The rules, from the county's published code
| Item | Jefferson County (unincorporated) |
|---|---|
| Codes | Code §500.010(A) adopts the 2015 International Building Code, "International Residential Building Code 2015," 2015 IFC/IFGC/IMC/IECC/IPC, NEC 2014 with the Jefferson County Electrical Code 2017, ISPSC 2015. The county's website blocked our read of any later adoption; confirm the current edition with Code Enforcement at application |
| Permit | Required; Code Enforcement, 725 Maple St., Hillsboro |
| Fees | "Building permit fees are based on the actual cost of construction (labor and materials)" (§500.250(A)(5)); the valuation used for fees is not less than $120 per square foot for new residential construction, $165 for remodels, $65 for finished basements, $38 for utility buildings; the fee itself follows a sliding table in §500.250(A)(7) |
| Design table | Not verified — the amended Table R301.2 was not in the portion of Chapter 500 we could read; expect values in line with Franklin and St. Charles (SDC C, 30-inch frost) and confirm |
| Zoning | Unified Development Order, adopted April 2, 2008, effective throughout unincorporated areas. RA-5 Rural/Agricultural: minimum lot 5 acres, one dwelling per 5 acres; cluster option minimum 1 acre (§400.1470(D)(5)) |
| Agricultural buildings | §400.1690(A)(1)(a): "agricultural buildings such as … barns, silos … but not including dwellings" — the farm-building treatment does not extend to a home |
| Minimum dwelling size | Not found in the text we read; unverified |
| Septic | County onsite sewage code in force; the permitting office was not confirmed — ask Code Enforcement or the Health Department |
| Floodplain | NFIP participant (CID 290808) |
| Electric | Crawford Electric Cooperative lists Jefferson among its counties (west and south); Ameren Missouri elsewhere |
| Growth | 226,573 → 233,132 (2020–2025, +2.9%) |
| Land | LandSearch, October 2026: 251 listings, asking average about $37,400 per acre across all types, median listing $195,000, average parcel 8.6 acres |
The $120-per-square-foot valuation
Jefferson County's fee rule is worth understanding because it is the only county in the ring that publishes a per-square-foot number for a house: for permit purposes, new residential construction is valued at no less than $120 per square foot. A 2,000-square-foot living area is valued at a minimum of $240,000 for the fee calculation whatever your contract says, and the fee is a percentage of that on the county's sliding table. It is a floor for fees, not a market price — our turnkey band is $160–$230 — but it is the county's official statement that a house here costs at least that.
Arnold
On September 24, 2020, Arnold's city council adopted a six-month moratorium on pole-building homes (5–0), extended it on March 4, 2021, and on June 17, 2021 voted 7–0 to prohibit pole buildings and metal as a primary siding material on one- to four-family dwellings, limit exposed foundation concrete to one foot on the front and three on the sides, and set masonry requirements for apartment buildings (as reported by the Jefferson County Leader). We have found no repeal. If your parcel is inside Arnold, a post-frame metal-sided home is not an option; a stick-built house with the barndominium look is — see post-frame vs. steel vs. stick.
Festus, Crystal City, Pevely, Herculaneum, De Soto, Hillsboro, Byrnes Mill and the other cities run their own zoning and permits; the county table does not apply inside them.
Soil, drainage and foundation considerations
Jefferson County sits on the Ozark border, with loess and alluvium along the river bottoms and, toward the Ozark edge, Hobson (fragipan 18–27 inches, slow permeability), Clarksville and Goss (35–75% chert, rock at depth). Fragipan soils push septic toward drip or lagoon designs; cherty soils drain well but dig hard — post holes may need a rock auger. The county's sewage code governs the permit; get the soil scientist's evaluation before you close. See land, septic and well.
Power and access
Crawford Electric lists Jefferson among its counties; Ameren serves the rest — the split is parcel-specific. Ameren's tariff gives an individual residential customer up to 1,000 feet of single-phase overhead line at no cost; Crawford's extension policy is not published — call before siting the building. Driveways onto Highway 21, 30, 61/67, A, B, BB, H, M, NN, W or Z need a MoDOT right-of-way permit; county roads, the county's entrance process.
What it costs here
Ring-wide bands — turnkey living $160–$230 per square foot, finished shop $55–$80, shell $45–$65 — plus a permit allowance of $1,000–$2,500 for the fee on the $120-per-square-foot valuation. A 1,600-square-foot living area with a 1,000-square-foot shop as a dry-in shell with slab is about $157,000–$223,500 before site work; turnkey with a typical site package, a 2,000/1,200 build is about $417,000–$638,500. The cost guide · instant estimate.
Not ready to talk yet?
Start with the complete St. Louis barndominium guide, HOA, covenants and city bans and permits and codes by county.
Frequently asked questions
How much land do I need in Jefferson County? Five acres in RA-5 zoning for one dwelling; one acre under the cluster option.
Can I build a barndominium in Arnold? Not a pole building or a metal-sided one — banned since June 2021.
Which code edition will my plans be reviewed under? The published county code adopts the 2015 family; confirm with Code Enforcement whether a newer edition has been adopted.
Is the five-acre rule a septic rule? No — it is zoning. Septic is permitted separately under the county's onsite sewage code.