ADU Pass
Highland County
ADU Pass helps homeowners in Highland County, Ohio navigate the permit paperwork for building an accessory dwelling unit. We cover 7 cities and 7 ZIP codes in this county.
Map
County ADU details
County ADU ordinance
Highland County, Ohio (south-central Ohio Appalachian-fringe county on the dissected glacial-till to unglaciated-plateau boundary; ~43,400 residents; county seat Hillsboro; seventeen townships - Brushcreek, Clay, Concord, Dodson, Fairfield, Hamer, Jackson, Liberty, Madison, Marshall, New Market, Paint, Penn, Salem, Union, Washington, and White Oak - and incorporated places Hillsboro (city), Greenfield (city, partly in Highland and partly in Ross County), and villages Leesburg, Lynchburg (partly in Highland and partly in Clinton/Brown), Mowrystown, Sinking Spring (partly in Highland and partly in Adams), and Highland) does NOT operate a countywide zoning ordinance and has not adopted county-tier zoning under O.R.C. Chapter 303. Ohio is a home-rule state under Article XVIII of the Ohio Constitution; counties may adopt county zoning under O.R.C. Chapter 303 but only a small minority do (Lake County is the canonical example), and Highland County is not among them. The Highland County Planning Commission handles subdivision plat review under O.R.C. § 711.10 for unincorporated parcels under Subdivision Regulations adopted in December 2005, plus parcel splits and adjoining-land matters, but does not regulate use or density. Townships in Highland County may adopt township-tier zoning under O.R.C. Chapter 519; as of 2026-05, township zoning coverage in Highland County is partial - the Hillsboro-fringe Liberty and New Market Townships and Greenfield-fringe Madison and Paint Townships are most likely to carry a township zoning resolution, while several outer southern and western townships (Brushcreek, Whiteoak, Concord, Dodson, Penn) operate without township zoning. Where neither county nor township zoning exists, an unincorporated parcel is effectively unzoned for land-use purposes; the Ohio Residential Code (OAC 4101:8) still governs construction standards for any dwelling, including an ADU, and the Highland County Health Department still governs septic permits under OAC 3701-29, but there is no local zoning prohibition on a second dwelling on a parcel. ADU permissibility in unincorporated Highland County therefore turns on (1) whether the parcel is in a township with zoning, (2) whether the parcel is inside Hillsboro, Greenfield, or one of the villages (in which case city/village zoning applies; Hillsboro's certified building department issues its own permits), and (3) whether the parcel can support a second septic field under OAC 3701-29 capacity rules. Highland County straddles the Wisconsinan glacial limit; northern townships sit on flat to gently rolling till plain while southern townships descend into the dissected Appalachian Plateau with shallower soils and steeper grades, materially affecting septic feasibility for a second dwelling.
County assessor
Assessment policy: The Highland County Auditor is the elected property-assessment authority under O.R.C. § 5713.01. Ohio assesses real property at 35% of true value (O.R.C. § 5713.03). New ADU construction is added to the parcel record on the next regular reappraisal or triennial update cycle (Ohio's six-year reappraisal / triennial update schedule under O.R.C. § 5713.01) and reassessed at true value as of January 1 following completion. The Ohio Owner Occupancy Credit (O.R.C. § 323.151 et seq.) provides a 2.5% reduction in tax on owner-occupied homestead. The Homestead Exemption (O.R.C. § 323.152) reduces taxable value by $25,000 for owners 65+ or totally and permanently disabled, subject to income tests. Highland County also administers Current Agricultural Use Valuation (CAUV) under O.R.C. § 5713.30 - § 5713.38; the Highland County Auditor's office publishes annual CAUV soil-rate updates and runs a renewal cycle (initial applications must be filed prior to the first Monday in March, and renewal applications are mailed annually). CAUV enrollment in Highland County is substantial but more uneven than in pure till-plain counties to the north - the northern till-plain townships (Madison, Concord, Liberty, New Market, Paint, Union) carry large CAUV-enrolled farm parcels while southern Appalachian-plateau townships (Brushcreek, Whiteoak, Penn, Dodson) carry a mix of CAUV cropland, CAUV-eligible woodland (the program covers managed forestland), and non-qualifying small holdings. An ADU built on a CAUV parcel may trigger a partial recoupment (rollback) if the dwelling reduces qualifying agricultural acreage below the CAUV threshold or removes the parcel's homesite area from agricultural use; CAUV reviewers in the Highland County Auditor's office examine ADU additions case-by-case. The county recently completed a triennial update / reappraisal cycle (Auditor Alex Butler reported a 'tentative abstract' submitted to and approved by the Ohio Department of Taxation), producing meaningful upward value movement on residential parcels. Under O.R.C. § 5713.17, the owner must notify the Auditor of any construction or improvement valued over $2,000.
County overlays (4)
Known county issues (6)
- other — Researchers should verify the township for any specific parcel before assuming ADU prohibition or permission. Parcels in unzoned townships have unusually permissive ADU conditions for Ohio, constrained mainly by septic capacity and floodplain. Parcels in the zoned townships, inside Hillsboro or Greenfield, or inside the five village limits require township- or municipal-specific review.
- other — An ADU applicant in Hillsboro has a single-counter experience (Hillsboro Building Department for permits and inspections; Health Department for plumbing and septic). An applicant in unincorporated Highland County must coordinate across three or four separate state and county agencies; DIC inspector trip routing from Reynoldsburg or a regional office adds friction relative to the city. Greenfield and the villages typically rely on their own building or shared-services arrangements depending on the village.
- other — ADU economics in unincorporated Highland County are commonly dominated by septic feasibility, not by permit fees or zoning. A site review for a second OSTS on a southern-township parcel can result in a no-permit outcome or a $20,000-$40,000+ advanced-treatment install, while a similar parcel in the northern till-plain townships typically supports a conventional drainfield in the $8,000-$15,000 range. Researchers should treat the Health Department site review as a first-order go/no-go gate for ADUs on parcels south of the glacial limit.
- other — ADU projects in Paint Township and northern Penn Township face higher demand and higher economic returns than elsewhere in the county, but materially higher regulatory friction (floodplain, Army Corps shoreline review, dense neighbor environment, frequent septic-capacity limits). Researchers should treat Rocky Fork Lake parcels as a distinct sub-market within Highland County.
- other — ADU economics on CAUV-enrolled parcels in unincorporated Highland County frequently include a one-time recoupment cost and a permanent shift of acreage out of CAUV; the parent parcel typically remains enrolled at the reduced acreage. Researchers should treat the CAUV consult as a first-order cost item for unincorporated ADUs on enrolled parcels, particularly in northern till-plain townships where enrollment is densest.
- policy-review — Researchers should monitor the Ohio General Assembly for any future ADU preemption proposal; until one passes, Highland County remains in the position where unincorporated unzoned-township parcels are de facto permissive without any state preemption being needed. Conversely, ADUs inside Hillsboro depend entirely on the city's zoning ordinance, with no state floor to fall back on.
Federal (United States) — ADU-relevant rules and programs
Federal ADU law
The United States has no federal statute that directly regulates accessory dwelling unit entitlement or design. Land-use authority over ADUs resides with states and local governments under the traditional police power. Federal engagement is limited to financing (Fannie/Freddie/FHA/VA/USDA), flood insurance (FEMA/NFIP), and discretionary housing programs (HUD), which are recorded in sibling sections of this file.
Federal financing programs
Federal housing-finance agencies and GSEs set nationwide underwriting rules that govern whether an ADU can be financed, appraised, and counted toward mortgage qualifying income. The relevant actors are Fannie Mae, Freddie Mac, FHA (HUD), VA, and USDA Rural Development.
Federal tax credits
There is no ADU-specific federal tax credit. ADUs may incidentally qualify for existing federal energy-efficiency and clean-energy tax credits when the ADU construction includes qualifying measures.
Federal housing programs
HUD administers several discretionary programs that can fund ADU-related activity at the grantee's election, but none is an ADU-specific program.