Twelve inches is the one depth Virginia sets for everyone
The statewide rule is a minimum, not an answer. Section R403.1.4 of the Virginia Residential Code sends every exterior footing at least 12 inches below undisturbed ground, meaning the surface as it sat before anybody cut or filled it rather than the grade the machine leaves behind. Measure from a spoil pile and the trench comes up short. That foot of depth has nothing to do with frost. It is there so the footing bears on ground that holds, and it applies whether winter reaches that far down or not.
The same section routes deck footings to Section R507.3, a separate rule with its own depth requirement and its own sizing table, and the sizing half of a deck question is answered under deck footing sizes.
One more figure from that chapter gets misread. Section R403.1.1 calls for a footing at least 12 inches wide and 6 inches deep, and deep in that line means thick, which is why the footing tables print sizes like 12 by 6, width by thickness. Nothing in it says how far down the trench goes.
The figure that usually decides is entered by your locality
The number that usually decides the dig is not printed in the state book at all. Virginia publishes Table R301.2, its climate and geography table, with the frost line column left for the jurisdiction to complete, and footnote b tells the jurisdiction to enter that figure as a minimum footing depth. The administrative rules back the handoff: 13VAC5-63-30 Section 103.4 item 3 lists the R301.2 provision that lets each locality set its own climate and geography criteria among the referenced code provisions Virginia counts as valid.
So whoever administers the building code where the concrete goes owns the number. Where that office has printed a figure and the work is the kind the frost rule reaches, the figure is the depth to dig to whenever it runs deeper than the 12 inch floor. Where it has printed nothing, the depth is whatever that office tells you, which is why the answer for your lot ends at a desk rather than on a chart.
Six inches apart, sometimes on either side of one county line
Where a figure is printed in our part of Virginia it is 18 inches or 24, and which one depends on the line you are standing behind: Floyd County prints 18 while Montgomery County next door prints 24, and one shared sheet gives the City of Roanoke 18 and Roanoke County 24. Galax sits at 18 and Pulaski at 24. Nothing in that spread averages into a regional figure, and a job a few minutes past a line can owe six more inches than the job before it.
Which document each figure comes from, the date on it, and which of those sheets deserve a phone call before anybody trusts them are laid out under frost depth and footing codes.
Six offices with no frost line figure on paper, and the numbers to call
Six of the eleven print no frost line depth: Carroll, Grayson, Wythe, Patrick, Smyth and Giles. Each was checked on the county's own website, and for Carroll and Grayson the posted documents were opened as well. In all six the depth comes from the office, not from whichever neighbor happens to print one.
Carroll, the county this crew calls home, needs the most care. The one depth figure Carroll prints is for manufactured home piers, and it does not carry over to a house, a garage or a shop footing. For a Carroll County address outside the Town of Hillsville the call goes to the county's Office of the Building Official, 276-730-3016, and inside the town limits to the Town of Hillsville, which issues its own building permits.
Grayson prints a footing and no frost line. Its plan documents call 18 inches the typical footing depth and say the actual requirement is settled case by case from the drawings, so treat that 18 as Grayson's typical footing rather than a frost figure, and put the question of what applies to your lot to the Building Department, 276-773-2322.
In Wythe, call Building Inspection on 276-223-4505. In Patrick, Building Inspections answers on 276-694-4143. In Smyth, it is Building and Zoning on 276-706-8317. Giles prints only an 18 inch footing minimum, on a plan requirements sheet created in March 2009, so confirm what applies with its building department on 540-921-2527. Give whichever office the address and what the footing will hold up, ask what depth it applies there, and write down who told you.
Digging below the figure is the usual route, not the only one
Virginia's frost rule, Section R403.1.4.1, is met most often by taking the footing below the local figure, but it names three other routes as well: an insulated shallow design under Section R403.3, a design to ASCE 32, or bearing on solid rock. The insulated route carries limits of its own, set out under what a frost protected shallow foundation is.
Two narrow exceptions let small free standing accessory buildings go without frost protection, and neither one lets them go without the footing or the 12 inch floor. Which buildings qualify is spelled out on the frost depth and footing codes page. What happens to concrete set too shallow is its own answer under what frost heave is, and our foundations and footers work covers what the footing is doing down there.