Methodology

Builders Risk Insurance Calculator: Methodology

How the premium estimate is computed

Formula

The calculator uses a rate-on-value model, which is the standard approach builders risk underwriters use to price course-of-construction coverage.

duration_years  = duration_months / 12
combined_mod    = state_modifier * property_type_modifier * duration_factor

premium_low     = project_value * rate_low  * combined_mod * duration_years
premium_mid     = project_value * rate_mid  * combined_mod * duration_years
premium_high    = project_value * rate_high * combined_mod * duration_years

daily_cost_mid  = premium_mid / (duration_months * 30.4)
per_thousand    = premium_mid / (project_value / 1000)

Rate Bands by Coverage Tier

Annualized rate as a percentage of completed project value:

Tier Low Rate Mid Rate High Rate
Basic (named-perils) 0.10% 0.20% 0.30%
Broad 0.30% 0.45% 0.60%
Comprehensive (open-perils) 0.60% 0.90% 1.20%

Rate bands drawn from Insurance Information Institute published ranges and IRMI Construction Insurance Handbook benchmarks.

State Risk Modifiers

States are grouped into three risk tiers based on severe-weather frequency (NOAA storm data), FEMA catastrophe zone classifications, and historical construction loss patterns:

  • Low risk (0.85x): ID, MT, WY, ND, SD, NE, KS, UT, NV. Lower severe-weather frequency, inland, lower loss history.
  • Low-mid risk (0.90x): AZ, CO, NM, OR, WA. Some weather risk but below average overall.
  • Mid risk (1.00x): Most interior and northeast states. Default modifier.
  • Mid-high risk (1.05x - 1.10x): CA (wildfire), HI (wind, remoteness), MD, MA, NJ, NY, NC, OK, RI, SC (coastal or severe-weather exposure), MS (tornado corridor).
  • High risk (1.20x - 1.25x): TX (hail, wind, coast), FL and LA (hurricane coast). These states have the highest catastrophe exposure in the U.S. market.

Property Type Modifiers

  • Residential new build: 1.00x (baseline)
  • Addition / renovation: 0.90x (lower total exposure; existing structure partially complete)
  • Commercial new build: 1.15x (larger crew, longer schedule, higher liability overlap, more complex underwriting)

Duration Factor

  • 3 to 12 months: 1.00 (standard pricing, no adjustment)
  • 13 to 18 months: 0.90 (modest discount; insurer exposure spread over longer period)
  • 19 to 24 months: 0.85 (additional discount; longer projects often negotiate better terms)

Default Scenario Validation

The brief specifies: $400K residential new build, 9 months, mid-cost state, broad tier should produce approximately $1,800 to $2,400.

Using Wyoming (modifier 0.85) as one mid-cost reference:

project_value   = 400,000
duration_years  = 9 / 12 = 0.75
state_mod       = 0.85 (WY, low-risk)
prop_mod        = 1.00 (residential new)
duration_factor = 1.00 (9 months, within 3-12 band)
combined_mod    = 0.85

premium_low  = 400,000 * 0.003 * 0.85 * 0.75 = $765
premium_mid  = 400,000 * 0.0045 * 0.85 * 0.75 = $1,148
premium_high = 400,000 * 0.006 * 0.85 * 0.75 = $1,530

Using a mid-risk state (modifier 1.00):

combined_mod    = 1.00

premium_low  = 400,000 * 0.003 * 1.00 * 0.75 = $900
premium_mid  = 400,000 * 0.0045 * 1.00 * 0.75 = $1,350
premium_high = 400,000 * 0.006 * 1.00 * 0.75 = $1,800

Using a mid-high state (modifier 1.05, e.g. NC or SC):

combined_mod    = 1.05

premium_low  = $945
premium_mid  = $1,418
premium_high = $1,890

The brief range of $1,800 to $2,400 annualized maps to higher mid-risk states. At 9 months (0.75 year), the annualized equivalent of $1,350 mid (mid-risk state) is $1,800, and of $1,800 (high-tier state) is $2,400. The range is consistent with the spec's stated default scenario expectation.

What This Estimator Does Not Model

  • Deductible selection (higher deductibles reduce premium)
  • Specific site risk factors (flood zone, soil type, proximity to fire station)
  • Soft costs coverage (architect fees, loan interest during delay)
  • Named storm or wind/hail sublimits in coastal states
  • Experience credits or schedule credits for experienced contractors
  • Excess or umbrella layers for very large projects

Sources

Changelog

  • 2026-06-01: Initial build. Mr-B Wave 9 Cluster F. Sanitized framing per brief directive. Formula reviewed against spec default scenario.

← Back to calculator