Do I need a permit for accessory dwelling unit (adu) in Tarrant County (Unincorporated)?

Most likely yes. Accessory dwelling unit (adu) usually requires a permit in Texas. Confirm the specifics with Tarrant County (Unincorporated).

An ADU is a dwelling. Texas Local Government Code 214.212 applies the residential code to all construction of residential structures in a municipality, and Section 214.211 defines residential to include accessory structures, so a city will review an ADU as new residential construction. Whether an ADU is allowed, and how large it may be, is set by local zoning. Cities may also amend the residential code by ordinance after a public hearing (Section 214.212(c) and (e)). In an unincorporated area, a county that has opted into Local Government Code Chapter 233, Subchapter F applies residential code standards to a new house or duplex on a vacant lot, with at least three inspections by a qualified third party, but that law does not require county approval before building. Building, altering, or extending an on-site sewage (septic) system needs a permit and approved plan from TCEQ or its local authorized agent under Health and Safety Code 366.051, which matters when an ADU adds wastewater to a septic lot. A new accessory dwelling unit, detached or added on, is new residential construction. It normally needs a building permit with full plans, plus separate electrical, plumbing and mechanical permits, and zoning approval for a second unit on the lot. Utility connections (water, sewer or septic, and electric service) are usually reviewed as part of the permit, so ask about capacity and connection fees early.