 to 5 feet (0 to 1.524 meters)
- Zone 2: 5 to 30 feet (1.524 to 9.144 meters)
- Zone 3: 30 to 100 feet (9.144 to 30.48 meters)
These zones are generated as ring geometries around the building and explicitly exclude:
- The interior of the building
- Overlapping inner zones - For example, Zone 2 only includes the area between 5-30 feet, not 0-30 feet. Each zone represents an exclusive ring.
A building is represented by the Roof class. If the Roof class represents a damaged or destroyed roof, the Building Lifecycle class will be used instead.
Risk objects
Risk objects are features detected within Defensible Space zones that may impact building vulnerability. The following risk object classes are included:
For each zone, the API provides:
- Total area covered by the risk objects detected
- Coverage ratio (risk object area / zone area)
- Individual breakdown by risk object type
- Clipped geometry of risk objects within the zone
Coverage Calculations
Defensible space coverage for a given zone is calculated using the following formula.
defensibleSpaceCoverageRatio = (zoneArea - totalRiskObjectArea) / zoneArea
This metric takes all risk object types in the zone into consideration. Higher coverage ratios indicate more defensible space (that is, less risk).
For example:
- Zone 3 Area: 4,349.7 sqm
- Total Risk Object Area: 2,039.7 sqm
- Defensible Space Area: 2,310.0 sqm
- Coverage Ratio: 2,310.0 / 4,349.7 = 0.531 (53.1%)
In the example above, the coverage ratio of 53.1% indicates that 53.1% of the Defensible Space area is free from risk objects while the remaining 46.9% area has risk objects.
Building-Level vs Property-Level Results
- Building-Level Results - Defensible Space results are provided individually for each building* on a property. This allows for detailed analysis of each structure's surrounding area.
- Property-Level Results (Aggregate) - The aggregate field provides a property-wide view based on the union of all buildings*. This aggregate calculation:
- Combines all footprints on the parcel
- Generates union of zones around the footprints
- Calculates overall defensible space and risk object coverage
- Provides a single comprehensive view of the entire property
Both views are available in the response to support different analysis needs.
* A building is represented by the Roof class. If the Roof class represents a damaged or destroyed roof, the Building Lifecycle class will be used instead.
Parcel Mode behavior
For information about the AI Feature API’s behavior when parcelMode=true is included in the request, see the article on Parcel Mode.