Washtenaw County, MI Accessory Structures: ADU Rules (2026)

Michigan's land-use framework gives general zoning authority to cities, villages, and townships under the Michigan Zoning Enabling Act, codified at MCL 125.3101 through 125.3702 (Public Act 110 of 2006). MCL 125.3201 authorizes local legislative bodies to enact zoning ordinances regulating land use and development. Counties may zone only in unincorporated, unzoned territory, and once a township adopts its own zoning ordinance the township ordinance controls (MCL 125.3209). Washtenaw County is fully covered by city and township zoning, so there is no county ADU rule. Michigan has no statewide ADU statute comparable to California's SB 9/SB 13 framework — bills such as HB 4221 of 2021 have been considered but none have passed into general preemption. Ann Arbor leads the county on ADUs: UDC Section 5.16.6 permits detached, attached, and internal ADUs in R1, R2A, R2B, R3, R4A-E districts where the lot has a single-family home, with size limits typically 600 sq ft on smaller lots and 800 sq ft on larger lots (or matching the main dwelling's ground floor area, whichever is smaller), minimum 3-foot setbacks for detached ADUs, and combined occupancy capped at four unrelated persons plus offspring. Critically, Ann Arbor expressly prohibits short-term rental of an ADU. Other Washtenaw jurisdictions vary considerably — Ann Arbor Charter Township amended its accessory structure ordinance in 2024, and Pittsfield, Ypsilanti Township, and Scio Township each maintain distinct rules. Building construction must comply with the Stille-DeRossett-Hale Single State Construction Code Act (PA 230 of 1972, MCL 125.1501) and the Michigan Residential Code regardless of zoning.