What is a septic reserve area?
A septic reserve area is ground set aside on a lot for a replacement drainfield. Virginia's rule requires one, equal to 50 percent of the required absorption area, where the estimated or measured percolation rate exceeds 45 minutes per inch. Carroll County's subdivision ordinance separately requires a 50 percent reserve for drainfields on subdivision lots, and no structure may be built on it.
A lot's percolation rate and whether it sits in a subdivision decide the question
Before anyone talks about square footage or setbacks, two facts about a lot settle whether a reserve area is required at all: how slowly its soil is rated to take water, and whether the lot is being created under a county subdivision ordinance that asks for one.
The first fact is the percolation rate. Under Virginia's onsite sewage regulations, a separate reserve area equal to 50 percent of the required absorption area must be provided where the estimated or measured percolation rate exceeds 45 minutes per inch. The regulation also requires sufficient suitable soils to install both the absorption system and the reserve area, so the reserve has to be good ground, not leftover ground.
The second fact is the subdivision. Carroll County's subdivision ordinance, as posted on the county's site in its April 14, 2014 text, gives drainfields a reserve area equal to 50 percent of the required and permitted drainage field, on which no structure may be constructed, so that a replacement facility can be built if the first one fails or proves inadequate. The county rule is local and broader than the state trigger. The one-paragraph version of the term is under septic reserve area.
| The lot | Virginia's rule (rate over 45 minutes per inch) | Carroll County subdivision ordinance |
|---|---|---|
| Slower than 45 minutes per inch, on a Carroll County subdivision plat | Requires a 50 percent reserve | Requires a 50 percent reserve |
| 45 minutes per inch or faster, on a Carroll County subdivision plat | Does not trigger | Requires a 50 percent reserve |
| Slower than 45 minutes per inch, not part of a subdivision | Requires a 50 percent reserve | Does not apply |
| 45 minutes per inch or faster, not part of a subdivision | Does not trigger | Does not apply |
What a required reserve area takes off the usable part of a lot
On the ground a reserve area need not look like anything in particular. What makes it real is the list of things that cannot happen there.
- No building on the absorption system. The state regulation says no structures may be placed over the subsurface soil absorption system. In a Carroll County subdivision the ordinance carries the same bar onto the reserve area itself.
- Distance from a new drinking water well. A private drinking water well must be at least 50 feet from an active or permitted drainfield for Class IIIA and IIIB wells, and 100 feet for Class IIIC, and the regulation's table counts the reserve drainfield in that distance.
- An easement where the ground belongs to someone else. Where any part of the absorption system is proposed on property other than the owner's, a perpetual easement is recorded before the construction permit issues, large enough for access, construction, the required reserve area and maintenance.
Put those together and a lot with a reserve has three footprints to fit: the drainfield, the reserve, and a circle around both that a new well stays outside. What is left for the house is the lot's buildable footprint, which is the subject of what a building envelope is. The term itself, building envelope, has a short definition of its own.
On a divided tract, the reserve is located on paper before the plat is recorded
In a subdivision the reserve area stops being a design detail and becomes something drawn and signed for, lot by lot.
12VAC5-610-360. Review of subdivision plats for individual sewage disposal systems when required by local ordinance.
Title of 12VAC5-610-360, Virginia Administrative Code
That title sets the section's reach: it applies where a local ordinance requires the health department to review a subdivision plat. Where it does, the intent is to give the health department enough information to decide whether each proposed lot has a suitable area and reserve area before the plat is recorded. No department employee signs approval for onsite sewage on the plat until a sewage disposal site, including the reserve area when one is required, has been identified, approved or disapproved, and recorded on each lot. The signature does not promise a construction permit later unless the conditions of the original approval still exist when someone applies, and each lot still needs its own construction permit before building begins.
Carroll County's ordinance puts the result on the final plat: the well site, the siting of the septic facilities and, when required, the location of the reserve area. Those sites and reserves feed the arithmetic of how many lots a tract can hold, which belongs to land development and feasibility, and the count itself to what lot yield is.
Finding out whether a particular lot carries a reserve, and where
For an existing lot the question is usually practical: is there a reserve, where is it, and has anything been put on it since.
- Pull the recorded plat. On a Carroll County subdivision lot, the county's ordinance calls for the final plat to show the septic siting and, when required, the reserve area's location.
- Ask the local health department what it has on file for the lot's sewage system, including whether the design's percolation rate brought the state reserve rule into play.
- Walk the lot against the plat. A shed, a garage or an addition standing where the plat shows the drainfield or reserve is a problem to raise before a contract, not after.
- Check the distance from any planned well to the drainfield and to the reserve, since the well regulations count both.
- If the drainfield or reserve sits on a neighbor's land, find the recorded easement.
How the tank, the drainfield and the reserve fit into a whole system is laid out in septic systems explained.
Where do the drainfield and reserve area sit on a lot you plan to divide or sell?
Send the road, the county and whatever plat you have, and Lowell will check what the recorded plat shows before you divide or list, and name the office that confirms the rest. No cost, no obligation.
Got it. Thank you.
We'll be in touch shortly with an honest read.
Questions this raises
Does a Carroll County subdivision lot with a fast percolation rate still need a reserve area?
Under the county's subdivision ordinance as posted, yes. It gives drainfields for on-site septic systems in a subdivision a reserve area equal to 50 percent of the required and permitted drainage field, without tying that to the percolation rate. The state rule's trigger, a rate over 45 minutes per inch, is a separate requirement that applies whether or not a lot is in a subdivision.
How large is a septic reserve area?
Both rules set it as a share of the drainfield, not as a fixed size. Virginia's regulation calls for 50 percent of the required absorption area, and Carroll County's subdivision ordinance for 50 percent of the required and permitted drainage field. The actual square footage follows the system's design, which comes from the licensed onsite soil evaluator, or an engineer working with one, and the health department's review.
Does a new well have to keep its distance from the reserve area as well as the drainfield?
Yes. Under Virginia's private well regulations, a Class IIIA or IIIB drinking water well must be at least 50 feet from an active or permitted drainfield, and a Class IIIC well at least 100 feet, and the regulation's table counts the reserve drainfield in that line. A reserve area limits where a new well can go even while nothing has been built on it.
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.
- 12VAC5-610-710: reserve area equal to 50 percent where the percolation rate exceeds 45 minutes per inch Virginia Department of Health
- 12VAC5-610-500: sufficient suitable soils for the absorption system and reserve area Virginia Department of Health
- 12VAC5-610-360: review of subdivision plats for individual sewage disposal systems when required by local ordinance Virginia Department of Health
- 12VAC5-610-700: no structures over the absorption system, and easements on another's property Virginia Department of Health
- 12VAC5-630-380: well separation distances, including the reserve drainfield Virginia Department of Health
- Subdivision Ordinance (April 14, 2014): soil evaluator, 50 percent reserve area and final plat contents Carroll County, Virginia