Methodology

Defensible Space Zones

California law (PRC § 4291) defines two primary defensible space zones around structures. Our assessment evaluates compliance with both. For more info on what defensible space is and why it matters, see the learning resources.

• Zone 0 — (0–5 ft)

  The area immediately adjacent to the structure. California AB 3074 (2021) established this zone, requiring all combustible materials to be removed including mulch, wood debris, and vegetation. Non-combustible hardscaping and ember-resistant vents are very relevant. This zone directly determines whether embers landing near your home ignite it.

• Zone 1 — Lean, Clean & Green (5–30 ft)

  The highest-priority vegetation zone surrounding your home. Plants must be well-irrigated and spaced to prevent fire from reaching the structure. Dead plants, dry leaves, and wood piles must be removed. This zone has the most direct impact on whether a home survives a wildfire.

• Zone 2 — Reduced Fuel (30–100 ft)

  A wider buffer where vegetation density and spacing are managed to slow the spread of fire toward the structure. Trees should be spaced so canopies don't touch, and shrubs should be pruned and thinned. This zone is especially important on slopes, where fire spreads faster uphill.

Inputs to the Assessment

The analysis draws from three sources of information. For technical details on how each is collected and delivered, see Data Sources.

• User-drawn polygons — the geometry you draw around your structures establishes the footprint from which the zone boundaries are computed
• Property questionnaire (Coming SOON) — answers about vegetation type and density, roof and siding materials, slope, access road width, and other site-specific factors that can't be derived from aerial data alone
• Building footprint data — known structure outlines used to cross-check drawn polygons and identify any unclaimed structures on the parcel
• Historical and regional data — we use the USFS Wildfire Hazard Potential (WHP) dataset to set a national baseline risk level for your area. WHP covers all 50 states at 270 m resolution and is updated annually by the U.S. Forest Service.

How the Score Is Calculated

The overall risk score runs from 0 to 100. A higher score means higher risk. It is the sum of four components, each capped independently:

• ML tree analysisup to 35 pts

  Points are added for trees physically touching your structure (13 pts each) and for spacing violations in each zone. This is the strongest individual predictor of structure loss in a wildfire, based on research across San Diego County fires 2001–2010.

• Zone 0 questionnaireup to 30 pts

  Questions about roof material, siding, firewood storage, mulch, and debris within the immediate 5-foot ember zone.

• Zone 1 questionnaireup to 20 pts

  Questions about foliage health and ladder fuels in the 5–30 ft zone.

• Wildfire Hazard Potential (WHP)up to 15 pts

  A landscape-level context multiplier sourced from the USFS WHP dataset — Very High (15 pts), High (12 pts), Moderate (8 pts), Low (4 pts), or Very Low (1 pt). WHP integrates fuels, weather, topography, and fire history to quantify the relative potential for wildfire to affect resources and assets across all 50 states.

Zone 2 questionnaire answers are scored and shown in the per-zone report cards but do not contribute to the overall score — research does not support individual-structure protection benefits beyond 30 ft.

Rating Bands

• Very High75 – 100
• High50 – 74
• Medium25 – 49
• Low0 – 24

If any tree is detected touching your structure, the score is raised to at least 25 regardless of other factors, guaranteeing a minimum Medium rating.

What the Score Means

The score is a relative risk estimate and is not a formal inspection result, legal certification, or insurance determination. It is designed to identify which aspects of your property most need attention and prioritize the highest-impact improvements. Always consult your local fire agency for authoritative guidance.

Actively Evolving

The scoring algorithm is under active development. If you have feedback on the methodology or access to relevant datasets, we'd love to hear from you.