ADU rules in Grand Junction

Detached ADUs (backyard cottages, carriage houses) are allowed with an administrative Planning Clearance on any lot with a single-unit detached dwelling, a single-unit attached dwelling, or a duplex. Table 21.04-2 lists ADUs as an allowed accessory use in every residential zone district (R-R, R-ER, R-1R, R-2R, RL-4, RL-5, RM-8, RM-12, RH-16, RH-24) plus the MU-1, MU-3, and P-2 districts. A lot with one single-unit dwelling may even have two ADUs, provided at least one of them is attached to the house; a single detached ADU on its own is fine. ADUs are not allowed with cottage court dwellings, and stricter ADU limits in older planned-development ordinances are unenforceable.

Attached and internal ADUs (an attic or basement apartment, or an addition) are allowed under the same administrative Planning Clearance process as detached units. The code's definition covers units attached to the principal structure or freestanding, and the ADU standards use attic, basement, and carriage house conversions as examples. Where a lot takes advantage of the two-ADU allowance, at least one of the two must be attached to the principal dwelling.

An ADU may not exceed 900 square feet of habitable space; the city's 2025 ADU Toolkit clarifies that habitable space excludes mechanical rooms, unconditioned space, and stairwells. ADUs are expressly exempt from the general accessory-structure cap of 75 percent of the principal structure's square footage (GJMC § 21.04.040(c)(4)(i)). There is no minimum size and no percent-of-primary-dwelling rule, but the design and location of the ADU must be clearly subordinate to the principal structure and the ADU cannot sit in front of it. The 900 sq ft cap comfortably exceeds the 500 to 750 sq ft range that Colorado law requires subject jurisdictions to allow.

Grand Junction's ADU standards contain no owner-occupancy requirement for either the main house or the ADU, so an ADU can be built and rented on a fully tenant-occupied property. Colorado HB24-1152 (C.R.S. 29-35-403(2)(b)) bars subject jurisdictions from imposing owner-occupancy mandates after June 30, 2025, and Ordinance 5263 (adopted June 18, 2025) recites that Grand Junction is a subject jurisdiction and updated the code accordingly. The city's short-term rental rules also impose no owner-occupancy test, though an STR permit may be issued only to the property owner. Owner occupancy matters only if you want the optional ADU Production Program Tier 2 incentive (up to $15,000), which is limited to income-qualified owner-occupants who commit for seven years.

Ordinance 5263 (adopted June 18, 2025) deleted the former requirement of one parking space per ADU (which on-street parking within 100 feet of the lot could satisfy) as part of the city's HB24-1152 compliance package. The current ADU standards say nothing about parking, and Table 21.08-2, the citywide minimum off-street parking schedule, has no line item for ADUs. State law (C.R.S. 29-35-403(2)(a) and (3)) now prohibits requiring a new off-street space for an ADU except on lots that have no existing off-street parking and where on-street parking is prohibited.

ADUs are approved administratively: a Planning Clearance, a permit issued by city staff, is required for any ADU, with no public hearing or discretionary board review. Even inside planned developments, ADU requests are processed through the administrative approval process of GJMC § 21.02.040, and any planned-development restrictions more restrictive than the code cannot be enforced. Building permits and inspections run through the Mesa County Building Department, which serves as the city's building authority.

An ADU may be rented long-term with no special license, and the city encourages it: the ADU Production Program (GJMC Title 43) pays city impact and water/sewer fees, plus up to $15,000 for income-qualified owner-occupants, in exchange for a recorded covenant barring short-term rental of both the ADU and the main house for five years (seven with the incentive). Renting any dwelling for fewer than 30 consecutive days requires an annual short-term rental permit issued to the property owner ($275 application per the city's STR page), and an ADU rented short-term is classified as a secondary short-term rental. The caps of 7 percent (downtown) and 3 percent (elsewhere) of residentially zoned lots apply only to primary STR permits, not to ADU secondary permits, but a residentially zoned building with four or fewer dwelling units may hold at most two STR permits, only one of them primary.