Washington adopts the 2021 International Residential Code statewide but publishes no frost depth of its own, leaving the number to each city or county to set in Table R301.2, so a reader digging in Washington has to ask the local building department for the frost line that applies to the parcel.
Washington leaves the frost line depth to the local building department, which is where the number for an address comes from.
R301.2 Climatic and geographic design criteria. Buildings shall be constructed in accordance with the provisions of this code as limited by the provisions of this section. Additional criteria shall be established by the local jurisdiction and set forth in Table R301.2.WAC 51-51-0301 Section R301 Design criteria, Washington State Building Code Council, read September 22, 2026. Read it at the source
That makes the building department issuing the permit the office with the number, and it is a short call: the counter gives the depth for the address, and the same call settles whether a deck footing or a slab edge needs a footing inspection before the concrete goes in. What Washington does publish is below, and it is worth having in hand first.
A frost line depth turns into two separate requirements in most codes: how far a footing has to bear below grade, and how far a buried water service has to sit below the same line. Washington writes one of them out.
Washington's own amendment to Section R403 sets only the minimum width and thickness of a footing, so the depth a footing has to reach is the frost line the local jurisdiction fills into Table R301.2 under the adopted 2021 International Residential Code.
R403.1.1 Minimum size. The minimum width, W, and thickness, T, for concrete footings shall be in accordance with Tables R403.1(1) through R403.1(3) and Figure R403.1(1) or R403.1.3, as applicable, but not less than 12 inches (305 mm) in width and 6 inches (152 mm) in depth.WAC 51-51-0403 Section R403 Footings, Washington State Building Code Council, read September 22, 2026. Read it at the source
The air freezing index is the quantity the frost provisions of the model codes are written on: add up how far each day’s mean temperature fell below freezing over a winter, in degree days, and take the deepest run the winter managed. It measures how much cold the ground had to absorb, which is why it is the honest way to set one place beside another. Spokane is the coldest of the 3 cities on the record in Washington.
394degree days
average freezing index a winter
Spokane Intl AP
| City | State | Freezing index | Once a century | Nights below freezing |
|---|---|---|---|---|
| Spokane | Washington | 394 | 1,184 | 134 |
| Yakima | Washington | 261 | 901 | 147 |
| Seattle | Washington | 21 | 95 | 30 |
Computed from NOAA National Centers for Environmental Information, Global Historical Climatology Network daily temperature records for each station, from 1951 onward. These are measurements of winter, not depths.
Only a handful of states answer with one number for the whole state, and they are the only codes that can be set beside each other directly. Ranked by the freezing index at their own reference cities, with Washington marked at 225 degree days for comparison against what those codes settled on:
Left to right, the mildest winters to the hardest. The figure on the right is what that state wrote into its code.
The nearest by winter are North Carolina at 12 inches, Massachusetts at 4 feet and Connecticut at 42 inches. That is a reading of other states’ codes rather than a depth for an address in Washington: soil, drainage, snow cover and local amendments all move the figure, and the building department issuing the permit is the office whose number binds.
Washington code read September 22, 2026 from WAC 51-51-0301 Section R301 Design criteria, Washington State Building Code Council. Codes are amended and a local amendment can be deeper than the state figure, so the building department issuing the permit has the number that binds. Every state is on the frost line depth hub.