Franklin County, OH Accessory Structures: ADU Rules (2026)
ADU rules in Franklin County, OH, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Max size
- 816 square feet
- Location
- Inside principal structure only
- Lot size
- 1.5x district minimum required
- Approval
- Board of Zoning Appeals
- Owner
- Must live on-site
Summary
Rural District homeowners can add an 816-square-foot accessory apartment inside the main house or attached garage, with Board of Zoning Appeals design approval and extra lot size and parking required.
302.039 - Accessory Apartment (Granny Flat) - An accessory apartment, as an independent dwelling unit, may be permitted, provided it meets the following conditions:
The apartment must be located in the principal structure (which includes attached garages or areas over garages).
Maximum size of the apartment dwelling unit shall not exceed 816 square feet.
The structure must maintain a single-family residential appearance which blends with the principal structure and the neighborhood. An architectural rendering and floor plan must be provided and approved by the Board of Zoning Appeals. Said plans shall include a landscape plan, which be followed as approved.
Public water and sewer must be provided, or the lot must be adequately sized for, and systems approved for water supply and wastewater disposal to serve both the principal residence and the accessory apartment.
Off-street parking on a hard all-weather surface must be provided, two (2) spaces for the principal residence and two (2) spaces for the accessory apartment, 9' x 18' per space. No one space shall block another. Garages count as parking spaces.
Maximum height of the accessory structure is twenty (20) feet at the peak. The accessory apartment may be located on the first or second floor.
Minimum lot size- One and a half (1.5) times the lot size for the district.
Minimum Road Frontage- One hundred and fifty (150) feet.
Maximum lot coverage by structures- Twenty percent (20%).
Full Breakdown
Section 302.039 caps accessory apartments, 'Granny Flats', at 816 square feet and requires the unit sit inside the principal structure, including an attached garage or space above one. The lot must be 1.5 times the district's minimum size with 150 feet of road frontage, four off-street parking spaces total, and the owner must live on-site. An architectural rendering and floor plan need Board of Zoning Appeals approval before construction.
Violations & Fines
Building or occupying an accessory apartment without Board of Zoning Appeals approval, exceeding 816 square feet, or renting it while the owner doesn't live on-site violates Section 302.039.
Frequently Asked Questions
Can I build a detached ADU in Franklin County's Rural District?
How big can a Granny Flat be?
Sources & Official References
Other rules in Franklin County
How Franklin County compares: Easiest Cities to Build an ADU·Compare Franklin County to another location·View the Ohio accessory structures overview
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