Barndominium with an In-Law Suite in Tennessee

That is the decision everything else follows from. The residential code defines a dwelling unit as a space with permanent provisions for living, sleeping, eating, cooking and sanitation, and Tennessee's own rule on dwelling units, 0780-02-23-.15, uses the same phrase, "complete independent living facilities." Without a permanent cooking facility the suite is normally part of your house, and the second-unit rules never engage. With one, the building normally holds two dwelling units, which brings a fire separation between them, a way out for each, and a zoning question about whether a second unit is permitted on your parcel at all. Sumner County publishes the clearest version of the test: indoor cooking appliances, a bedroom, a utility room with washer and dryer hookups, or full sanitation — any combination triggers an accessory dwelling unit permit, and the Planning and Engineering Director decides borderline cases. A sink and a small fridge sit on one side of that line. A range sits on the other.

Usually, and two counties say why in their own ordinances. Williamson County treats an accessory structure within 20 feet of the principal structure and connected via roofline as part of the principal structure — so a connected suite leaves the accessory regime altogether and then has to clear the principal-structure setbacks instead, which in the county's Rural Preservation and Rural Development districts means a 100 ft front yard. Maury County's accessory standards turn on the structure being subordinate to the principal one, and a single building holding dwelling and shop is one principal structure. Detached is where the caps and the siting rules live: Chattanooga confines a detached accessory dwelling unit to the rear or interior side yard, caps it at 800 sq ft and 24 ft, and separately forbids any detached accessory structure from exceeding the footprint or the height of the principal building. For a barndominium the attached answer is usually also the cheaper one, because it shares the frame, the slab and the services.

Smaller than most people expect, and the number is local. Chattanooga caps an accessory dwelling unit at 800 sq ft gross. Wilson County allows 1,000 sq ft of heated area for a detached accessory dwelling unit and 600 sq ft for internal accessory dwelling quarters, one of each per property. Williamson County allows 750 sq ft or 25% of the principal dwelling, whichever is greater, and never more than 75% of it. Maury County allows 750 sq ft or 30% of the principal dwelling, whichever is greater. Rutherford County sets no size cap on an accessory dwelling unit but does set an appearance standard. A useful way to see the squeeze: a 900 sq ft suite sits inside Wilson County's 1,000 sq ft detached ceiling, over Chattanooga's 800 sq ft cap, and in Williamson County would need a principal dwelling of at least 1,200 sq ft just to clear the 75% test. Note also that Metro Nashville sets no minimum floor area for a dwelling at all — it regulates size through lot area, floor area ratio and impervious surface ratio.

In the jurisdictions on the 2018 and 2021 editions, which share Chapter 3 numbering, R302.3 requires a 1-hour fire-resistance rating between the units, dropping to half an hour only where the building is sprinklered throughout, with the wall running from the foundation to the underside of the roof sheathing. There is an exception that lets the wall stop below a protected attic, but a clear-span shell has no attic to use it on, so the wall goes to the sheathing, and R302.3.1 requires whatever supports a rated floor to carry the same rating — which in a post-frame or steel shell is a framing decision, not a lining one. Smoke alarms are R314 and carbon monoxide alarms R315 in those editions. The 2024 edition renumbers parts of Chapter 3, so on a parcel in Nashville, Knoxville city, Maryville, Columbia, Sevier County or Rutherford County the section numbers come from that book instead. On sprinklers, the state rule makes R313 non-mandatory but expressly leaves local governments free to be stricter, so the honest answer is that it is a local check — Murfreesboro, for instance, has deleted R313 outright.

This is the question to settle before anything is drawn, because in Tennessee it genuinely varies. Jefferson County's zoning FAQ answers it flatly: "You are not allowed to have more than one dwelling unit on any one piece of property. In order to place a second home on your property you will have to hire a licensed surveyor to cut a tract off for that home." Robertson County allows one principal building per zone lot, but on agricultural lots over fifteen acres permits up to two additional dwelling units for immediate family or full-time farm employees, each on a site that independently meets the district's lot and yard requirements so it could be subdivided later. Montgomery County lists a guest house as permitted with conditions in its agricultural district. Knoxville takes a different route again, providing that accessory structures other than accessory dwelling units "shall not be used for occupancy" — which closes the live-in-the-shop plan inside the city. Where a research file for a jurisdiction records no second-unit rule, that is a question for the planning office, not a permission.

Yes, and it is the constraint people forget, because septic sizes on bedrooms and a suite bedroom counts like any other. Shelby County's regulations require a minimum 370 sq ft of trench bottom area per bedroom plus an area for full duplication of the field, so a four-bedroom building needs about 1,480 sq ft of trench bottom and the same again in reserve. Rutherford County puts it directly: if you are adding a bedroom, the number of bedrooms in the house has to match the number listed on your septic records. In Hamilton County the zoning regulations name Hamilton County Groundwater Protection as the body that sets the required disposal area and note it may limit the number of bedrooms on the basis of the effective capacity of the proposed system. Knox County asks for bedroom count and basement details on the application alongside soil maps from a licensed soil consultant. Sumner County requires an accessory dwelling unit to meet the applicable TDEC septic standards as well as building codes. So the suite's bedroom is designed against the field, and on a tight lot the field can be what decides the plan.

The state rule that governs this is Tenn. Comp. R. & Regs. 0780-02-23-.15, effective 25 February 2024. A single dwelling unit providing complete independent living facilities may meet the requirements for a one- and two-family dwelling where it is three storeys or fewer, has a maximum occupancy of twelve or fewer transient occupants, and is under 5,000 gross sq ft — and a transient occupant is one who stays not more than thirty days. An in-law suite sits comfortably inside all three limits, which is why letting one normally keeps the building under the residential code rather than pushing it into the commercial one. The same rule adds a detail almost nobody mentions: smoke alarms in dwelling units must be no more than ten years old from the date of manufacture. Zoning is a separate question from the code and has to be checked on its own — Sevier County, for example, requires one parking space per bedroom with a two-space minimum for a transient dwelling, against two spaces plus one per two bedrooms over four for an ordinary one. Several counties also attach conditions to the second unit itself, so the recorded deed restriction Williamson and Maury require is worth reading before any letting is planned.