How much of a tract is actually buildable?
No published Virginia or county rule sets a buildable percentage for a tract here, so the answer comes from subtraction. Map out the drainfield and its reserve area, the distances a well must keep, the slopes too steep for septic trenches, and any floodplain, dam break zone or setback line. The ground left after those marks is the part that can actually hold a house.
No percentage exists, so the map starts where the sewage goes
There is no published figure for this. The state's sewage, well and erosion rules do not give one, Carroll County's posted subdivision ordinance does not, and a percentage offered as a rule of thumb has no published source behind it.
What does exist is a sequence of things that cannot share ground with a house. The first to mark, on land without public sewer, is the septic site, because in Carroll County the reserve area that goes with it is written off for building entirely. The subdivision ordinance the county posts says:
Drainage fields for all on-site septic or sewage systems shall have a reserve area equal to 50% of the required and permitted drainage field upon which no structure may be constructed.
Carroll County Subdivision Ordinance (posted April 14, 2014)
The same ordinance has developers use an AOSE for the soil evaluations, layouts and sewage designs on each lot, so the drainfield and its septic reserve area are located by soil work, not placed wherever a sketch leaves room. How the reserve rule works in detail is under what a septic reserve area is.
The actual size and placement of the drainfield on your tract come from the local health department and a licensed onsite soil evaluator, and how much ground a single house site claims is set out under how much land you need to build a house here.
Draw the well's distances around the house and the drainfield
A private well does not take much ground itself. The distances it has to keep take a great deal.
| From | Class IIIA/B and IVA/B wells | Class IIIC and IVC wells |
|---|---|---|
| A building foundation | 15 feet (10 feet if the structure has borate-based termite treatment) | 15 feet (10 feet if the structure has borate-based termite treatment) |
| An active or permitted drainfield, including the reserve drainfield | 50 feet | 100 feet |
Slope stretches those circles. Under 12VAC5-630-380 C a well placed directly downslope of the septic system needs more separation, or deeper grout and casing, for every 5.0 percent of slope. Put the well above the drainfield instead of below it and the added distance does not apply, which is why the well site and the septic site get chosen together.
Carroll's subdivision ordinance puts the result plainly: lots must be large enough for all buildings and structures, any well and any onsite sewage system.
Shade the slopes a trench cannot use and the ones a county reviews
Steep ground comes off the map in two ways: through the sewage rules, and through a county review on land being divided.
The one fixed slope limit in these rules belongs to the sewage regulations: septic trenches may not go on slopes over 50 percent unless the ground is terraced.
Steep ground is common here, and the county plans' own figures for how much of their land is steep are collected under slope and grade. None of those figures is a building rule.
One county clause does carry a number: Carroll County's posted subdivision ordinance sends areas with slopes greater than 33 percent to County Engineer review when land is divided, which is part of subdivision approval and not a building permit rule for a lot that already exists. That clause, and whether a single steep homesite works, are covered under whether you can build on a steep lot.
Add the floodplain, any dam break zone and the setback lines
The last marks come from maps and ordinances rather than from the soil.
Virginia requires subdivision ordinances to provide for drainage, flood control and dam break inundation zones and for identifying soil characteristics, and Va. Code 15.2-2258 adds that when any part of land proposed for subdivision lies in a mapped dam break inundation zone, that fact goes on the plat. Ground inside a floodplain carries its own permit questions for the county or city floodplain administrator, set out under whether you can build in a floodplain.
Setbacks trim the edges. Floyd County's building office, for example, lists setbacks of 35 feet from the edge of roadways (state-maintained and private), 60 feet from the center of the road, and 5 feet from structures and property lines (residential only). Other localities set their own figures, and their offices are the source for them.
Read what survives as building area first, then as lots
Once the septic site, the well circles, the steep ground, the mapped zones and the setbacks are shaded, the unshaded ground is the answer to this page's question.
Look at shape, not just size
Pockets of level ground too small or too scattered for a house, a drive and a yard do not add up to one building area, however many acres they total.
Check the pieces connect
A building area the drive cannot reach at a workable grade is ground that still has to be tested before anyone counts on it.
Fit the house inside it
On a single lot the usable footprint is the building envelope, read from the plat, the setback lines, the sewage permit and anything recorded against the lot.
Count lots last
Whether the remaining ground divides into several lots, each with its own septic site and well distances, is the arithmetic of lot yield.
Doing this on paper before buying is exactly what a feasibility study is for. The soil evaluation, the well class and the floodplain determination each belong to a licensed professional or a public office, and the map is only as good as those answers.
Weighing how much of a tract will actually take a house?
Send the acreage, the road and whatever soil or survey work exists, and Lowell will set out what the records already mark off on that tract and which answers still belong to the health department or the county. No cost, no obligation.
Got it. Thank you.
We'll be in touch shortly with an honest read.
Questions this raises
Can a house's well or septic system sit on a neighboring lot?
In Carroll County, yes, under its posted subdivision ordinance: wells and septic systems are not required to be on the same lot as the building they serve, but the easements must be shown on the plat. That can free up building area on a tight lot. The health department still permits the system itself, and drafting the easement is an attorney's work.
Is subtracting constrained land a legal requirement in Carroll County?
Not as an ordinance. The idea that constrained land should be subtracted from the acreage used to calculate development density appears in Carroll County's comprehensive plan as policy, alongside a policy to limit development on high slopes and poor soils. Binding rules include the subdivision ordinance and the health department's sewage and well regulations, so confirm current requirements with Carroll County Land Use and Planning.
Where this page's facts come from
Every rule, office and figure above traces to one of these. Rules change; check the office before you act on one.
- Subdivision Ordinance, posted April 14, 2014: AOSE designs, reserve area, lot size, slopes over 33 percent Carroll County, Virginia
- 2024 Comprehensive Plan: slopes and constrained land Carroll County, Virginia
- 12VAC5-630-380: private well separation distances and downslope precautions Virginia Department of Health
- 12VAC5-610-593: trench systems and slope Virginia Department of Health
- Building Inspections: setbacks Floyd County, Virginia
- Va. Code 15.2-2241: subdivision ordinance provisions for drainage, flood control and dam break zones Virginia General Assembly
- Va. Code 15.2-2258: a mapped dam break inundation zone shown on the plat Virginia General Assembly