Frost Line Depth in Snohomish County, Washington
- Snohomish County’s adopted code
- Checked Oct 2026
- Sources
Verified
18in
46 cm · 1 ft 6 in
Set in the building code Snohomish County has adopted.
Source and how we checked
Document: snohomishcountywa.gov
Climate: 53 nights a year at or below 32 °F (NOAA 1991–2020, Startup 1 E, 13.2 mi).
Checked Oct 2026. Report a correction
How deep is the frost line in Snohomish County?
Snohomish County's building authority sets the frost line depth at 18 in (46 cm). Footings, piers and other permanent supports must reach at least this depth (IRC R403.1.4.1).
Who sets the frost line depth in Snohomish County?
Washington leaves the frost line depth to each city or county, and Snohomish County fills it in when it adopts the residential code. The value above comes from Snohomish County’s own document on snohomishcountywa.gov. See how Washington sets frost line depth.
How deep do footings, decks and water lines need to be in Snohomish County?
House or garage footing in Snohomish County: dig at least 18 in (46 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).
| Project | Depth | Rule |
|---|---|---|
| House or garage footing | 18 in | Below the frost line and at least 12 in deep (IRC R403.1.4) |
| Deck footing (attached) | 18 in | Below the frost line and at least 12 in deep (IRC R507.3) |
| Water service line | 24 in | 6 in below the frost line and at least 12 in deep (IRC P2603.5) |
| Fence post bottom | 18 in | No code rule; common practice is below the frost line |
Work out posts, concrete and footing sizes with the fence post depth calculator or the deck footing calculator.
How cold does it get in Snohomish County?
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Snohomish County never averages a low below freezing. 53 nights a year reach 32 °F or colder; Dec is coldest, with an average low of 33.6 °F.
NOAA 1991–2020 climate normals, Startup 1 E station, 13.2 mi away, elevation 170 ft.
Monthly temperatures as a table
| °F | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Low | 34 | 34 | 36.1 | 39.3 | 44.4 | 48.8 | 51.6 | 51.4 | 47.3 | 41.6 | 37.1 | 33.6 |
| High | 47.4 | 50.7 | 54.7 | 59.9 | 66.6 | 70.7 | 77.6 | 78.4 | 72.5 | 61.7 | 52.1 | 46.5 |
Snohomish County has fewer freezing nights than 25 of the 47 other Washington places we track. The Washington median is 54. Coldest: Chelan County; mildest: Lakewood.
What frost line depth do towns near Snohomish County use?
5 official codes near Snohomish County are verified; each links to its document.
| Place | Verified frost line | Distance |
|---|---|---|
| Lake Stevens | 12 in | 16 mi away |
| Marysville | 18 in | 18 mi away |
| Everett | 18 in | 21 mi away |
| Lynnwood | 18 in | 29 mi away |
| Redmond | 12 in | 31 mi away |
How likely is the soil in Snohomish County to heave?
USDA rates the sampled soil around Snohomish County as 32% low, 58% moderate, 10% high for frost action (main soils: Typic Udivitrands, Skykomish, Aquandic Dystrudepts).
USDA NRCS soil survey, 9 sample points within 2 mi. Higher frost action means more risk of heave; see frost heave.
How do you confirm the frost line before you dig in Snohomish County?
Snohomish County at a glance
- Frost line depth
- 18 in
- Population (2023 est.)
- 844,761
- Mean annual temperature
- 51.6 °F
- ZIP codes (30)
- 98012 · 98020 · 98021 · 98026 · 98036 · 98037 · 98043 · 98072 · 98077 · 98087 · 98201 · 98203 · 98204 · 98207 · 98208 · 98223 · 98241 · 98251 · 98252 · 98256 · 98258 · 98270 · 98271 · 98272 · 98275 · 98282 · 98290 · 98292 · 98294 · 98296