Boulder, CO Accessory Structures: ADU Rules (2026)

Section 9-6-3(n) allows one accessory dwelling unit per lot with a single detached dwelling unit, subject to Table 6-3's floor area caps. A detached ADU tops out at 800 square feet, rising to 1,000 square feet if it qualifies as an affordable accessory dwelling unit. An attached ADU is capped at 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 attached ADU maximum drops to 750 square feet.

An affordable attached ADU gets a larger allowance of two-thirds of the principal structure's floor area or 1,200 square feet, whichever is less, again with a 750-square-foot floor if the principal structure is small. The Board of Zoning Adjustment may grant a variance to these floor area limits under Section 9-2-3. Subsection (n)(1)(A)(iii) requires any owner renting out the ADU or the principal dwelling unit on the same lot to hold a valid rental license from the city manager under Chapter 10-3, and Subsection (n)(1)(A)(iv) flatly prohibits short-term rental of an ADU, and of the principal dwelling unit on a lot that has an ADU, except as specifically authorized under Section 10-3-19.

An owner also cannot convey an ADU separately from the principal dwelling unit, and an attached ADU built into the rear yard setback area is capped at 500 square feet of building coverage and 20 feet in height in the RR, RE, RL, and RMX-1 districts, per Subsection (n)(2)(B).