Boone County, MO Accessory Structures: ADU Rules (2026)
ADU rules in Boone County, MO, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- State Preemption
- None: counties zone under Mo. Rev. Stat. Ch. 64
- Accessory Structure Limit
- 3,000 sq ft, 24 ft, 3 stories max
- Side/Rear Setback
- 2 ft (if 10+ ft from main building)
- Front Setback
- 60 ft from front street line
- Separate Inspection
- Setback inspection required from June 1, 2026
- Permitting Office
- Boone County Resource Management, (573) 886-4330
Summary
Missouri has no statewide ADU mandate; ADU regulation is delegated to counties under Mo. Rev. Stat. Ch. 64. The Boone County Zoning Ordinance (administered by Resource Management at 801 E. Walnut St., Columbia) treats accessory dwellings as accessory structures or as conditional uses depending on district. The County is actively considering expanded ADU allowances under its 2026 Affordable Housing public hearing process; until adopted, second dwelling units typically require a conditional use permit and must meet the underlying district's lot size and setback rules.
7.1.3 A single accessory dwelling is allowed on legally created individual parcels in the A-1, A-2, A-R, and R-S Districts subject to the following: 7.1.3.1 There shall be a minimum separation of 10 feet between the primary and accessory dwelling unless they are attached by a common wall. 7.1.3.2 The accessory dwelling shall not exceed 75% of the total square feet of the primary dwelling or 1,500 square feet, whichever is less. 7.1.3.3 One additional off-street parking space shall be required for any accessory dwelling with more than three bedrooms. If the primary dwelling has more than two off-street spaces, one may be counted toward the accessory dwelling. 7.1.3.4 The accessory dwelling shall meet all of the requirements of the applicable Building Code, Zoning Regulations, and Code of Heath Regulations. 7.1.3.5 The primary dwelling and the accessory dwelling shall be owned by the same entity.
Full Breakdown
Boone County's authority to zone unincorporated areas is rooted in Mo. Rev. Stat. Chapter 64 (county planning and zoning). Per the Boone County Building Code Addendums, an accessory structure is one that is not greater than 3,000 square feet, not more than 3 stories, and not exceeding 24 feet in height with separate means of egress. Accessory buildings located more than 10 feet from the main building must be set back at least 2 feet from side or rear lot lines and at least 60 feet from the front street line.
Beginning June 1, 2026, Resource Management requires a separate setback inspection for any building permit that adds square footage, including new homes, accessory dwellings, pools, additions, and ground solar arrays. The Planning & Zoning Commission has held public hearings (Jan/Feb 2026) on a draft amendment that would allow one accessory dwelling on a legally created lot under common ownership with the primary dwelling, and would permit ADUs above or attached to commercial uses in commercial districts. Cities inside Boone County (Columbia, Ashland, Centralia, Hallsville) administer their own zoning codes and may have different ADU rules.
Violations & Fines
Building or occupying a second dwelling without a permit or conditional use approval is a zoning violation under the Boone County Zoning Ordinance. Resource Management enforces by notice of violation, stop-work order, and (if unresolved) referral to the Prosecuting Attorney for civil injunctive relief and misdemeanor charges. Each day a violation continues may be treated as a separate offense. Unpermitted electrical, plumbing, or structural work may also trigger separate building-code enforcement.
Frequently Asked Questions
Can I build a second small house for my mother on my rural Boone County lot?
Do Boone County ADU rules apply inside Columbia or Ashland?
Sources & Official References
Other rules in Boone County
How Boone County compares: Easiest Cities to Build an ADU·Compare Boone County to another location·View the Missouri accessory structures overview
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