Foundations
Frost Line Depth: Who Sets It and What It Covers
5 min · Updated 2026-09-28
Who sets the frost line depth in the building code, how to find yours, Austin's and Los Angeles's 12 inches, and which footings must be frost-protected.
The building code has no single frost line depth. Its climate table, Table R301.2, leaves the frost line column for each city or county to fill in, with the minimum depth of footing below finish grade, so the depth that applies to a building is the one in its own building department’s version of that table.
How to find yours
Look in the residential code your city or county has adopted, at its Table R301.2, or ask the building department for the figure. The same table carries two other local climate figures: the air freezing index, which the jurisdiction takes from the code’s 100-year map or from a National Climatic Data Center table, and the mean annual temperature. If the adopted table cannot be found, ask the building department before digging.
Frost line depth in Austin and Los Angeles
Austin, which builds under the 2024 International Residential Code, and Los Angeles, which builds under the California Residential Code with its own amendments, both write 12 inches into their tables. A separate rule puts every exterior footing at least 12 inches below the undisturbed ground surface, frost or no frost (R403.1.4). The two are measured from different surfaces, finish grade and undisturbed ground, so where soil has been filled or cut, ask the building department which one sets the depth.
| Figure | Austin | Los Angeles |
|---|---|---|
| Frost line depth, below finish grade | 12 in | 12 in |
| Air freezing index | 30 °F-days | 0 °F-days |
| Mean annual temperature | 68.1 °F | 61.1 °F |
Where the frost line and a footing sit
- Frost line depth, measured below finish grade: the figure in the jurisdiction’s own Table R301.2. Austin and Los Angeles both write 12 inches.
- Every exterior footing sits at least 12 inches below the undisturbed ground surface, frost or no frost (R403.1.4).
- Extending the footing below the frost line is one of four ways to protect it from frost; the others are a frost-protected shallow foundation, the ASCE 32 standard and solid rock (R403.1.4.1).
Not to scale. Finish grade and undisturbed ground differ where soil was filled or cut; ask the building department which one sets the depth.
The air freezing index is measured in degree-days: the degrees the air stays below 32 °F, added up over the days it stays there, across a year running July to June. The more and the longer it freezes, the higher the number; the code reads the figure expected once in 100 years. The model code’s county list puts every Texas county, and every California county but Nevada and Sierra, at 1,500 °F-days or less (Table R403.3(2)).
What must be protected from frost
Unless something else already protects them, foundation walls, piers and other permanent supports of buildings and structures must be protected from frost in one or more of four ways: extended below the frost line in Table R301.2, built as a frost-protected shallow foundation under Section R403.3, built to the ASCE 32 standard, or set on solid rock. A footing must not bear on frozen soil unless the ground stays frozen for good (R403.1.4.1).
A frost-protected shallow foundation uses buried insulation instead of depth. The model code allows it only for a building kept at a monthly mean of at least 64 °F, not for unheated spaces such as porches, utility rooms, garages and carports, and not attached to a basement or crawl space kept cooler than that; insulation used below grade must be labeled as meeting ASTM C578. Where the air freezing index is 1,500 °F-days or less, as in Austin, the model code’s table calls for a footing at least 12 inches deep with R-4.5 insulation on the wall and no horizontal insulation (Table R403.3(1)). This page did not compare the California code’s version of that section; in Los Angeles, ask the building department.
Sheds and other small structures
The model code waives frost protection for free-standing accessory structures no bigger than the table below (R403.1.4.1, Exceptions 1 and 2), and the California code prints the same exceptions. The exception waives only the frost protection. Whether a particular shed counts as a free-standing accessory structure of light-frame or other construction is the building official’s call, so ask the building department.
| Construction | Area up to | Eave height up to |
|---|---|---|
| Light-frame | 600 sq ft | 10 ft |
| Any other construction | 400 sq ft | 10 ft |
Whether the shed needs a permit at all is a separate question: the shed permit brief gives Austin’s and Los Angeles’s lines, and the shed setback guide covers where it may sit.
Deck footings and the frost line
Deck footings must go at least 12 inches below undisturbed ground, and where a deck is attached to a structure whose own foundation is protected from frost, its footings must be protected too: extended below the frost line, set on solid rock or protected by another approved method (R507.3.2, R507.3.3). The deck footing guide gives the code’s footing sizes and post heights.
Fence posts, pipes and everything else
The frost rule covers the foundations of buildings and structures, and this page found no depth in it for a fence post; ask the building department. The fence calculator gives the concrete depths some cities write down for fence posts and a concrete maker’s steps for the hole. How deep buried water and sewer lines must go was outside what this page read; ask the city or a licensed plumber.