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Williamson County, TN Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Williamson County, TN, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

ADUs per parcel: One, regardless of dwelling count
Size cap: Greater of 750 sq ft or 25% of house
Absolute size cap: Never over 75% of principal dwelling
Deed restriction: Recorded with Register of Deeds first
Mobile-home ADUs: Only where mobile homes are zoned
Construction method: Interior apartment or converted structure

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Tennessee under recent ADU streamlining laws. The Williamson County, TN rules below cover local size, height, and setback requirements.

Summary

Williamson County allows one accessory dwelling unit per parcel, built as an interior apartment or as a separate or converted accessory structure such as a garage, carriage house or stable. It is capped at 750 square feet or 25 percent of the principal house's size, whichever is greater, and can never exceed 75 percent of the house.

City-specific rules exist: Franklin has its own adu rules rules that differ from Williamson County's county-level regulations. If you live in Franklin, check the city-specific page instead.

(1) Accessory Dwelling Units a) Accessory dwelling units may be constructed within an existing dwelling unit (interior apartment) or as a separate or converted accessory structure (e.g., converted garage, carriage house, or stable). b) Only one accessory dwelling unit, regardless of the number of principal dwellings located on a single parcel, may be permitted. c) Accessory dwellings shall be limited in size to 750 square feet or 25 percent of the square footage of the principal dwelling, whichever is greater. In no case shall the accessory dwelling be more than 75 percent of the square footage of the principal dwelling. d) Accessory dwellings classified as mobile homes by the terms of this Ordinance are only allowable in zoning districts in which mobile homes are allowed.

Source: Tennessee Code AnnotatedView official code

Official source re-checked September 8, 2026: the cited page had not changed since it was quoted.

Full Breakdown

04(D)(1) governs residential accessory dwelling units countywide, in unincorporated Williamson County only. Only one ADU is permitted per parcel no matter how many principal dwellings already sit there, and it can be built inside the existing house as an interior apartment, attached to the exterior, or created as a separate or converted accessory structure such as a converted garage, carriage house, or stable. Size is capped at 750 square feet or 25 percent of the principal dwelling's square footage, whichever is greater, but the ADU can never exceed 75 percent of the principal dwelling's size.

Mobile-home ADUs are allowed only in zoning districts where mobile homes are permitted outright. Before a Zoning Certificate and building permit issue, the applicant must record a deed restriction with the Register of Deeds stating that no additions to the ADU will be permitted unless the property and structure meet current dimensional standards for dividing the property, and a copy must reach the Community Development Department first. Interior apartments must preserve the single-family exterior appearance and cannot add a second front door; any addition to the living quarters must still meet the applicable zoning district's dimensional rules.

04(D)(2), allows one commercial accessory dwelling unit per parcel, capped at 750 square feet, located inside the principal structure and limited to use by the owner or employees of the commercial use on site.

Violations & Fines

Occupying or building an ADU without the recorded deed restriction and approved Zoning Certificate is a violation under Section 22.03. The Codes Compliance Director enforces the Ordinance and can revoke a Zoning Certificate found out of compliance under Section 22.06. Conviction is a misdemeanor with a fine of ten to fifty dollars under Section 22.07(A) and Tenn. Code Ann. § 13-7-111, and each day the violation continues is a separate offense; decisions can be appealed to the Board of Zoning Appeals.

Frequently Asked Questions

Can I convert my garage into an accessory dwelling unit?

Yes. Section 11.04(D)(1)(a) allows an ADU built within the existing house as an interior apartment or as a separate or converted accessory structure such as a converted garage, carriage house, or stable, provided it meets the size and deed-restriction requirements below.

How big can an ADU be?

Section 11.04(D)(1)(c) caps it at 750 square feet or 25 percent of the principal dwelling's square footage, whichever is greater, but in no case may the accessory dwelling exceed 75 percent of the principal dwelling's size.

Do I need to record anything before building an ADU?

Yes. Section 11.04(D)(1)(e) requires a deed restriction recorded with the Register of Deeds stating no additions will be permitted unless the property meets current division requirements, with a copy delivered to Community Development before a Zoning Certificate or building permit issues.

Can I have more than one ADU on my property?

No. Section 11.04(D)(1)(b) permits only one accessory dwelling unit per parcel, regardless of how many principal dwellings already sit on that parcel.

Sources & Official References

[1]Williamson County, TN Zoning Ordinance (current through amendments)

Other rules in Williamson County

Shed & Carport RulesSome Restrictions Noise Ordinances: Decibel LimitsSome Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSome Restrictions Parking Rules: Loading ZonesSome Restrictions Fence Regulations: Retaining WallsSome Restrictions Fence Regulations: Fence Permit RequirementsSome Restrictions Animal Ordinances: Veterinary Clinic ZoningSome Restrictions

All Williamson County rules

How Williamson County compares: Easiest Cities to Build an ADU·Compare Williamson County to another location·View the Tennessee accessory structures overview

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ADU Rules in Cities Across Williamson County

Franklin, TN