ADU rules in Casper

Casper allows accessory dwelling units in exactly one zoning district, R-2 (One Unit Residential), and only with a conditional use permit: CMC 17.32.030 lists "Accessory dwelling unit" at subsection J among the R-2 conditional uses. No ADU appears anywhere in the permitted or conditional use lists for R-1 Residential Estate (17.28.020 and 17.28.030), AG Urban Agriculture (17.24.020 and 17.24.030), R-3 (17.36.020 and 17.36.030), R-4 (17.40.020 and 17.40.030) or R-5 (17.44.020 and 17.44.030). A detached unit in R-2 needs the permit on a second, independent ground as well: CMC 17.12.250(D) bars more than one principal building on a lot in R-1 and R-2 unless a conditional use is approved, while permitting multiple principal buildings outright in every other district. The provisions are old and stable; they predate the code's online disposition tables and no ordinance through Ord. No. 03-26 of April 21, 2026 has touched them.

The same single listing covers attached and internal units, because CMC 17.08.010 defines an accessory dwelling unit as a habitable living unit "added to, created within, or detached from a single-family dwelling unit." A basement conversion, an internal unit or a rear addition in R-2 is therefore the identical conditional use as a backyard cottage and needs the same Planning and Zoning Commission hearing; Casper's code offers no lighter administrative track for interior conversions. Outside R-2 there is no ADU pathway at all, but in R-3, R-4 and R-5 a second attached unit is simply a permitted two-family dwelling (17.36.020(B), 17.40.020(C), 17.44.020(B)) needing only a building permit, which in practice makes those zones an easier place to add a unit than R-2.

Neither the definition at CMC 17.08.010 nor the R-2 conditional use listing states any square footage, percent of the primary dwelling, or height limit for an accessory dwelling unit. The closest binding standard is CMC 17.12.121(F)(4), which caps detached garages and accessory buildings at one thousand five hundred square feet measured at the maximum exterior wall dimension, or fifteen percent of the total lot area associated with the primary residence, whichever is less, with a Planning and Zoning Commission conditional use permit required to exceed either; 17.12.121(G) requires a separate conditional use permit for exterior wall height above twelve feet from the finished floor. That chain is an inference rather than an express rule: the code defines an accessory building as a "subordinate and detached structure, the use of which is incidental to the use of the principal building," and 17.12.121(F)(5) allows accessory buildings to be used "for those accessory uses allowed in the respective zone," but no section says in terms that a detached ADU is an accessory building. For an attached or internal unit only the R-2 bulk standards apply (17.32.040: four thousand sq ft minimum lot area, forty-foot lot width, fifteen-foot front setback with an alley or eighteen without, ten-foot rear, five-foot side). In every case the commission may condition the "area and height to be occupied by buildings" under 17.12.240(H)(1) and (I), so the permit, not the code, sets the real number.

Casper writes owner occupancy into the definition itself, not into a permit condition: "Either the principal dwelling unit or the accessory dwelling unit must be occupied by an owner of the property, or an immediate family member of the property owner" (CMC 17.08.010). The family carve-out is broad; the same section defines immediate family as husband, wife, child, stepchild, brother, sister, stepbrother, stepsister, parent, stepparent, grandchild, grandparent, mother-in-law and father-in-law, so a non-resident owner whose parent or adult child lives in either unit complies. Because the requirement lives in the definition it is ongoing rather than an at-application affidavit: a property where neither unit houses an owner or qualifying relative no longer has a conforming accessory dwelling unit. The conditional use agreement follows the land and binds later owners, and the city manager may revoke a permit whose continuing conditions go unmet (17.12.240(A) and (J)). Wyoming has no statute preempting or limiting local owner-occupancy rules.

Casper's parking table has no accessory dwelling unit line, so an ADU falls under item 39, "Residential: 2 parking spaces per dwelling unit" (CMC 17.12.080), the same demand the code applies to the house itself. Two extra spaces is high by national standards and is often the binding constraint on an R-2 lot as small as four thousand square feet. CMC 17.12.070(A)(10) lets the Planning and Zoning Commission grant an exception reducing the minimum after written staff findings and recommendations, but the reduction "shall not exceed fifty percent," and 17.12.240(I)(4) separately lets the commission fix the "location and amount of off-street parking" as a permit condition. Spaces must be surfaced in concrete or asphalt to the city's street specifications and reachable from a street, paved alley or driveway (17.12.070(A)(8) and (A)(12)(a)). Inside the Old Yellowstone District and South Poplar Street Corridor form-based code the residential standard drops to one space per dwelling unit (17.94.100(B)).

Every accessory dwelling unit in Casper needs a conditional use permit from the seven-member Planning and Zoning Commission after a noticed public hearing. For a site under three acres, written notice goes to every private property owner within a three hundred foot radius thirty days ahead of the hearing, with published notice at least fifteen calendar days out (CMC 17.12.240(D)). Since Ord. No. 18-25 took effect on January 20, 2026, an affirmative decision requires "the concurring vote of a majority of all members" of the commission, four of seven regardless of how many attend (17.12.210(B) and 17.12.190(A)), so absences now function as no votes. The commission must find the use consistent with the title's spirit, purpose and intent and compatible with adjacent land uses (17.12.240(G)), may impose continuing conditions, and the permit is void if the work is not completed within one year of issuance (17.12.240(I)). An aggrieved party may appeal to the city council within ten calendar days (17.12.240(L)). The city's current Conditional Use Permit application carries a $600 non-refundable fee and applications are due about thirty days before the commission's monthly meeting, the second Thursday. Once the conditions have been performed the use "shall be considered to be a permitted use" (17.12.240(J)).

Nothing in Casper's code forbids renting an accessory dwelling unit, and the owner-occupancy rule presupposes that one unit is leased: only "either the principal dwelling unit or the accessory dwelling unit" must house an owner or immediate family member, leaving the other free to rent. The same definition forecloses selling the unit off: "An accessory dwelling unit shall not be subdivided or otherwise segregated in ownership from the principal dwelling unit" (CMC 17.08.010). Casper has no short-term rental ordinance, registration or licensing regime in Title 17 or Title 5. The only overnight-lodging license, CMC 5.24.020, reaches a "lodging facility," defined at 5.24.010 as a business "where four or more rooms or structures on one parcel of real estate are rented for overnight accommodations," a threshold a single ADU does not cross. A hosted operation would instead be the separate "bed and breakfast" use, itself a conditional use in R-2 (17.32.030(A)) and limited by definition to stays of no more than seven consecutive days. Because a conditional use permit may carry continuing conditions, an individual ADU's permit can restrict rental more tightly than the code does.