The rules, from the county's own documents
| Item | Franklin County (unincorporated) |
|---|---|
| Codes in effect | 2021 IBC, IRC, IMC, IPC, IFC, ISPSC; 2020 NEC — effective April 23, 2024 |
| Permit | Required for new dwellings; issued and inspected by the Building Department, Union |
| Fees | $25 non-refundable application fee; permit fee by square footage (the schedule is not published online — "contact us for list of our fees"); $350 escrow for one- and two-family dwellings, $500 if the permit fee is $1,500 or more, $200 for sewer and other structures — refunded when the final inspection passes |
| Permit term | 365 days |
| Design table (R301.2, amended) | Ground snow 20 psf; roof snow 20 psf; wind 105 mph; ice barrier not required; Seismic Design Category C; weathering Severe; frost depth 30 inches; termite Moderate to Heavy; decay Slight to Moderate; winter design temperature 0°F; mean annual temperature 56°F; air-freezing index 1,000 |
| Foundations | Wood foundations are not allowed (IBC 1807.1.4 amended) — posts set in concrete collars or on a slab |
| Agricultural exemption (R105.2 amended) | A structure is exempt only if "designed and constructed to house farm implements, hay, grain, poultry, livestock" and used exclusively for that; it must be at least 200 linear feet from any residence; a notarized certification and plot plan are filed within 30 days; use for anything else requires a permit. A dwelling is never exempt. |
| Septic | County of Franklin On-Site Sewage Disposal Systems Ordinance, adopted September 23, 2025, effective October 1, 2025; sewer permits issued by the Building Department with a $200 escrow |
| Driveway | County Highways Department Entrance Permit; MoDOT permit on state routes (44, 50, 100, 47, 30, 185) |
| Floodplain | Franklin County Flood Damage Prevention Ordinance, administered by Planning & Zoning; NFIP participant |
| 911 address | Planning & Zoning is the county addressing authority (since 2018); apply at P&Z |
| Zoning minimum lot | Not verified at this update — confirm with Planning & Zoning (the P&Z site blocked our read) |
Sources: franklinmo.gov/building and the county's code-adoption and amendment PDFs, read October 13, 2026.
What the 2024 code change means in practice
Since the April 2024 adoption:
- Plan review. The county reviews the plan against the 2021 IRC; for a post-frame building that means the engineering has to show the lateral system for Seismic Design Category C and the wind and snow values, not just the truss sheet.
- Inspections. The county's inspection schedule applies through the build to the final; each is a scheduled visit.
- Escrow. The $350 (or $500) sits with the county until the final passes — a real incentive to finish the inspection sequence.
- The ag exemption is closed to homes. The 200-foot-from-any-residence rule and the exclusive-use language make it impossible to permit a dwelling as a farm building, which is as it should be for the lender and the insurer.
- Wood foundations. The amended code prohibits them; how that applies to embedded treated posts is confirmed at plan review, and we set posts in concrete collars or on a slab.
For a buyer, the change is good news at resale: a 2024-or-later Franklin County barndominium has a permit record and inspections behind it, which the appraiser and the next buyer's lender will want. For an owner with a pre-2024 unpermitted shell, a finish-out now has to be permitted; see shell finish-out.
Soil, drainage and foundation considerations
Franklin County is where the Union soil series was established, in 1909 — loess over clayey residuum with a fragipan 18 to 36 inches down and water perched above it in wet seasons. Where Union, Hobson and similar fragipan soils occur, septic systems run toward drip, mound or lagoon designs and slab drainage is the design problem; on Menfro loess along the Missouri River hills, conventional systems usually work but the 60–75% montmorillonite subsoil wants roof water kept away from the slab edge. The county's own sewage ordinance (effective October 2025) governs the permit; the soil scientist's pit tells you which case you have. See land, septic and well and slab, soils and frost.
Power and access
Three electric providers serve the county: Crawford Electric Cooperative (which lists Franklin among its seven counties), Three Rivers Electric Cooperative (west, with Gasconade and Osage), and Ameren Missouri in and around the cities. Ameren's tariff provides an individual residential customer up to 1,000 feet of single-phase overhead line at no cost; the co-ops' line-extension policies are not published online — call before you choose the building site. Driveways onto I-44 outer roads, Highway 50, 100, 47, 30 or 185 need a MoDOT right-of-way permit with stopping sight distance; onto county roads, the Highways Department's entrance permit.
Land and growth
Franklin County grew from 104,680 to 108,090 people between 2020 and 2025 (+3.3%) — steady, not the Lincoln County boom. Land listings on LandSearch in October 2026: 165 listings, an asking average of about $34,700 per acre across all parcel types, median listing $225,000, average parcel 12.2 acres. Small improved parcels pull that average up; larger rural tracts list lower per acre.
City rules within the county
Union, Washington, St. Clair, Sullivan, Pacific, New Haven, Gerald, Berger and the villages each run their own zoning and permits; the county table does not apply inside them, and some have residential design standards. USDA's last rural-area review flagged the city limits of Union and Washington for possible loss of USDA loan eligibility — check the address.
What it costs here
Our bands are the same across the ring — turnkey living area $160–$230 per square foot, finished shop $55–$80, shell $45–$65 — plus the Franklin-specific adders: the $25 application, the square-footage permit fee, the $350 escrow (refunded) and the $200 sewer escrow, which the estimate carries as a $1,000–$1,900 allowance. A 2,000-square-foot home with a 1,200-square-foot shop, turnkey, with a typical site package, runs about $417,000–$638,000 on Franklin County land. The cost guide · instant estimate.
Not ready to talk yet?
Start with the complete St. Louis barndominium guide and permits and codes by county.
Frequently asked questions
Do I need a permit for a barndominium in Franklin County? Yes, since April 23, 2024, for any dwelling on unincorporated land.
Can I call it an agricultural building? Not a dwelling. The exemption requires exclusive agricultural use and 200 feet of separation from any residence.
Is the escrow a fee? No — it is refunded when the final inspection passes.
What code are the cities on? Each city adopts its own; ask Union, Washington or the relevant city.