Boulder County, CO Accessory Structures: ADU Rules (2026)
ADU rules in Boulder County, CO, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Governing code
- BCLUC Article 4-516 (unincorporated)
- Ownership
- Must stay in common ownership
- Floor area added
- 675-1,000 sq ft by parcel size
- Wastewater
- 150 GPD min per ADU (CDPHE Reg 43)
- Cities differ
- Boulder/Longmont set own ADU rules
Summary
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
An accessory use must be a use customarily incidental to and on the same parcel as the main use. A use listed in 4-500 may be an accessory use if the Director determines that the use is customarily incidental to a main use.
Full Breakdown
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.
Violations & Fines
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.
Frequently Asked Questions
Can I sell the ADU separately from my house?
Do I need a septic upgrade for an ADU?
Sources & Official References
Other rules in Boulder County
How Boulder County compares: Easiest Cities to Build an ADU·Compare Boulder County to another location·View the Colorado accessory structures overview
See something wrong?
Help us keep this page accurate. If you notice an error or outdated information, let us know.