Aptos, CA, United States
What service or documentation resource can I use to determine what the frost-line is for my area? I'd be inclined to ask the county building/planning office but I suspect that they'll want to charge for answering a seemingly simple (and hopefully well documented) question.
In general you can use these guidelines.
These are very good guidelines that are widely recognized. However if getting inspected I would just ask your building department for their code requirements.
**NOTE***** This is how deep your footings would be in undisturbed soil. So for example if there was a regrading of the land - as often the case in new builds - you would have to go this deep past the disturbed soil or consult an engineer that would sign off on something else. So if they regraded your land to 3 feet and you lived in Arkansas you would need to go to 54 inches or gets clearance from an engineer to put the footings shallower.
This map shows
the frost line depth for each state.
I say be mighty careful with vague national or even state line drawings of frost depth. Every county has those variations that have been recorded for some time and thus county records will show those zones that are hit hardest by extreme temperatures. This is especially true when extremes are wind driven into hillsides and when below 0 Fahrenheit air settles into valley floors for a week or more. This is exactly why seeking a professional who will apply the county zoning information to your exact location is vital.
The 2010 California Residential Code has a beautiful table (TABLE R301.2(1)) to list the frost lines; however, that is empty.
The Santa Cruz County Municipal Code states "Frost line depth = 12 inches below undisturbed soil". Search for "frost line".
Contact your local Building Code Department. They have minimum requirements for various conditions in your area.
