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

Summit County's zoning is administered by the Summit County Community Development - Planning Department under the Summit County Land Use and Development Code. Each single-family lot is zoned for one unit of density; the ADU is treated as a workforce-housing exception that adds a second unit of density when the property owner records the required restrictive covenant. To approve an ADU in unincorporated Summit County, the property owner submits a building permit application together with a signed and notarized covenant; no separate planning approval is required for most properties. Exceptions that still trigger Class 2 planning review include ADUs proposed on a duplex lot and ADUs in a Planned Unit Development (PUD) where the PUD document calls for Class 2 review. The covenant restricts occupancy of either the ADU or the primary residence to a Qualified Occupant - someone who works in Summit County at least 30 hours per week on average - or to a Relative of the property owner. The ADU may be incorporated into the primary residence or detached as a stand-alone structure or above or below a garage or outbuilding. Water and wastewater capacity must be verified, and 2018 IRC fire and sound separation standards apply between the units. To encourage construction, Summit County waives all county-administered fees (building permit, planning, engineering) provided the property continues to comply with the covenant and the Land Use and Development Code for a minimum of 10 years; tap fees and design fees outside county purview are not waived. The Summit County Housing Department also operates the ADU Assistance Program, which reimburses up to 25% of total ADU construction cost (design, materials, labor, utilities, associated construction costs) for new ADUs in unincorporated Summit County; building permits already issued or completed are ineligible, so applications must be filed before permit issuance. Program-funded ADUs carry an additional 110% of Area Median Income rental cap in the recorded covenant. Inside town limits, town zoning controls instead of county rules: Breckenridge Development Code Policy 51 (Section 9-1-19-51A) limits an ADU to the lesser of one-third the density of the primary unit or 1,200 square feet and applies its own Town deed restriction; Dillon's Municipal Code allows secondary residential units in the RE, RL, RM, and RH zones subject to a Level III Development Permit and Planning and Zoning Commission site-plan review; Frisco and Silverthorne run their own programs. Summit County is a non-MPO county - it lies outside the Denver Regional Council of Governments, North Front Range, Pikes Peak, Grand Valley, and Pueblo Area MPOs - so HB24-1152's mandate that 'subject jurisdictions' allow one ADU by administrative review in any zone where single-unit detached dwellings are permitted does not apply. Summit County and its towns continue to regulate ADUs under their own home-rule and CRS Title 30 county powers.