Building permits, zoning and land rules in Jefferson County, Missouri

Yes. Unincorporated Jefferson County, Missouri requires building permits for regulated construction¹. Jefferson County has adopted the 2015 International Residential Code and International Building Code, plus the fire, fuel gas, mechanical, plumbing and energy conservation codes and a county electrical code, all subject to local amendments in Chapter 500^2,10. Code Enforcement regulates construction, alteration and occupancy in unincorporated areas⁴. Applications go through the Citizen Portal, where users can submit permits, schedule inspections and check status after registering online first^8,9.

A one-story detached storage shed no larger than 200 square feet, a fence no higher than 8 feet and a grade-supported water tank within the county's stated limits do not need a building permit¹⁵. Jefferson County requires inspections before framing, electrical, plumbing, mechanical or drain systems are covered or used; request inspections at least one business day ahead²⁰. Final inspection follows finish grading and completion²¹. Ask Code Enforcement before treating a barn, workshop or other accessory structure as exempt.

The Jefferson County building department is the Code Enforcement Division at 725 Maple Street in Hillsboro, phone 636-797-5310, open Monday through Friday from 7:30 am to 5 pm⁴. Jefferson County's building-permit fee schedule took effect September 6, 2021; ask Code Enforcement for the total for your project².

A qualifying owner or immediate-family member may receive a homeowner permit for work on an owner-occupied detached single-family home or accessory structure, but this is not a waiver of permits, plans, fees or inspections¹⁰. The permit holder must personally perform the work, pass the required written test with at least 75%, keep only one such permit open, and complete the work within the permit's one-year validity¹⁰. The building application still requires verification for the homeowner or general contractor and applicable licensed trades²².

County building rules apply to unincorporated Jefferson County; Hillsboro administers its own residential building permits inside city limits^6,23. Septic permits inside incorporated areas go to the Missouri Department of Health and Senior Services (DHSS)⁷.

Jefferson County applies its Unified Development Order to unincorporated land³. Crystal City and Hillsboro have their own zoning; septic permits inside incorporated areas go to DHSS^7,24,25. District-specific controls apply, and every unincorporated parcel has property-line setbacks based on its zoning district and lot shape^3,11,12.

Accessory dwelling units are allowed in residential zoning districts, including planned and non-planned districts and the PM Planned Mixed Use District, but they must be permanent, obtain a building permit, meet the principal use's setbacks and have off-driveway parking²⁶. An accessory dwelling cannot be sold separately; a mobile home can serve as one only in the RA-5 and LR-2 districts²⁶. Minimum lot size depends on the zoning district: 5 acres in RA-5, 2 acres in LR-2, and 40,000, 20,000, 10,000 or 7,000 square feet in R-40, R-20, R-10 and R-7²⁷. We could not confirm minimum dwelling sizes, lot coverage or complete height limits; give Planning the parcel number and proposal before relying on a listing.

Jefferson County Code Enforcement administers onsite sewage permits and inspections in unincorporated Jefferson County^5,13. Construction cannot begin until Code Enforcement issues the septic permit⁵. Jefferson County septic applications require bedroom count and a licensed onsite contractor; new systems need soil morphology reports and stamped engineered drawings, with sealed plans and soil reports submitted for plan review¹³. Commercial/industrial facilities and subdivision developers must contact the Missouri Department of Natural Resources before seeking county approval⁵.

Jefferson County septic systems need inspection approval before backfill and a final inspection after backfill; alternative designs may require owner-paid special inspections^5,13. Only 2.3% of Jefferson County soils are rated free of severe limitations for a conventional drain field, so establish system feasibility and cost before committing to a building site¹⁴. The Missouri Department of Health and Senior Services says its application fees for onsite sewage construction are temporarily suspended effective August 28, 2026, but the county sets its own fees, so ask Code Enforcement what it charges⁷. Onsite sewage is for sites without central sewer; an addition, alteration or replacement permit generally cannot be issued when centralized sewer is available within 400 feet unless Code Enforcement grants a written exception supported by a reason and two detailed bids showing connection costs at least twice the onsite-system cost; the county application asks whether public sewer is available and which sewer district serves the parcel⁵.

An RV generally may not be used as a residence in unincorporated Jefferson County unless another county ordinance provides otherwise¹⁹. An RV may be placed under a permit in an authorized RV park, and an RV may be occupied temporarily while a principal dwelling is under construction in a residential zoning district; that construction use ends when the dwelling receives its occupancy permit¹⁹. Applicable sewer, water, electrical and other county permits still apply¹⁹. We could not confirm a broader temporary-camping rule, so ask Planning before staying on vacant land.

We could not confirm a separate tiny-home category or minimum dwelling size in unincorporated Jefferson County; ask Planning and Code Enforcement how your proposed home would be classified. A tiny home intended as a dwelling must still satisfy the parcel's zoning, setbacks, building permit and sanitation requirements^1,3,5,11. A permanent accessory dwelling is expressly allowed in residential zoning districts subject to its permit, utility, setback, parking and ownership conditions²⁶.

1. Ask the Planning Division to confirm the parcel's zoning district, allowed use and setbacks^3,11.
2. Determine whether the site is in a special flood hazard area; each proposed structure or other development there requires a separate floodplain development permit²⁸.
3. Establish sewer availability or septic feasibility before fixing the house site; central sewer within 400 feet generally blocks an onsite-system addition, alteration or replacement permit unless the documented exception is granted⁵.
4. Obtain a county driveway permit for work on county right-of-way or a MoDOT permit for access on state right-of-way^16,17,18.
5. Register in the Citizen Portal and submit the building application and related plans and approvals^8,9.
6. Pay the applicable published fees, then schedule required building and septic inspections and obtain final approvals before backfilling the septic system or occupying the structure^2,5,13,20,22.

Start with the exact zoning district, setbacks and legal-lot status; unincorporated parcels are subject to district controls, and preliminary subdivision plats require a completed checklist and a Planning and Zoning Commission hearing^3,11. A residential driveway on Jefferson County right-of-way needs a county permit with $500 refundable escrow; state right-of-way access requires a MoDOT permit^2,16,17,18. Jefferson County floodplain approval covers construction and manufactured-home placement in special flood hazard areas, with a separate permit for each structure or development²⁸.

Resolve wastewater before selecting the house site because only 2.3% of Jefferson County soils are rated free of severe limitations for a conventional drain field¹⁴. Terrain adds cost: 77.5% of the county is wooded, and 42.5% of its land has slopes of 15% or less¹⁴. The effective property-tax rate is about 0.8%, and the median annual bill is $1,835¹⁴.