Why the word has no single number behind it in Virginia
Virginia hands this one to the locality on purpose, and says so twice. Its administrative code names the referenced-code section that lets localities fix their own climatic and geographic design criteria as a valid provision of the Virginia code, and the residential code table where the number is supposed to live ships with its cells unfilled and a footnote instructing the jurisdiction to enter a figure of its own. So there is no Virginia frost depth to look up. There are as many as there are building departments.
The spread across this corner of Southwest Virginia is the proof, and it is not subtle. The published frost line figures run from eighteen inches in Galax and Floyd County to twenty four in Montgomery and Pulaski counties, neighbors disagree with each other, and six of the eleven Virginia jurisdictions checked around here print no frost line figure at all, our home county among them. Which one publishes what, how old each document is and what it is worth are set out on our frost depth and footing codes page. This entry is about the word.
One word doing three jobs in the same chapter
Most of the confusion around this term is the code's own doing, because it uses the word depth for three different measurements within a few paragraphs of each other. One is how thick the concrete itself is, which is why footing tables print two numbers side by side. One is a floor on burial that every exterior footing meets whether or not frost is anywhere in the question, and Virginia takes that one from the undisturbed ground rather than from finished grade, which is a meaningfully different line on a lot about to be cut or filled. Only the third is frost depth.
Keeping them apart is most of what this term is for, because reaching for the thickness figure in a frost conversation produces a badly wrong answer. A footing carries all three at once, and the provisions themselves, with whatever figure your county has actually published, sit on how deep footers go in Virginia.
One figure for the whole state, and it does not travel
North Carolina enters one figure and applies it statewide, so under the code in force a homeowner there does have a single number to look up. The edition sitting behind it softens that: the entry becomes a minimum and picks up a footnote routing the reader to the local jurisdiction, which edges that state toward Virginia's arrangement without arriving at it.
Never carry either state's figure across the line. They are arrived at by completely different machinery and they are measured from different datums, so a number that is right in Surry County is not evidence of anything in Carroll. Our guide to permits there keeps that side separate on purpose. What freezing does to concrete already in the ground is a different subject again, and the frost heave answer covers it.