Site Design Criteria by ZIP Code

Look Up Your Site Design Criteria

Enter a ZIP code. The result lists the county, the 2021 IECC climate zone, the seismic design category, the EPA radon zone and the mapped soil at that ZIP, plus the statewide frost line, ground snow load and wind speed with links to the pages that hold the local values.

What Each Number Is Used For

Every value in the lookup comes up on a residential permit. Most of them live in IRC Table R301.2(1), the climatic and geographic design criteria table that each jurisdiction fills in when it adopts the code. The others come from the energy code, the soils section and the EPA. This table says where each number is set, what it changes, and where on this site to use it.

Site Design Criteria by State

One row per state and DC. Climate zones and seismic categories are the set that occur in the state. Radon is the most common EPA zone among the state's counties. Soil orders are the two most common at ZIP centers. Frost and snow link to the pages that hold the city-level values. A statewide range is a planning number; the lookup above gives the county and point values for a ZIP.

Seismic design category assumes Risk Category II and Site Class D under ASCE 7-16. Wind is the ASCE 7-16 Risk Category II range across the state; coastal and special wind regions run higher.

How to Read Your Result

A ZIP is not a parcel

The lookup works on ZIP Code Tabulation Areas, the Census Bureau's map of ZIP codes. Each one is matched to the county that holds most of its land, and point values are read at the center of the area. A ZIP that straddles a county line, a ridge or a fault can have different values at your lot. PO box and single-building ZIPs have no area, so they return statewide values.

Three kinds of rows

County rows: climate zone and radon zone are assigned by county, so these are exact for any ZIP inside the county.

Point rows: seismic category and soil type are read at the ZIP center. They are right for most of the ZIP and can be off at its edges.

Statewide rows: frost line, ground snow load and wind speed are shown as the state average or range here. The frost line page has the code depth for 47 cities and the state guides, the snow load page has capitals and major cities, and the ASCE Hazard Tool has the site wind speed.

What the seismic row assumes

The category comes from the USGS design-value service for ASCE 7-16, Risk Category II, Site Class D. Site Class D is the default ASCE 7-16 Section 11.4.3 allows when the soil has not been tested; softer soil (Site Class E) or a soils report showing rock can move the category. Where S₁ is 0.2 g or more, the service leaves the category blank, because ASCE 7-16 Section 11.4.8 asks for a site-specific study on Site Class D. The category is still fixed by Tables 11.6-1 and 11.6-2: D, or E once S₁ reaches 0.75 g. About 3,800 ZIPs, most in California, Washington, Oregon and Utah and along the New Madrid zone, get their letter that way. The 2018 and 2021 IRC and IBC reference ASCE 7-16. The 2024 editions reference ASCE 7-22, which uses multi-period spectra, and some ZIPs change category between the two. Check which edition your jurisdiction has adopted.

What the soil row means

The soil row names the dominant soil series in the USDA soil survey (SSURGO) at the ZIP center, its texture at 36 in, and its Unified Soil Classification group. The group maps to a presumptive load-bearing value in IRC Table R401.4.1: 3,000 psf for gravel, 2,000 psf for sand and silty or clayey sand and gravel, 1,500 psf for clay and silt. Dense cities often show "no mapped soil" because the survey maps urban land as disturbed. The presumptive value is a floor. The building official can accept it or require a test under R401.4.

The permit number wins

Every value here is a planning number. The jurisdiction's adopted Table R301.2(1), its amendments and the inspector's reading of them are what the permit is checked against. Use this page to know what to expect, size the pour with the calculators, and confirm the numbers with the building department before you order concrete.

Where the Numbers Come From

ZIP geography. The Census Bureau's 2020 ZIP Code Tabulation Area to county relationship file gives each ZIP's counties and how much land falls in each; the 2024 Gazetteer gives the center point. Territories are left out.

Climate zone. The county list behind 2021 IECC Table R301.1, published by Pacific Northwest National Laboratory with report PNNL-33270 (September 2022). About 370 counties changed zone or moisture letter from the 2015 and 2018 editions, most to a warmer zone.

Seismic. The USGS design-value web service for ASCE 7-16, queried once per ZIP center for Risk Category II and Site Class D. The service returns the mapped spectral accelerations S_S and S₁, the site-adjusted S_DS and S_D1, and the resulting category.

Radon. The EPA Map of Radon Zones county table. Zone 1 counties have a predicted average indoor level above 4 pCi/L, Zone 2 between 2 and 4, Zone 3 under 2. The EPA map predates several county renamings in Alaska and South Dakota; those are matched to their current names.

Soil. The dominant component of the SSURGO map unit at each ZIP center, from the University of California, Davis SoilWeb service, widened to 2 and 5 km where the center falls on unmapped land. Texture and the Unified classification for the horizon at 36 in come from the USDA Soil Data Access service.

Frost, snow and wind. Frost line averages and ground snow load ranges are the values on this site's frost line and snow load pages, with their citations. Wind is a statewide range read from ASCE 7-16 Figure 26.5-1B.

Data vintages: IECC 2021 Table R301.1. EPA Map of Radon Zones, county table (epa.gov, 2024-05 spreadsheet). USGS design-value web service, ASCE 7-16, Risk Category II, Site Class D (default); where S1 >= 0.2 leaves the service category blank (§11.4.8), the category is D, or E when S1 >= 0.75, per §11.6. SSURGO dominant component at the ZCTA centroid via SoilWeb (UC Davis); texture and USCS class from USDA Soil Data Access for the horizon at 36 in. ZIP geography: Census 2020 ZCTA to county relationship file; 2024 Gazetteer centroids. Coverage: 33,642 ZIP Code Tabulation Areas; seismic values for 33,641; a mapped soil component for 32,498, with 918 centroids on unmapped (mostly urban) land. Built 2026-10-25.

Frequently Asked Questions

Sources

1. U.S. Census Bureau. 2020 ZCTA to County Relationship File and 2024 Gazetteer Files, ZIP Code Tabulation Areas. census.gov. Retrieved October 24, 2026.
2. Pacific Northwest National Laboratory. Guide to Determining Climate Zone by County: Building America and IECC 2021 Updates, PNNL-33270, September 2022, and its county data files. basc.pnnl.gov.
3. International Code Council. 2021 International Energy Conservation Code, Section R301.1 and Table R402.1.3; 2021 International Residential Code, Table R301.2(1), R301.2.2, R401.4, Table R401.4.1, R403.1.4.1, Appendix AF.
4. U.S. Geological Survey. Seismic Design Web Services, ASCE 7-16. earthquake.usgs.gov. Queried October 2026.
5. American Society of Civil Engineers. ASCE 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Sections 11.4 and 11.6, Figure 26.5-1B; ASCE 7-22, Chapter 7. ASCE Hazard Tool.
6. U.S. Environmental Protection Agency. EPA Map of Radon Zones, county table. epa.gov.
7. USDA Natural Resources Conservation Service. Soil Survey Geographic Database (SSURGO) via SoilWeb (University of California, Davis) and Soil Data Access.