Blog · Industry
First Street vs. Buildability Score™: Two Scores, Two Questions
Landon Reid · Founder, Buildability™2026-09-10 · 7 min
TL;DR: First Street scores natural hazard exposure at the parcel level — flood, fire, wind, heat, and air quality — using peer-reviewed models built with more than 80 scientists. The Buildability Score™ composes hazard and permission into one number: nine weighted factors that include those same physical risks plus zoning standards, lot geometry, soil suitability, and infrastructure access. First Street answers "what are the odds this property floods?" Buildability answers "can I build here, and what?" Both questions matter. The right one depends on the decision you are making.
The two questions
| First Street | Buildability Score™ | |
|---|---|---|
| Core question | What natural hazards face this property? | Can I build here, and what? |
| Axis | Hazard only | Hazard + permission, composed |
| Score range | 1–10 per factor | 0–100 composite |
| Factors scored | Flood, Fire, Wind, Heat, Air | Flood, wildfire, seismic, radon, soil, terrain, zoning, utilities, lot geometry |
| Zoning and permitted uses | Not scored | Yes — FAR, setbacks, lot coverage, ADU eligibility |
| By-right building rights | Not scored | Yes |
| Source model | Peer-reviewed physical science models | Weighted overlay suitability analysis (GIS-MCDA) |
| Distribution | Redfin, Realtor.com, API, bulk data | buildability.us, API, embeddable widget |
| Primary users | Buyers, lenders, insurers screening climate exposure | Developers, investors, homeowners screening build feasibility |
The overlap is intentional: hazard is a real input to feasibility. A lot fully inside a FEMA Special Flood Hazard Area carries both a First Street Flood Factor consequence and a Buildability Score consequence — the first tells you the probability and magnitude of the flood risk; the second tells you how that risk, combined with everything else on the parcel, affects whether building there makes financial and regulatory sense.
What First Street does extremely well
First Street's models are built to a scientific standard that is genuinely hard to match. Their Flood Model was developed in partnership with more than 80 scientists, validated against historical FEMA flood insurance claims, and has undergone peer review for publication in scientific journals. The same rigor applies to their Fire, Wind, Heat, and Air models.
The result is parcel-level hazard scores that appear on Redfin and Realtor.com listings — meaning a buyer, broker, or lender can see physical risk exposure directly on a property listing, before they ever contact a flood insurance agent or commission a site study. That distribution reach is significant: it put peer-reviewed climate risk data in front of millions of transactions.
First Street also publishes their methodology openly. The flood model documentation, the assumptions behind future climate scenarios, and the validation process are all available at firststreet.org/methodology. This transparency is what you want from a tool making parcel-level risk claims.
If the decision is "does this property face climate risk, and how severe is it?" — First Street is the right tool.
What Buildability Score™ does that First Street does not
First Street's scope is intentional: they score natural hazard exposure. They do not score what you can build.
That second question requires a different data layer entirely — the permission axis. Zoning ordinances, dimensional standards, overlay districts, ADU eligibility rules, lot coverage limits, floor area ratios, setbacks, and by-right building rights are not physical science problems. They are regulatory records: municipal codes, parcel databases, and GIS zoning layers that have to be queried, parsed, and interpreted against the physical site.
The Buildability Score™ composes both axes:
- Hazard factors (flood, wildfire, seismic, radon, soil, terrain) — drawn from FEMA, USGS, EPA, and USDA, queried live per fresh run
- Permission factors (zoning standards, utilities access, lot geometry) — drawn from Regrid for parcel geometry, Zoneomics for zoning codes and permitted uses, and OpenStreetMap for infrastructure
The nine factors combine into one 0–100 score using a weighted linear combination, with spatially varying weights by state. Florida weights flood at 24% (vs. a 14% national default) because flood exposure dominates feasibility decisions there. California weights terrain and seismic higher because hillside grades and fault zones are the constraints that actually kill projects in that market. Missing factors drop out of the calculation entirely — they are not replaced with assumed values.
One mechanism worth naming: if a single factor is severe enough to determine the outcome regardless of everything else, it caps the composite score at its own level. A lot rated "Very limited" for septic absorption fields gets a named cap banner in the report, with its USDA source cited. A strong average does not bury the finding that changes the decision.
A worked scenario: the ADU on a coastal lot
A homeowner in Sarasota, Florida wants to add a detached ADU to a lot they already own. Two questions follow immediately.
Question 1: What is the flood risk?
First Street's Flood Factor answers this. Sarasota County has significant coastal flood exposure. The homeowner can see their property's 1–10 Flood Factor on Redfin or Realtor.com, understand the 30-year cumulative flood probability, and decide whether flood insurance cost is a deal-breaker for the project.
Question 2: Can I build an ADU here, and under what constraints?
First Street does not answer this. The Buildability Score™ does. It checks whether the parcel's zoning district permits ADUs by right under Florida law and local ordinance, what setback clearances apply (Florida's rear and side setback minimums for ADUs vary by city), whether the existing structure plus ADU footprint would exceed lot coverage limits, what FAR the zone allows, and — critically in a coastal location — how much of the lot sits inside the mapped Special Flood Hazard Area. The SFHA overlap factors into both the flood sub-score and the overall composite.
The homeowner who checks both scores before calling a contractor arrives with the full picture: the physical risk they are accepting and the regulatory envelope they are building inside. Neither score alone gives them that.
A note on method lineage
The Buildability Score™'s underlying method is not new. Weighted overlay suitability analysis was formalized by Ian McHarg in Design with Nature (1969) and has been the methodological foundation of GIS-based land suitability assessment for more than fifty years. The modern form — GIS multi-criteria decision analysis (GIS-MCDA), with weighted linear combination and spatially varying weights — has been documented in more than 300 peer-reviewed papers and is the method your city's planning department uses when it models development suitability across its jurisdiction.
What is new here is the operationalization: parcel granularity across all 50 states, live-queried federal sources, and the composition of the hazard and permission axes into a single score per parcel. Prior productizations solved one axis at a time. First Street solved hazard at national scale. The Wharton Regulatory Land Use Intensity Index (WRLURI) solved permission at the jurisdiction level, periodically. The Buildability Score™ runs both axes on demand, per parcel, with sources cited per finding.
More detail on the method, including the Malczewski (2006) literature survey and the spatial sensitivity work behind the weights, is on the Technology page.
Frequently asked questions
Is Buildability™ trying to replace First Street?
No. First Street answers a hazard question at a level of scientific rigor and peer review that took years and more than 80 scientists to build. We are not in the hazard-modeling business. We are in the feasibility business — which requires composing hazard data (including FEMA's, which we query directly) with permission data. The products answer different questions for different decisions.
Which score matters more?
It depends on the decision. If you are a lender underwriting flood insurance cost, First Street's Flood Factor is the relevant number. If you are a developer deciding whether to make an offer on a parcel, the Buildability Score™ is more decision-relevant because it prices in both the physical constraints and the regulatory ones. For a buyer who wants to build an ADU: they need both.
Do you use First Street data inside the Buildability Score™?
The complete data-sources list is on the Data Sources page — federal APIs, commercial partners, and any modeled layers are named there. The hazard factors in the Score query FEMA, USGS, EPA, and USDA directly, which are the same underlying government datasets First Street's peer-reviewed models are built on top of.
If I have a high First Street Flood Factor, will my Buildability Score™ be low?
Not necessarily. Flood is one of nine factors, weighted at 14% by default (24% in Florida). A lot with significant flood exposure can still score well if the other eight factors are strong and the SFHA overlap is partial rather than total. What you will see is a low flood sub-score in the breakdown, a named finding in the report citing the FEMA source, and an overall composite that reflects the drag. If the flood exposure is severe enough — a lot fully inside a mapped floodway, for example — it can cap the composite regardless of other factors.