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

Jefferson County's ADU rules are codified in Section 19 (Accessory Uses) of the Jefferson County Zoning Resolution, last amended January 28, 2025. ADUs are allowed only as accessory to a single-family detached dwelling in Agricultural (A), Residential (R, MR, SR), and comparable Planned Development (PD) zone districts. Attached ADUs use the underlying zone minimum lot size; detached ADUs require at least 7,500 sq ft. Maximum size for detached ADUs scales by lot tier: 600 sq ft (or 40% of primary, whichever is less) on 7,500-12,499 sq ft lots; 800 sq ft on 12,500 sq ft to under 1 acre; 1,200 sq ft on 1+ acre lots. Attached ADUs may be 1,200 sq ft or 40% of the primary unit, whichever is less. Height is capped at the height of the primary residence or 25 ft, whichever is less. Owner-occupancy is required (the owner must live in either the primary home or the ADU). Maximum 3 residents per ADU and no more than 6 unrelated individuals total on the property. Properties on a well must obtain a Colorado Division of Water Resources letter confirming the existing well permit can serve both units; septic systems must be sized for total bedrooms. ADUs cannot be sold separately without subdivision. Colorado HB24-1152 (signed 2024, effective June 30, 2025) preempts certain ADU restrictions in 'subject jurisdictions' but applies primarily to municipalities; unincorporated Jefferson County is not a subject jurisdiction.