What kind of instrument it is, and who holds it
The setback this entry is about is a zoning creature. It comes out of an ordinance a county, city or town chose to adopt, under a statute in each state that says a locality may adopt one, and it lives with the zoning administrator or the planner rather than with the building official. Two separate questions, and a job can be square with one and crosswise with the other.
Because zoning is optional and local, a zoning setback belongs to the locality, not to the state. Both state zoning statutes hand a locality the power to fix dimensions and neither prints a dimension itself, and some counties around here have not adopted county-wide zoning at all. Setbacks outside zoning are another matter, and both states write some, among them the distances their health rules keep between a septic system and what is around it. Virginia's 12VAC5-610-592, titled Setback distances, says "Setback distance to various structures and topographic features and an absorption area are contained in Table 4.2." North Carolina's 15A NCAC 18E .0601(a) says "Every wastewater system shall be located the minimum setbacks from the site features specified in Table IX," and building foundations and property lines are among the features that table lists. If there is a septic system anywhere near the pour, ask the local health department where those lines fall before anything is formed.
It is measured from a line, and the line is not always the one you expect
The ordinances read for this page measure from the lot line or from the edge of the street right of way. Surry County, North Carolina adds a third possibility that catches people out. Its setback runs to whichever is nearer of the lot line, the street right of way line, or the line where NCDOT's street maintenance begins. On a road out in the county those three can sit in three different places, and the closest one governs.
Then there is what the distance is measured to. Pulaski County, Virginia takes it to the vertical wall of a building or structure, and its definition of a structure reaches "Anything that is constructed or erected with a fixed location on the ground," so how that measurement treats a flat slab is a question for its zoning administrator. The Town of Hillsville runs its setback line against the principal building. Different ordinances, different reference points, and both of those are on file and readable.
Whether it even reaches a slab is a definitions question
Since the distance is measured to a structure, the whole homeowner question turns on how that one word was defined in that one ordinance, and the definitions in this corner of the two states disagree with each other.
Surry County, North Carolina carries an exclusion list inside its general definition of structure, and it names at-grade slab patios and driveways among the things a structure is not. Whether its setback then reaches them is the county zoning office's call. The same ordinance keeps separate driveway standards of its own, and the exclusion leaves them untouched.
Giles County, Virginia, in the copy of its ordinance posted on the county's own site, defines a structure as anything constructed or erected whose use requires a location on the ground, or that is attached to something having one, and we found no exclusion list following that definition. A poured slab is constructed and does require a location on the ground. The copy is captioned 1992 as amended and was posted in 2019, so confirm with the county before you build anything on it. Two counties, two different definitions, and whether the Giles one reaches a patio is a question for its zoning administrator.
So ask three things of the planning office and ask them in this order. Is this lot zoned at all. Which district is it in. Does your definition of structure reach a slab at grade. That third question is the one almost nobody asks and the one that decides it, on a patio or on a driveway alike. Nothing on this page is a figure, because no zoning dimensional table for any jurisdiction here was read for it.
Why your neighbor's slab proves nothing
Concrete poured tight to a line down the road tells you nothing about your ground. It may be in another district, or inside a town, or over a county line in a county without zoning. It may predate the ordinance in force today. It may sit on a variance granted on facts belonging to that lot. Or nobody ever asked anyone.
Keep the setback separate from the two things it gets confused with, and do not work either of them out from here. A recorded easement is a private right out of a deed, and the easement entry has it. The strip along a state maintained road answers to the state transportation department, which keeps its own rules for it. All this page will say about them is that clearing the setback clears neither. Our Carroll County page shows what one local desk actually puts in writing.