Jefferson County, CO Accessory Structures: Garage Conversions (2026)

Under Section 19.D, an ADU may be created by converting an existing qualifying dwelling into an ADU (for example, by building a larger primary dwelling). Table 1 notes that basements may be converted to an ADU regardless of the square footage or percentage-of-primary-unit limits that otherwise cap ADU size, and that detached units include units above a detached garage or other outbuilding. Any ADU may be at least 500 sq ft regardless of the percentage cap. A converted unit must still meet accessory-structure setbacks (if detached), height limits, one extra off-street parking space per ADU bedroom, architectural-compatibility standards, and owner-occupancy. A building permit is required and reviewed by Planning & Zoning.