Fort Collins, CO Accessory Structures: ADU Rules (2026)

The City adopted the 2024 International Residential Code's Appendix BC, Accessory Dwelling Units, in its entirety through City Code § 5-30, with local amendments spelled out at item 85 of that section. 2, an ADU is permitted without triggering a change-of-occupancy review only when it satisfies all six listed conditions together. It must be located within an existing single-family detached dwelling or an existing townhouse unit that falls within the scope of the International Residential Code, so a brand-new stand-alone ADU built without a qualifying existing home does not fit this appendix.

Only one ADU is allowed per lot. The ADU must carry a house number separate from the primary dwelling's address, letting utilities and emergency responders tell the two units apart. Size and function must remain secondary to the primary home, and the unit must also satisfy the City's separate Land Use Code, which is where zoning-based floor-area, height and district-eligibility limits actually live; the building code amendment itself sets no square-footage cap. An attached ADU needs its own separate entrance, either from the building's exterior or from a common hallway shared with the primary unit.

7. Every ADU, attached or detached, must have its own provisions for electricity, water supply and sewage disposal rather than tapping the primary home's service. 2 conditions before issuing a permit.