Detached ADUs and backyard cottages

Can my lot have a detached ADU?

Maybe, and the answer changes at every city line. A detached accessory dwelling unit, a DADU in Metro’s shorthand, is a second, smaller home on a single-family lot. Here is what each local code said when we last checked it in October 2026. We confirm your parcel with the codes office before anything is drawn.

Metro Nashville and Davidson County

• Allowed where the zoning table permits it, on a lot that meets the minimum area: in the Urban Services District, a DADU overlay, or a plan with DADU standards.
• One per parcel, on a lot with one single-family house.
• Living space up to 700 square feet on a lot under 10,000 square feet, or 850 square feet on a larger lot. The footprint can’t exceed the main house’s.
• Eaves no higher than 10 feet for one story or 17 feet for two; the ridge stays below the main house and no higher than 27 feet.
• One owner for both homes, and one of them owner-occupied. Owner-occupied short-term rental isn’t permitted in a new DADU on single-family lots.
• A registered contractor has to pull the permit, through the Zoning Help Desk or ePermits.

Franklin

• One accessory dwelling per lot, with a footprint no more than half of the principal building’s.
• On a single-family lot, the owner has to live on the property.

Unincorporated Williamson County

• One accessory dwelling per parcel.
• Up to 750 square feet or 25 percent of the house, whichever is greater, and never more than 75 percent of the house.

Brentwood

We haven’t found a provision for accessory dwellings in Brentwood’s code, and it bars living quarters in accessory buildings except on parcels of three acres or more. If you live in Brentwood, we ask the city’s planning office about your lot before we draw anything.

Tennessee

There’s no statewide ADU law. A 2026 bill, HB 2031, was taken off notice in March, so each city’s and county’s own rules are the ones that apply.

How does a DADU project go?

1. Feasibility and zoning reviewYour parcel against the code: lot size, setbacks, overlays, height.
2. ConceptPrefab or custom, sized to what the lot allows and what you need it for.
3. Engineering and permittingDrawings for the codes office, and the permit pulled by us as your contractor.
4. Construction and utilitiesFoundation, framing and roof, and the utilities connected.
5. Interior finishThe inside finished like a house, because it is one.

Why ask a sunroom company to build one?

Because the hard parts of a backyard cottage are the same ones we handle on every room addition: the permit, the foundation, tying in heat and air, and finishing the inside to a standard you’d live in. We’ve been building onto Middle Tennessee houses for more than 60 years, and a DADU is that work on its own foundation.

• Metro Nashville Codes, Detached Accessory Dwelling Unit (page updated December 22, 2025).
• City of Franklin Zoning Ordinance §5.2.7.A, effective January 13, 2026.
• Williamson County Zoning Ordinance §11.04.
• City of Brentwood Municipal Code §78-22(b).