The order is soil, septic, then house. Every year people in the ring buy a parcel for a barndominium, close on it, and then learn the soil will not take a conventional septic system, the county wants three or five acres, or the nearest power line is 2,000 feet away. All three are checkable before you sign. This guide is the checklist, with the Missouri rules that apply and the county numbers that differ.
How much land does the county require?
| County | Minimum lot for a dwelling (unincorporated, rural zoning) |
|---|---|
| Jefferson | 5 acres (RA-5), one dwelling per 5 acres; 1 acre under the cluster option |
| St. Charles | 5 acres ("A" district); a 3-acre tract around an existing residence; 800 sq ft minimum dwelling |
| Warren | 3 acres (AG), one dwelling per 3 acres; 50-ft setback from road right-of-way |
| Lincoln | No county zoning; cities have their own |
| Franklin | Not verified at this update — confirm with Planning & Zoning |
Separately, if you want a Farm Credit loan, FCS Financial requires a barndominium to sit on ten acres or more. See financing.
The three-acre septic rule — what it is and isn't
Missouri's state permit requirement for onsite wastewater excludes "the owner of a single-family residence lot consisting of three acres or more" where every part of the system is more than ten feet from any property line (RSMo 701.031; 19 CSR 20-3.015). That is the rule people quote. What it does not do: exempt you from your county. Franklin (ordinance effective October 1, 2025), Warren (Sanitarian) and St. Charles (private sewage code) regulate septic systems on every lot regardless of acreage; Jefferson has a county sewage code (confirm the permitting office); and in Lincoln County the Health Department issues the permit. In Warren County the septic permit must be issued before the structure permit; in Franklin the sewer permit runs through the Building Department with its own escrow.
What the state rules require everywhere a permit applies (19 CSR 20-3.060): a site evaluation by percolation test or soil morphology, a profile pit for new installations, and a soil morphology examination done by a soil scientist (or qualified engineer, geologist or sanitarian). Installers must be registered with the state and, in Warren County, with the county.
Lagoons. The state rule says lagoons are "not generally suitable in subdivisions with lots less than three acres," and sets setbacks of 75 feet from a property line, 200 feet from a neighbor's residence, 100 feet from your own, 100 feet from a water supply and 50 feet from a stream, with 440 square feet of surface per bedroom and 900 square feet minimum. On a 3-acre lot the 200-foot neighbor setback often decides it.
What the soil will do
The ring's soils fall into a few families, and each one sends the septic design in a different direction. From the NRCS official series descriptions:
| Soil | Where | What it means for septic and slab |
|---|---|---|
| Menfro (loess, well drained) | Ridgetops and bluffs along the Missouri and Mississippi — St. Charles, Warren, Franklin river hills | Conventional lateral fields usually work; subsoil is 60–75% montmorillonite clay, so roof water and slab-edge moisture matter |
| Winfield (loess, moderately well drained) | Uplands in Lincoln, St. Charles, Warren | Perched water table 2.0–3.5 feet down from November to April — shallow or pressure-dosed systems, drainage around the slab |
| Union (loess over clayey residuum) | Franklin County (the series was established at Union in 1909), Washington County | Fragipan 18–36 inches down with water perched above it — the hard case; drip, mound or lagoon designs |
| Hobson | Ozark border — south Franklin, Jefferson | Fragipan 18–27 inches; slow permeability |
| Clarksville / Goss | Ozark border | 35–75% chert fragments; drains fast, hard to excavate, rock at depth |
The soil scientist's pit tells you which one you have. Order it with a contingency in the land contract; an evaluation that typically costs a few hundred to around a thousand dollars is cheap insurance on a $500,000 project. Site-specific shrink-swell and depth-to-water ratings are in the USDA Web Soil Survey for the parcel.
Well
A domestic well must be drilled by a Missouri-permitted well installation contractor (RSMo 256.603 — the owner-drills-own-well exception exists but is not a practical path). Depth and yield vary across the ring; the driller's log from a neighbor's well is the best predictor. Our planning allowance is $12,000–$25,000.
Access, address, power
- Driveway. Onto a state route, a MoDOT permit to work in the right-of-way is required; a residential drive is a Type I entrance; stopping sight distance must be met or the permit is refused; drainage may not cross the driveway surface. Onto a county road, Franklin's Highways Department and Warren's Road & Bridge issue entrance permits.
- 911 address. Required before Warren's structure permit; Franklin's Planning & Zoning is the addressing authority. Get it early — the septic and electric applications want it.
- Electric. Ameren's tariff gives an individual residential customer single-phase overhead service with up to 1,000 feet of distribution line at no cost (at most 500 feet on private property); beyond that you pay in advance, and underground service costs the overhead-to-underground difference. Crawford Electric (Franklin, Jefferson, Washington and west), Three Rivers (west Franklin) and Cuivre River (parts of the northern ring; its county list is not published) are the co-ops; their extension policies are not published online — call before you choose the building site.
- Floodplain. All five counties require a floodplain development permit in a mapped flood hazard area, including the counties with no building code. Warren checks every parcel at application.
What land costs
Asking-price averages across all listings on LandSearch's county pages (October 2026, improved and unimproved mixed): Franklin about $34,700 per acre (median listing $225,000, average 12.2 acres); Jefferson $37,400 ($195,000, 8.6 acres); Warren $36,500 ($299,000, 11.7 acres); Lincoln $58,900 ($420,000, 10.5 acres). Those averages are pulled up by small improved parcels; larger rural tracts list far lower — recent Lincoln County examples ran $14,000–$24,000 per acre for 13–20-acre parcels. Missouri farm real estate averaged $5,000 per acre statewide in 2025 (USDA). The growth counties are where the pressure is: Lincoln +11.9% and Warren +9.8% in population since 2020.
The checklist, in order
- County and city: which rules apply to the parcel, and is it inside a municipality or covenant? (permits by county, HOA and city bans)
- Floodplain map.
- Soil evaluation with a profile pit — before closing, with a contingency.
- Septic design and permit path for that county.
- Power: distance to the line, utility or co-op, extension estimate.
- Access: state route or county road, sight distance.
- 911 address.
- Then the building.
Frequently asked questions
Do I need three acres for a septic system? Not by state rule — the three-acre figure is an exclusion from state permitting, not a minimum. The county's zoning minimum (three to five acres) and the soil decide.
Can I put in a lagoon? On three acres or more with the setbacks, and where the county allows it. Fragipan soils are where they come up most.
Will Ameren run power to my building for free? Up to 1,000 feet of single-phase overhead line for an individual residential customer, 500 of it on your property, by tariff. Beyond that you pay.
Who does the soil evaluation? A soil scientist meeting DHSS requirements; Warren County names that requirement in its permit form.
Next step
Custom barndominiums carries the soil evaluation, septic, well and access permits as part of the project. Related: slab, soils and frost · the complete guide.