Boulder County, CO Accessory Structures: ADU Rules (2026)

Boulder County's Land Use Code (LUC) regulates accessory dwellings in unincorporated areas through Article 4-516.G. The historic county standard limited detached accessory dwellings to roughly 800 sq ft, with attached units up to 1,000 sq ft, and required common ownership with the primary dwelling so that no new building lot or condominium ownership is created. After the December 2021 Marshall Fire, the Board of County Commissioners adopted Docket DC-22-0003 (Resolution 2022-062, August 4, 2022) creating a Disaster Recovery Unit category up to 900 square feet, eligible for expedited administrative review and allowed as standalone or incorporated structures. Disaster Recovery Units may not be used as short-term or vacation rentals. At the state level, HB24-1152 declares ADUs a matter of mixed statewide and local concern and requires Subject Jurisdictions to allow at least one ADU as an accessory use to a single-unit detached dwelling through administrative review by June 30, 2025. The statute defines Subject Jurisdiction as a municipality of 1,000+ population within a Metropolitan Planning Organization, or a county portion within both a 40,000+ census-designated place and an MPO area. Most unincorporated Boulder County is reviewed under the county LUC; HOA bans on ADUs are also limited under HB24-1152 in subject jurisdictions. Applicants apply through Boulder County Community Planning & Permitting (303-441-3930).