ADU rules in Centennial

Detached ADUs (a backyard cottage, or a unit above a detached garage) are allowed by right. Table 12-2-302 lists 'Accessory Dwelling Unit' as a Permitted use (P) in the Agriculture (AG), Suburban Residential (RS), Auto-Urban Residential (RA), Urban Residential (RU), Neighborhood Conservation (NC) and Neighborhood Infill (NI) districts, and as a Limited use (L), still approved by the Director, in General Commercial (CG), Activity Center (AC) and Urban Center (UC). The Land Development Code defines a detached ADU separately from an attached or interior one, and Sec. 12-3-603(H)(5)(c)(iii) sets a specific 17-foot height cap for detached ADUs. Only one ADU is allowed per single-family detached dwelling, and the lot must contain one single-family detached dwelling.

Attached and interior ADUs are allowed on the same by-right basis as detached ones; the use table has a single 'Accessory Dwelling Unit' row covering all three forms. Sec. 12-16-201 defines 'ADU, Attached/Interior' as a unit inside the principal dwelling (a finished basement or attic) or attached to it (a converted garage or a structural addition). Sec. 12-3-603(H)(5)(a) gives interior conversions their own size allowance, and Sec. 12-3-603(H)(5)(e) requires a separate entrance, which may be reached through a shared foyer or mud room rather than a separate exterior door. Attached ADUs use the principal dwelling's zone-district height limit instead of the 17-foot detached cap.

An ADU may not exceed fifty percent of the footprint of the principal dwelling unit. As an alternative, an interior ADU may take up the entire attic or basement area, including the internal stairwell serving it, so long as no construction expands those footprints. Only habitable area counts, and for a detached ADU a garage attached to the unit is not counted as habitable. On top of the size cap, an ADU must meet the same setbacks and maximum building coverage as a principal dwelling in that zone district; a detached ADU may be no more than seventeen feet tall, while an attached ADU follows the zone district's normal height limit. Because the cap is tied to the house's footprint rather than a flat square footage, a small ranch house yields a small ADU.

Centennial does not require the owner to live in either unit. Instead, Sec. 12-3-603(H)(8) sets up a registration program that applies precisely when the owner of record will not occupy the principal dwelling or the ADU as a primary residence: before the ADU is occupied the owner must register it with the Community Development Department, pay a registration fee, and certify that written notice with the owner's contact information was mailed or hand delivered to every immediately adjacent property owner. The registration must be renewed annually, with renewal notice sent to adjacent owners by certified mail, and failure to register, renew or notify is a Municipal Code violation charged as a separate offense for each day. The city's ADU page states the rule plainly: if the property owner does not live on the property, the owner must register the ADU with the City. HB24-1152 bars subject jurisdictions from imposing owner-occupancy mandates, and Centennial's code has none, so there is no conflict on this point.

Table 12-5-202A requires one off-street parking space per accessory dwelling unit, on top of the two spaces per dwelling unit already required for a single-family detached home. Sec. 12-3-603(H)(5)(f) simply cross-references that table. The city's ADU page adds the practical detail that the ADU space may be provided in a driveway if space is available, so an existing driveway usually satisfies the requirement without new paving. Note that state law (C.R.S. 29-35-403(2)(a) and (3)(a)) bars a subject jurisdiction from requiring construction of a new off-street space for an ADU while allowing it to require designation of an existing driveway, garage or tandem space, so Centennial's flat one-space rule is in tension with the statute where no existing off-street space is available.

ADUs are approved administratively, with no public hearing. Sec. 12-3-603(H)(3) routes them through a building permit application subject to the building permit referral process, after a required pre-submittal meeting with staff under Sec. 12-14-303 (ADUs are specifically carved out of the exemption that spares ordinary residential accessory buildings from that meeting). In the six residential districts the use carries a 'P' in the use table, meaning permitted and approved by the Director; in CG, AC and UC it carries an 'L', which Sec. 12-2-301(B)(3) defines as a Limited Use that is still 'permitted as of right and approved by the Director' subject to the extra standards in Sec. 12-2-401(D). Applicants also apply for a separate address assignment, since every ADU must have its own address, and non-owner-occupied ADUs must be registered before occupancy.

Renting an ADU on a normal lease is allowed and is the use the code plainly contemplates; the only string attached is the Sec. 12-3-603(H)(8) registration and neighbor-notice program that kicks in when the owner does not live on the property. Short-term rental (under thirty days) of an ADU is not allowed. Both the city's ADU page and its short-term rental program page state it outright, and the code hook is Sec. 6-9-40(3), which lets the Director deny an STR license where the application asks to license 'any other accommodation other than the permanent primary dwelling structure at the address'. STR licenses remain available for the primary house itself under Chapter 6, Article 9 (annual license, two renters per bedroom up to eight people, two off-street spaces, 24-hour reachable representative). Home-based businesses are allowed in an ADU under Sec. 12-2-404. HB24-1152 expressly preserves local authority to regulate the short-term rental of ADUs, so this ban is consistent with state law.