ADU rules in Rock Springs

A detached accessory dwelling is a principally permitted use, but only in the R-2 Low Density Residential Zone. The § 157.086(B) table of uses lists 'Accessory dwelling' under Residential Accessory Uses with a single entry, 'P (13)', in the R-2 column; the R-E, R-1, R-3, R-4, R-5, R-6 and B-R columns are blank, and § 157.007 provides that any use not expressly permitted or conditionally permitted in a district is excluded. The R-2 purpose statement backs this up: the zone exists for 'single-family detached dwellings, accessory dwelling units and directly related complementary uses.' So the gate is your zoning designation rather than a hearing: check the city zoning map first, because R-1 and R-E, the other low-density single-family zones, do not allow ADUs at all. An owner outside R-2 has only the § 157.224 route of asking the Planning and Zoning Commission to grant an exemption for a use consistent with the district.

The definition forecloses attached, internal and basement units. § 157.008 defines an ACCESSORY DWELLING as 'A detached structure which is accessory to a single-family dwelling, located on the same lot as a single-family dwelling and placed on a permanent foundation, with no more than one bedroom.' The same definitions section adds that 'Where any part of an accessory building is connected to part of the principal building, such accessory building shall be deemed to be a part of the principal building', and § 157.129(A) treats an accessory building joined by a covered passageway as an integral part of the principal building. A second unit inside or attached to the house would therefore be a two-family dwelling, which the § 157.086(B) table permits only in R-3, R-4 and B-R, never in R-2 where accessory dwellings are allowed. § 157.128 separately bars using a cellar, garage, or a basement under an unfinished structure as a dwelling unit, though it does let the basement of a finished home be used for living, eating and sleeping as part of that home.

§ 157.086(B)(13)(b) caps a new or newly converted accessory dwelling at 'no more than 75% of the square footage of the principal building or 800 square feet, whichever is less', with a hard floor of 300 square feet. The percentage test bites on smaller houses: below roughly 1,067 sq ft of principal building, 75% governs instead of the 800 sq ft ceiling. The § 157.008 definition adds a separate ceiling of no more than one bedroom, which is unusually restrictive. Height is capped below the principal building by the R-2 development-standards row for structures accessory to single-family residences, and § 157.129(G) requires a conditional use permit to exceed the principal building's height. Watch the combined accessory-building cap too: § 157.129(D) and (E) allow no more than 1,500 sq ft of residential accessory building floor area in total, so an existing detached garage plus an 800 sq ft ADU can trip the conditional use permit requirement.

§ 157.086(B)(13)(e) is one sentence: 'Owner occupancy. Either the principal building or the accessory dwelling shall be owner-occupied.' It is an ongoing condition of the use, not a one-time test at application, and the owner may pick either unit to live in and rent the other. There is no hardship exemption, no term of years, and no recorded-covenant mechanism spelled out in the code. Wyoming has no statute preempting local owner-occupancy conditions on accessory dwellings, so nothing at the state level softens this.

§ 157.086(B)(13)(d) requires 'At least one off-street parking space ... provided solely for use by the accessory dwelling occupants and in addition to off-street parking for the principal building.' Because § 157.133(E)(1)(a) already requires two off-street spaces per single-family dwelling unit, a lot with an ADU needs three spaces in practice. The ADU's space may sit inside the accessory dwelling structure (a garage bay under the unit) provided the non-parking part of the accessory dwelling is at least 300 square feet, which interacts with the 300 sq ft minimum unit size. Required spaces and their driveways must be paved with asphalt, concrete or asphaltic concrete under § 157.133(B)(12)(a), and a driveway serving a detached structure must be paved at least to the fence line or building line.

In R-2 an accessory dwelling carries a 'P' in the use table, and the table's own legend reads 'P - Principally permitted use', so review is administrative with no conditional use permit and no Planning and Zoning Commission hearing. Construction goes through an ordinary building permit from the Building Official under § 150.02, applying the International Residential Code, 2024 edition, adopted by reference at § 150.04; the § 157.220 zoning permit covers only structures the International Codes exempt (fences, decks under 30 inches, sheds up to 200 sq ft and the like), which an ADU is not. Because Rock Springs zones under Wyo. Stat. §§ 15-1-601 through 15-1-611 (§ 157.002) and has adopted an IRC version, Wyoming's new Fast Track Permits Act applies to permit applications filed on or after July 1, 2026: written completeness notice within 10 business days, approve or deny within 30 calendar days of a complete application, written denial reasons within 7 days, and deemed approval as submitted if the 30-day deadline lapses. Conditional use permits still surface at the edges: exceeding the principal building's height (§ 157.129(G)) or exceeding 1,500 sq ft of combined accessory-building floor area (§ 157.129(E)), which must be resolved before a building permit issues.

The code sets no minimum lease term and imposes no rental license or registration on accessory dwellings; the only occupancy condition is § 157.086(B)(13)(e), which requires either the house or the ADU to be owner-occupied, leaving the other unit free to rent long term. Short-term rental is a different story: Rock Springs has no short-term-rental ordinance at all, and the code's transient-lodging uses (hotel or motel, tourist home, lodging house) appear nowhere in the residential use table, so under § 157.007 nightly rental of an accessory dwelling in R-2 is an excluded use. The single transient pathway in residential zones is a conditional-use bed and breakfast inn, defined at § 157.008 as 'An owner occupied dwelling unit that contains no more than two guest rooms' and limited by § 157.086(D)(2) to breakfast only, two guest rooms, an unaltered single-family exterior, and guests who 'may stay no more than six consecutive days'; that is rooms inside the house, not a separate accessory dwelling. Separate sale is not contemplated: the ADU must sit on the same lot as the single-family dwelling and § 157.086(B)(13)(f) allows 'a maximum of one accessory dwelling ... per parcel or lot.'