ADU rules in Mandan

Since Ordinance No. 1490 (adopted 9-1-2026) re-enacted Mandan's Land Use Code as Chapter 101 and repealed Chapter 105, a detached ADU in a new or existing accessory building is a 'permitted use with additional standards' in the RR, R7, R4, R3.2 and RMH residential districts, which take in nearly all of the city's single-family land (R7 and R3.2 alone are about 2,400 mapped acres). In R3.2 the ADU must accompany a detached dwelling, not a duplex. ADUs are not listed in the A, RM, RH, commercial, downtown, mixed-use or industrial districts, and the code bars any use a district does not list. Before the rewrite, ADUs were by right only in R4 (mapped nowhere) and RMH and needed a conditional use permit in R7 and R12; Municode still prints that superseded Chapter 105.

Section 101-7-4(6) lets an ADU sit within the principal building or in or attached to an accessory building, and lists conversion of part of an existing house, expansion of an existing structure that meets setback, coverage and height limits, and a new house built with an ADU as ways to add one, so basement, internal and addition units are allowed in the same five districts (RR, R7, R4, R3.2 with a detached dwelling, RMH). Every ADU needs its own outside entrance, must be completely separate from the primary unit with facilities for one family, and only one is allowed per lot. Review is administrative, the same as for detached units.

Section 101-7-4(6)(a)(iv) caps every ADU at 40 percent of the principal dwelling's habitable floor area and 800 sq ft. The old Chapter 105 applied the 40 percent cap only to ADUs inside the primary structure and set a 300 sq ft minimum for all ADUs; Ord. 1490 dropped the minimum and extended the percentage cap to detached units. A detached ADU's building must also meet the accessory-structure rules: in R7, R4, R3.2, RH and RMH (and RM lots with two or fewer units) all accessory buildings together may not exceed 10 percent of the lot area and none may exceed 75 percent of the size of the primary structure; height is 16 ft to the roof midspan or 25 ft to the peak, whichever is less, with 10 ft maximum walls (RMH district tables cap accessory buildings at 15 ft; RR allows accessory structures up to 3,200 sq ft). Accessory buildings go in the rear yard, 3 ft from side and rear lot lines, and the principal structure must be under construction first.

The owner of the parcel must occupy either the primary dwelling or the ADU as a legal residence for more than six months of any given year, an ongoing requirement rather than a one-time test at application; the owner may be a beneficiary of a private trust. The rule carried over unchanged from the repealed §105-1-3(3)c. North Dakota has no ADU statute, so no state law limits it.

The parking table in §101-12-1(8) requires 1 space per accessory dwelling unit and 2 per single-unit detached dwelling, so a house with an ADU needs 3 off-street spaces. Ord. 1490 dropped the old per-lot district minimums (3 per lot in R7, 4 in R12) and the rule that half of required residential spaces be exterior. The City Planner may grant reductions of up to 25 percent in listed circumstances such as a parking study, and larger reductions need a variance (§101-12-1(9)). The repealed §105-1-3(3)d carried the same one-space ADU rule.

Each eligible district lists the ADU under 'Permitted Uses with Additional Standards', a category Ord. 1490 created to move some former conditional uses to administrative approval: staff check the §101-7-4(6) standards through the building permit (§101-5-1) and certificate of occupancy (§101-5-2), with no public hearing. Under the repealed Chapter 105, an ADU in R7 or R12 needed a conditional use permit with a Planning and Zoning Commission hearing and City Commission ratification; that route no longer applies to ADUs. A drafting slip in §101-7-4(6)(a)(i) still says the ADU must be 'a listed an accessory or conditional use' in its district, but the district lists themselves name it as a permitted use with standards.

Nothing in Chapter 101 bars renting the ADU (or the house) long-term, but the owner must live in one of the two units more than six months a year, only one ADU is allowed per lot, and each unit houses one family (related persons or up to four unrelated individuals). The code defines 'short term rental housing' as stays of one night to 30 days and names attached and detached ADUs as examples, yet no district lists it as a permitted, conditional or accessory use, the 'dwelling' definition excludes short-term rentals, and §101-1-3(1)(a) bars any use not listed for a district, so nightly rental of an ADU is not a permitted use (an inference from the use lists; there is no express ban). Mandan's code has no rental registration or short-term rental license; owner-occupied, state-licensed bed and breakfasts are a separate conditional use. The ADU stays on the same lot as the primary dwelling, and the code makes no provision for selling it separately.