ADU rules in Commerce City

A detached ADU (backyard cottage, alley-loaded unit or garage conversion) is allowed as a use-by-right in the R-1 Single-Family Detached Residential district, the R-2 Single-Family Attached Residential district and the AG Agricultural district, which are the only districts where a single-family detached dwelling is itself allowed by right, plus Planned Unit Development districts where a single-family detached dwelling is allowed. Ordinance No. 2737, passed on second and final reading January 5, 2026, added Accessory Dwelling Units to Table V-3 of Sec. 21-5400 with an 'R' in those three columns. A detached ADU must be sited so its front wall sits behind the rear wall of the principal building, must be shorter than the principal building or 22 feet, whichever is less, and must meet 5 foot side and rear setbacks with front and street-side setbacks matching the principal building. The city's own ADU page confirms detached additions are one of the four permit paths.

Commerce City recognizes three ADU types and permits all of them on the same terms: an Attached ADU shares at least eight feet of a common wall with the principal dwelling, a Contained ADU is fully inside the existing space of the principal dwelling (a basement finish, attic conversion or interior remodel), and a Detached ADU shares no wall. Attached and contained units may not alter the principal dwelling so it looks like multi-unit housing from a public or private street, which rules out multiple street-facing doorways or walkways to the sidewalk (an alley connection is allowed) and multiple mailboxes. Access must use an existing side or rear door unless the Building Code requires a new entrance, and no new doorways or stairways to upper floors may be added on a street-facing side. Water, sanitary sewer and electrical utilities may not be provided separately to any ADU.

The code text reads: the floor area of the ADU shall not exceed 75 percent of the floor area of the principal unit or 1,000 square feet, whichever is less, however a minimum of 750 square feet of floor area is permitted regardless of the size of the principal unit. So a 900 sq ft house still supports a 750 sq ft ADU, a 1,600 sq ft house supports 1,200 sq ft on the percentage test but is capped at 1,000 sq ft. A detached ADU must also be shorter than the principal building, and never more than 22 feet tall. The 750 sq ft floor exists because HB24-1152 makes it unlawful for a subject jurisdiction to bar ADU sizes between 500 and 750 square feet.

The code says in plain terms that an accessory dwelling unit may be owner-occupied or occupied by a person who is not the property owner, so no one has to live on site. Ordinance No. 2737 also carved ADUs out of the unity-of-ownership rule in Sec. 21-5420(4) that otherwise ties accessory uses to the principal use. This tracks C.R.S. 29-35-103(2)(b) from HB24-1152, which bars a subject jurisdiction from requiring that an ADU or any other dwelling on the same lot be owner-occupied. One narrow exception survives under state law and the city's own STR rules: a short-term rental cannot be run in the principal house by someone whose only residence is the ADU, and the principal house must be occupied as a long-term residence when it is not being rented short term.

Ordinance No. 2737 deleted the only row in the off-street parking table (Table VII-3 in Sec. 21-7233) that had applied to an accessory dwelling, the 2 spaces per dwelling unit requirement for a 'dwelling in conjunction with and supplementary to the principal use', and added no ADU row in its place, so the 2009 code now sets no minimum for an ADU. The 2025 Land Development Code is even more explicit: Sec. 21-4310.A.2.b lists Accessory Dwelling Units among the uses that shall have no minimum parking requirement. Staff told City Council that the city has no authority to require parking here because C.R.S. 29-35-103(2)(a) bars a subject jurisdiction from requiring construction of a new off-street space for an ADU. Vehicular access still cannot be separate from the principal dwelling's access unless the ADU is reached from an alley and the house from a street.

In Table V-3 an 'R' means the accessory use or structure is allowed in the district as a use-by-right, while a 'P' would mean use-by-permit; Ordinance No. 2737 gives Accessory Dwelling Units an 'R' in R-1, R-2 and AG, so there is no public hearing and no discretionary land use approval. The process is administrative: contact the city's GIS division at GIS@c3gov.com first to get a unit designation for the ADU (the principal dwelling keeps its address), then apply for the building permit through the eTRAKiT portal. The permit type depends on the work: Residential Alterations - Basement Finish for a basement ADU, Residential Alterations - Interior Remodel for any other contained ADU such as an attic or garage conversion, Residential Addition - Attached Addition for an addition, and Residential Addition - Detached Addition for a standalone structure. Zoning questions go to the Planner of the Day at 303-227-8777 or cdplanner@c3gov.com. HB24-1152 requires that a subject jurisdiction's ADU approval be subject to an administrative approval process.

Long-term renting is unambiguous: the ADU may be occupied by a person who is not the property owner, and Ordinance No. 2737 struck the old rule tying accessory uses to unity of ownership. Short-term rentals, meaning stays under 30 days, have been legal in Commerce City since January 1, 2025 but require a short-term rental unit license under Chapter 9, Article III, Division 9 of the Revised Municipal Code; the license costs $150, runs one year, is applied for through eTRAKiT, and requires $500,000 of liability insurance, a responsible agent within 30 miles who is reachable 24/7, advance written notice to adjacent and across-the-street neighbors, and a self-inspection. On a lot with both a house and an ADU, the STR may be in either unit, but not in both at once, and an STR cannot be operated in the principal house by a person whose primary residence is only the ADU; the principal house must be occupied as a long-term residence when it is not being short-term rented. No more than one licensed short-term rental is allowed within 500 feet of another in any residentially zoned district, stays may not exceed 30 consecutive days, occupancy is capped at two persons per bedroom plus two, and no exterior signage is allowed. HOA covenants that are stricter than city rules still apply.