ADU rules in Williamson County, TN — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Tennessee sets no statewide ADU mandate, so a second or accessory dwelling in unincorporated Williamson County is controlled entirely by the county Zoning Ordinance and your residential district. A building permit and septic approval are required where one is allowed.
Tennessee has no law forcing counties to permit accessory dwelling units, so whether unincorporated Williamson County allows a detached guest house or second dwelling depends on the Zoning Ordinance and your residential district's standards. An accessory structure must stay incidental and subordinate to the main home and, with other accessory uses, cover no more than 20 percent of the lot. Where a dwelling unit is allowed, the Building Codes Department issues the permit and the Sewage Disposal Management Department must approve septic capacity, since most of the county is on septic. Franklin, Brentwood, and Nolensville set their own ADU rules.
Building or occupying a second dwelling where the zoning district does not allow it is a zoning violation that draws enforcement and denial of occupancy. Building without a permit is unlawful.
Other ordinances people look up for this city. Green dot = verified primary-source excerpt.
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See how Williamson County's adu rules rules stack up against other locations.
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