ADU rules in Boulder

A detached ADU (a separate and complete housekeeping unit inside an accessory structure, such as a backyard cottage or converted garage) is allowed by right on any lot that has one detached dwelling unit. Table 6-1 of the Use Table marks accessory dwelling units [A] (allowed by right, subject to the Section 9-6-3(n) use standards) in every zoning district except RH-6, MH, BMS, BCS, IS-1/IS-2 and IMS, so all of the Residential Rural, Estate, Low, Medium, Mixed, and High districts qualify, as do the Mixed Use, Business Transitional, Business Community, Business Regional, Downtown, Industrial General, Industrial Manufacturing, Public, and Agricultural districts. Only one ADU is allowed per lot, and the lot must hold exactly one detached dwelling unit, so duplex and attached-dwelling lots do not qualify. Detached ADUs are capped at 800 square feet and 20 feet in height (25 feet if the roof pitch is 8:12 or steeper), must sit at least 6 feet from other buildings, and the city requires automatic fire sprinklers in them.

Attached and internal ADUs are allowed on the same by-right basis as detached ones: the code defines an accessory dwelling unit as a separate, complete single housekeeping unit either within the detached dwelling unit itself (a basement, upper-floor, or addition unit) or within an accessory structure. An attached ADU must be physically separated from the principal dwelling by a wall or a lockable door, and if an interior connection already existed it must be kept along with that lockable separation. Attached ADUs get relief from the principal-building rear yard setback: the ADU portion may come within 5 feet of the rear lot line if only ADU floor area sits in the setback, that portion stays under 20 feet tall, and (in the RR, RE, RL and RMX-1 districts) building coverage in the setback area stays under 500 square feet. Overall height still follows the district maximum in Table 7-1, which is 35 feet in every district that allows ADUs.

Table 6-3 in Section 9-6-3(n)(1)(A)(ii) sets the caps. A standard detached ADU may not exceed 800 square feet; a standard attached ADU may not exceed one-half of the principal structure's total floor area or 1,000 square feet, whichever is less, except that if the principal structure has less than 1,500 square feet of floor area the cap becomes a flat 750 square feet. Two incentive categories share a larger allowance: an affordable ADU (rents capped at 75 percent of area median income, with a recorded declaration of use) and an ADU on a property designated as an individual landmark or contributing to a historic district. Those get 1,000 square feet detached, or two-thirds of the principal structure or 1,200 square feet, whichever is less, attached (a flat 750 square feet if the principal structure is under 1,125 square feet). Floor area is measured under the Chapter 9-16 definition, which excludes uninhabitable space such as areas with ceilings 6 feet or lower. The Board of Zoning Adjustment may grant a variance to the floor area cap under Section 9-2-3.

Boulder repealed its ADU owner-occupancy mandate through Ordinance 8650, adopted in February 2025 and applicable to any ADU proposed on or after March 8, 2025; the city's ADU FAQ states plainly that ADUs are no longer required to be owner occupied, in accordance with state legislation. The current Section 9-6-3(n) contains no owner-occupancy standard at all. State law reinforces this: HB24-1152 (2024) bars a subject jurisdiction from requiring an ADU, or any other dwelling on the same lot, to be owner-occupied, and the Colorado Department of Local Affairs lists Boulder as a subject jurisdiction. Two related requirements survive: a short-term rental license anywhere in Boulder is only issued for the licensee's principal residence, and a detached dwelling with a second kitchen that is not an ADU is a separate use that still requires owner occupancy plus conditional use or use review approval.

Section 9-9-6 sets only parking maximums, not minimums: subsection (b) caps off-street motor vehicle spaces at one per dwelling unit for attached units and duplex units in the MU-4 and RH-7 districts and sets nonresidential maximums in RH-3, RH-6, RH-7 and MU-4, and nothing in the section requires a minimum number of car spaces for a residential use. Subsection (c)(5) goes further and voids any minimum off-street motor vehicle parking requirement, other than accessible spaces, carried in a planned development, planned residential development, planned unit development, site review or use review approval. Table 9-4, the minimum off-street bicycle parking table, lists 'Accessory dwelling units: no requirement'. Boulder therefore adds no parking obligation for an ADU, which also puts it inside the HB24-1152 limit on requiring new off-street parking for an ADU.

An ADU is an allowed use by right. In Table 6-1 an 'A' means the use is permitted by right in that district subject to the applicable specific use standards, and the bracketed [A] in the accessory dwelling unit row simply flags that the Section 9-6-3(n) standards apply. Since Sept. 1, 2023 (Ordinance 8571) ADUs are reviewed and approved through the ordinary building permit process and no longer need a prior administrative review approval; there is no public hearing and no saturation or density cap. A building permit is required even when no construction happens, as a change of use, so the unit is recorded and life-safety standards are checked. Two extra steps can apply: if the lot sits in a planned development, planned residential development, planned unit development or site review approval, a minor modification approval under Section 9-2-14(k) must be obtained before the building permit, and an affordable ADU needs a recorded declaration of use. A Board of Zoning Adjustment variance under Section 9-2-3 is available for the floor area cap.

Section 9-6-3(n)(1)(A)(iii) forbids renting either the ADU or the principal dwelling for consideration unless that unit holds a valid city rental license under Chapter 10-3, so long-term renting is expected and simply licensed. Short-term rental is the hard restriction: Section 9-6-3(n)(1)(A)(iv) prohibits short-term rental of an ADU and of a principal dwelling on an ADU lot except as authorized in Section 10-3-19, and Section 10-3-19(o) authorizes it only where both units were legally established by Feb. 1, 2019, a current short-term rental license exists, only one unit on the property is licensed or rented, and the ADU is rented short-term for no more than 120 days per calendar year. Section 10-3-19(o)(7)(A) bars any new short-term rental license application for these properties after Feb. 1, 2019, so existing licenses can only be renewed, and a renewal filed more than 90 days after expiration is lost. Citywide, a short-term rental license is only issued to a natural person, qualifying trust, or 501(c) nonprofit, the unit must be the licensee's principal residence, and a principal-residency affidavit is due annually. The ADU may also not be sold separately from the principal dwelling.