ADU rules in Boulder County, CO — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Unincorporated Boulder County allows accessory dwelling units by right in most residential districts under the Land Use Code (Article 4-516). An ADU must stay in common ownership with the main home, meet setbacks and floor-area limits, and comply with wastewater (OWTS) sizing. Inside Boulder, Longmont, Louisville and Lafayette, city rules
Boulder County directly zones the unincorporated area under CRS Title 30, Art. 28 through the Boulder County Land Use Code (BCLUC). An Accessory Dwelling is a permitted accessory use (Article 4-516) and must be customarily incidental to, and on the same parcel as, the main residence. The ADU cannot create a separate building lot or convert the property to condominium ownership, and counts toward the parcel's maximum Residential Floor Area (an added 675-1,000 sq ft over existing floor area as of May 13, 2025, depending on parcel size). Every ADU with plumbing must be sized for onsite wastewater (OWTS) at 150 gallons/day minimum per CDPHE Reg 43. In 2026 the county opened text amendment DC-26-0002 to broaden ADU allowances.
Building without required Land Use and building permits is a zoning violation subject to Article 4-1300 enforcement, stop-work orders, and daily fines; unpermitted ADUs may be ordered removed.
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