Onslow County, NC Accessory Structures: ADU Rules (2026)

8(A) of the Onslow County Code of Ordinances governs accessory dwelling units on lots outside the towns of Jacksonville, Swansboro, Richlands, and Holly Ridge. Only one ADU is permitted per lot containing a single-family dwelling, and it must remain clearly secondary to the principal home. In the R-15 and R-20 zoning districts, the ADU's heated gross floor area may not exceed 900 square feet or 60 percent of the principal dwelling's heated area, whichever is smaller; property owners in the Rural Agricultural (RA) district get a larger ceiling of 1,200 square feet.

A detached ADU has to sit in the side or rear yard, never the front, and must meet the same setback lines that apply to the principal structure on the lot. The ordinance forecloses a common workaround: no recreational vehicle and no manufactured home may be used to satisfy an ADU, so a camper parked behind the house does not count no matter how it is wired or skirted. Because Onslow County folds its land-use code into the general Code of Ordinances rather than a stand-alone unified development ordinance, the same Article 9 standards apply whether the ADU is attached above a garage or built as a freestanding cottage.

A Zoning Permit confirming the floor-area math and setback compliance goes to the Planning and Development Department before construction begins. If the Zoning Administrator finds a completed or occupied ADU exceeds the size cap or sits on a nonconforming setback, the property is treated as a zoning violation under Article 5, subject to a notice of violation, a correction deadline, and civil penalty citations if the owner does not bring the structure into compliance.