Frost Line Depth in King County, Washington
- King County’s adopted code
- Checked Oct 2026
- Sources
Verified
12in
30 cm · 1 ft
12 in Below 1,000 ft, Deeper Above
Set in the building code King County has adopted.
Source and how we checked
Document: aqua.kingcounty.gov
Climate: 54 nights a year at or below 32 °F (NOAA 1991–2020, Snoqualmie Falls, 3.5 mi).
Checked Oct 2026. Report a correction
How deep is the frost line in King County?
King County's building authority sets the frost line depth at 12 in (30 cm) up to 1,000 ft elevation, and 18 in (46 cm) or more at higher sites. Footings, piers and other permanent supports must reach at least this depth (IRC R403.1.4.1).
Who sets the frost line depth in King County?
Washington leaves the frost line depth to each city or county, and King County fills it in when it adopts the residential code. The value above comes from King County’s own document on aqua.kingcounty.gov. See how Washington sets frost line depth.
How deep do footings, decks and water lines need to be in King County?
House or garage footing in King County: dig at least 12 in (30 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).
| Project | Depth | Rule |
|---|---|---|
| House or garage footing | 12 in | Below the frost line and at least 12 in deep (IRC R403.1.4) |
| Deck footing (attached) | 12 in | Below the frost line and at least 12 in deep (IRC R507.3) |
| Water service line | 18 in | 6 in below the frost line and at least 12 in deep (IRC P2603.5) |
| Fence post bottom | 12 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 King County?
- J
- F
- M
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- D
King County never averages a low below freezing. 54 nights a year reach 32 °F or colder; Feb is coldest, with an average low of 35.3 °F.
NOAA 1991–2020 climate normals, Snoqualmie Falls station, 3.5 mi away, elevation 440 ft.
Monthly temperatures as a table
| °F | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Low | 35.8 | 35.3 | 37.2 | 40.1 | 46.1 | 50.6 | 54 | 53.7 | 49.1 | 43.5 | 38.3 | 35.7 |
| High | 46.4 | 49.6 | 53.7 | 58.9 | 64.1 | 69 | 75.8 | 76.7 | 70.4 | 59.5 | 51.1 | 45.8 |
King County has fewer freezing nights than 23 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 King County use?
4 official codes near King County are verified; each links to its document.
| Place | Verified frost line | Distance |
|---|---|---|
| Redmond | 12 in | 18 mi away |
| Lynnwood | 18 in | 32 mi away |
| Pierce County | 18 in | 34 mi away |
| Lake Stevens | 12 in | 37 mi away |
How likely is the soil in King County to heave?
USDA rates the sampled soil around King County as 28% none, 13% low, 34% moderate, 25% high for frost action (main soils: Elwell, Edgewick, Tokul).
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 King County?
King County at a glance
- Frost line depth
- 12 in Below 1,000 ft, Deeper Above
- Population (2023 est.)
- 2,271,380
- Mean annual temperature
- 51.7 °F
- ZIP codes (89)
- 98001 · 98002 · 98003 · 98004 · 98005 · 98006 · 98007 · 98008 · 98010 · 98011 · 98014 · 98019 · 98022 · 98023 · 98024 · 98025 · 98027 · 98028 · 98029 · 98030 · 98031 · 98032 · 98033 · 98034 · 98038 · 98039 · 98040 · 98042 · 98045 · 98047 · 98050 · 98051 · 98052 · 98053 · 98055 · 98056 · 98057 · 98058 · 98059 · 98065 · 98068 · 98070 · 98072 · 98074 · 98075 · 98077 · 98092 · 98101 · 98102 · 98103 · 98104 · 98105 · 98106 · 98107 · 98108 · 98109 · 98112 · 98115 · 98116 · 98117 · 98118 · 98119 · 98121 · 98122 · 98125 · 98126 · 98133 · 98134 · 98136 · 98144 · 98146 · 98148 · 98154 · 98155 · 98158 · 98164 · 98166 · 98168 · 98174 · 98177 · 98178 · 98188 · 98195 · 98198 · 98199 · 98224 · 98288 · 98354 · 98422