California's First Cat Model Approval Reshapes Property Rate Filings

From tracking California rate filings for over a decade, this is the most consequential shift in property pricing methodology since Proposition 103 became law in 1988. For 36 years, California regulators required insurers to set homeowners rates using the average of actual losses over the prior 20 years. That constraint collapsed in late 2024 when the California Department of Insurance finalized regulations under Commissioner Ricardo Lara's Sustainable Insurance Strategy, permitting forward-looking catastrophe models and reinsurance cost inclusion in rate filings for the first time. The CDI completed its evaluation of the first wildfire catastrophe model on July 24, 2025, and by August 15, Mercury Insurance had submitted California's first rate filing under the new framework. The market implications are already visible: surplus lines homeowners policies surged from roughly 50,000 in 2023 to 320,000 in 2025, FAIR Plan policies jumped 45% in 2024 alone, and five admitted carriers have filed identical 6.9% rate increases signaling the first wave of model-based pricing.

Wildfire cat models approved by CDI: Verisk (July 24), KCC (August 1), Moody's RMS (August 4, 2025)

6.9%

Average rate increase filed by Mercury, CSAA, USAA, Pacific Specialty, and California Casualty under the new framework

540%

Growth in California surplus lines homeowners policies from 2023 to 2025 (50,000 to 320,000)

How Catastrophe Models Generate Average Annual Loss

The core output that replaces historical incurred losses in a California rate filing is the average annual loss, or AAL. A catastrophe model produces this estimate by simulating tens of thousands of synthetic wildfire seasons, each drawn from a stochastic event set that represents the full range of plausible fire ignition, spread, and suppression outcomes. Each simulated season generates a set of fire events with specific locations, intensities, and wind-driven spread patterns. The model then overlays each event against the insurer's geocoded exposure data, applying vulnerability functions that translate fire intensity into structural damage ratios based on construction type, roof material, defensible space, and vegetation proximity.

The result is a loss distribution across all simulated seasons. The mean of that distribution is the AAL: the probability-weighted expected loss per year. For a given territory or ZIP code, the AAL captures not just the average historical fire frequency but also the conditional severity given ignition, accounting for topography, fuel loading, wind corridors, and structural density that backward-looking data captures only indirectly through whatever fires happened to occur during the 20-year experience window.

Three models have now completed CDI review. Verisk's Wildfire Model for the United States was first, finishing the six-month evaluation on July 24, 2025. Karen Clark and Company's US Wildfire Reference Model Version 3.0 followed on August 1, and Moody's RMS U.S. Wildfire Model v2.0 completed review on August 4. Each model incorporates climate change impacts and community-level mitigation efforts, factors that historical loss data inherently lags by years or decades.

Replacing Historical Incurred Losses in the Rate Indication Formula

The standard property rate indication formula under the new regime becomes:

Indicated Rate = (AAL + Non-Cat Expected Loss + ALAE Loading + ULAE Loading + Fixed Expenses + Net Cost of Reinsurance) / (1 − Variable Expense Ratio − Profit & Contingencies)

The shift from backward-looking historical losses to forward-looking AAL eliminates two components that pricing actuaries previously spent significant effort selecting: catastrophe loss development factors and catastrophe loss trend factors for the wildfire peril. Under the old framework, the actuary had to develop historical catastrophe losses to ultimate using standard development triangles, then trend those losses from the midpoint of the experience period to the midpoint of the prospective policy period. Both steps introduced judgment and uncertainty, particularly when the 20-year window contained either an anomalously active wildfire period or an anomalously quiet one.

With AAL from a cat model, the modeled loss already represents a fully developed, fully trended expected annual loss. The model's stochastic event set is calibrated to current conditions, not historical conditions at some midpoint year. This is a genuine simplification for the wildfire component.

Non-catastrophe losses, including theft, water damage, and liability, still require traditional development triangles and trend selections. A California homeowners filing under the new framework is therefore a hybrid: cat model output for wildfire (and potentially earthquake, if a separate model is used), combined with standard actuarial methods for non-catastrophe perils. The actuary must clearly delineate which portion of the expected loss emerges from each methodology, a documentation requirement that CDI reviewers will scrutinize.

Reinsurance Cost Inclusion Changes the Expense Loading

The second pillar of the Sustainable Insurance Strategy, finalized on December 30, 2024 and effective January 2025, permits insurers to include the net cost of reinsurance in rate filings. Under Proposition 103, reinsurance costs were excluded from the expense loading for all lines except earthquake and medical malpractice. The new regulation treats reinsurance like other operating expenses: claims handling, agent commissions, and general overhead.

The net cost of reinsurance is calculated as ceded premium minus expected ceded recoveries. For a property cat excess-of-loss treaty, the ceded premium is the treaty rate applied to the subject premium base, and the expected ceded recoveries represent the expected value of claims that pierce the attachment point. The difference, the net cost, reflects the margin that reinsurers charge above expected losses for bearing tail risk, covering their own expenses, and earning a profit.

The allocation of net reinsurance cost across territories is where the pricing mechanics become consequential. Because the cat excess treaty responds primarily to wildfire losses, the net cost should be allocated in proportion to each territory's contribution to the modeled AAL. A territory generating 5% of the portfolio's total AAL should bear 5% of the net reinsurance cost. This creates a direct link between wildfire exposure and the expense loading at the territorial level, amplifying the rate differential between high-fire and low-fire territories beyond what the AAL alone would produce.

Before this regulation, insurers bore reinsurance costs as an unrecoverable overhead item that compressed margins uniformly across their book. Now that the cost can be passed through, territories with high wildfire AAL face a compounding effect: higher expected losses from the cat model and a proportionally higher reinsurance cost allocation. The CDI has established a standard cost of reinsurance and caps the amount that can be charged, preventing unlimited pass-through, but the directional effect on territorial rate dispersion is significant.

The 85% Writing Mandate as a Cross-Subsidy Mechanism

The third pillar of the Sustainable Insurance Strategy introduces a regulatory constraint that directly limits how territorial relativities can be set. Under Cal. Code Regs., tit. 10, section 2644.4.8, major insurers utilizing catastrophe models must increase their writing of comprehensive policies in wildfire-distressed areas to at least 85% of their statewide market share, with a two-year compliance timeline and a three-year coverage maintenance requirement for qualifying policyholders.

This writing mandate functions as a cap on territorial relativities in wildfire zones. Consider an insurer whose cat model indicates that wildfire-territory AAL is four times the statewide average. Without the writing requirement, the insurer could set wildfire-territory rates at the full actuarially indicated level, or simply decline to write in those territories. The 85% mandate removes the second option and constrains the first: if the insurer must maintain substantial market share in wildfire-distressed areas, setting rates at the full indicated level risks pricing itself out of those areas and failing the writing test.

The practical result is an implicit cross-subsidy. The insurer absorbs some portion of the wildfire cost through two channels. First, the territorial relativity for wildfire zones is set below the actuarially indicated level. Second, the residual cost is spread to lower-risk territories through a higher base rate. Policyholders in Sacramento, Fresno, and other low-fire-risk cities pay more than their territory's standalone actuarial indication would warrant, subsidizing the insurer's obligation to write in the wildfire interface zones of the Sierra Nevada foothills, Sonoma County, and the greater Los Angeles basin.

Commissioner Lara framed the mandate as a market access guarantee: "For the first time in California history, insurance companies will be required to write more policies in wildfire-distressed areas." From a pricing perspective, it is a regulatory constraint on the rate indication that forces departures from cost-based territorial pricing.

The E&S Wildfire-Exposure Paradox

While the admitted market restructures around cat models and writing mandates, the surplus lines market has absorbed a historically unprecedented volume of displaced homeowners. Surplus lines homeowners policies surged from approximately 50,000 in 2023 to over 320,000 in 2025, a 540% increase in two years. First-half 2025 transactions alone hit 171,551, a 119% increase over the same period in 2024.

The composition of this growth contradicts the assumption that E&S expansion reflects worsening wildfire hazard. A wildfire exposure metric measuring wildland fuel proximity on a 0-to-1 scale fell from 0.44 in 2020 to 0.34 in 2023 to 0.20 in 2025 across the E&S homeowners book. In 2023, approximately 80% of E&S placements were urban; by 2025, that share had risen to roughly 90%, with suburban and rural placements in the low single digits.

This pattern reveals that the surplus lines surge is driven by coverage scarcity, not by hazard-priced risk. Ordinary suburban and urban homeowners who lost admitted market access, whether through carrier non-renewals, underwriting tightening, or geographic withdrawal, migrated to E&S carriers. The policies they bring are, on average, lower-risk than the wildfire-zone exposures that originally defined the California E&S homeowners market. Average premiums per policy declined 25% in 2025 versus 2024, consistent with a book that is becoming less hazard-concentrated even as it grows.

For pricing actuaries at surplus lines carriers, this compositional shift means the portfolio's expected loss ratio is improving through mix change even as policy counts multiply. The strategic question is whether the Sustainable Insurance Strategy's admitted-market reforms will eventually pull these urban displacements back into the admitted market, shrinking the E&S book as quickly as it grew.

FAIR Plan Rate Adequacy and the Pricing Floor

The California FAIR Plan sits at the intersection of all three market segments. FAIR Plan policies rose 276% from 2018 through 2024, with a 45% year-over-year jump in 2024 alone. Premium reached $1.4 billion in 2024, up more than 15 times from $87.2 million in 2018. Reinsurance ceded premium escalated from $8 million to $169 million over the same period.

The FAIR Plan filed a 35.8% statewide rate increase effective April 1, 2026, the first FAIR Plan filing to incorporate forward-looking cat models and net cost of reinsurance. Territory-level dispersion ranged from cuts of up to 78% in low-risk Central Valley ZIP codes to increases exceeding 300% in some Sonoma and Sierra Nevada areas. That dispersion reflects the full actuarial indication from cat model output without the cross-subsidy constraint that the 85% writing mandate imposes on admitted carriers.

FAIR Plan premiums effectively establish a pricing floor for surplus lines carriers. An E&S carrier writing in a territory where the FAIR Plan charges $5,000 can price above that level with confidence that the policyholder's alternative is either the FAIR Plan (limited coverage, assessment risk) or no coverage at all. As the FAIR Plan rate level adjusts upward to reflect cat model output, the E&S pricing floor rises in tandem, supporting rate adequacy across the non-admitted market even as portfolio-wide hazard metrics decline.

Mercury Insurance's filing illustrates the admitted-market response. Mercury targets growing its high-risk book by 15% and shifting 6.5% of FAIR Plan policyholders to its own coverage over eight years. Combined with expanded wildfire mitigation discounts that could reduce the wildfire premium portion by up to one-third, Mercury is positioning to compete for FAIR Plan depopulation on a model-informed pricing basis. CSAA has already written 18,300 more policies in high-hazard areas than required, and its "My Home Hardening" program offers up to 12.5% discounts for completing wildfire mitigation.

What This Means for California Property Pricing

The transition from 20-year historical averages to stochastic cat model output is not a marginal adjustment. It changes the information set that determines the indicated rate level. Historical data reflects whatever fires happened to burn during the experience period, weighted toward recent years through trend factors but fundamentally backward-looking. Cat model output reflects the full distribution of fires that could burn given current conditions, weighted by probability rather than historical accident.

For territories where wildfire risk has increased since the midpoint of the 20-year experience period (due to development patterns, vegetation changes, or climate shifts), cat model AAL will exceed trended historical losses. Rates in those territories will rise. For territories where historical experience included anomalously severe fire seasons that the model treats as tail events rather than base expectations, rates may moderate. The net effect across the state depends on the model's statewide AAL relative to the trended historical loss level, a comparison that will become visible as more filings move through CDI review.

The combination of model-based pricing, reinsurance cost pass-through, and the 85% writing mandate creates a regulatory equilibrium that differs fundamentally from every other state's approach to property cat pricing. Insurers can now price wildfire risk forward, but they cannot fully act on that pricing by concentrating their book in low-risk territories. The writing mandate ensures that admitted carriers remain exposed to wildfire-zone losses at volumes proportional to their statewide presence, while the reinsurance cost provision allows them to recover some of the tail-risk transfer cost they incur as a result.

Travelers notified CDI of expansion plans in April 2026, and Farmers pledged to market to 300,000 consumers in high-risk zones starting in 2026. These commitments suggest the Sustainable Insurance Strategy is achieving its stated goal of expanding admitted-market capacity in wildfire-distressed areas. Whether the 6.9% average rate increases prove adequate as cat model output replaces historical loss data across more of the book is the pricing question that will define California property insurance through 2027 and beyond.