ADU rules in Jefferson County

Section 19.D of the Jefferson County Zoning Resolution lets Planning and Zoning permit one accessory dwelling unit, attached or detached, in every agricultural and residential zone district and in comparable Planned Development districts, as an accessory use to a single family detached residence. A detached unit may be a standalone cottage or a unit above a detached garage or other outbuilding, and it may be built with the primary house or added later. The one hard gate is lot size: detached ADUs are not permitted on lots smaller than 7,500 square feet, and the size cap steps up at 12,500 square feet and again at 1 acre. On a qualifying lot there is no hearing, just a building permit reviewed by Planning and Zoning staff.

The same subsection states that the accessory dwelling unit may be attached or detached, and the county's guide and FAQ describe an attached ADU as one in a basement or in an attached addition to the existing house. For an attached unit the only minimum lot size in Table 1 is the minimum lot size of the underlying zone district, so an attached or basement ADU is available on small lots where a detached one is not; the county's FAQ gives the example of a 4,000 square foot lot where an attached ADU is allowed but a detached one is not. Basement conversions are expressly exempt from the square footage and percent-of-primary caps. An attached ADU still needs its own water and sanitation approval, still counts against the total accessory square footage cap, and still triggers the owner occupancy rule.

Table 1 in Section 19.D pairs a minimum lot size with a maximum unit size, and the detached caps step down as lots get smaller. The percentage is measured against the total square feet in the primary unit including basements but excluding attached or detached garages, and where the flat cap and the percentage disagree the smaller number governs, subject to a 500 square foot floor that applies to any ADU. Two separate caps in Section 19.B often bind before the ADU table does: all accessory square footage on the lot combined (the ADU plus every attached and detached garage, shed and other accessory use) may not exceed the primary residence's square footage, and an accessory structure's footprint may not exceed 75 percent of the primary structure's footprint. Height is limited to the accessory structure limit of the underlying zone district, which is 25 feet in the residential districts, or the height of the primary structure, whichever is less.

The owner of the property as reflected in the title records must occupy one of the two dwelling units as their primary residence, shown with voter registration, vehicle registration or similar documentation. The county's FAQ states the practical effect plainly: you may live in the house and rent the ADU, or live in the ADU and rent the house, but you may not rent both. The Director of Planning and Zoning may waive the requirement for temporary absences once the owner has occupied the primary or accessory unit for at least two years, and because owner occupancy sits among the Lot and Building Standards rather than the no-relief Requirements, an applicant may also seek relief through the administrative exception process or a variance. HB24-1152's statewide ban on owner occupancy mandates does not reach unincorporated Jefferson County: the state Division of Local Government's applicability table classifies Jefferson County as exempt from that law with an option to opt in, and the county has not opted in.

Section 19.D.2.b(10) requires one additional off-street parking space for each bedroom in the ADU, on top of the primary residence's own requirement. Because the Zoning Resolution defines an accessory dwelling unit as a unit limited to two bedrooms, the practical ceiling is two extra spaces. Section 14 credits an 18 foot space in front of a garage, meaning a full driveway, toward parking requirements, and Section 19.D.2.b(12) requires the ADU to use the same driveway as the primary unit. The requirement is a Lot and Building Standard, so an owner who cannot fit the spaces may request an administrative exception or a variance; HB24-1152's limits on new ADU parking mandates do not apply here because the county is not a subject jurisdiction.

Approval is administrative. Section 19.D.2.c requires a building permit for the accessory dwelling unit, and Planning and Zoning staff evaluate the ADU criteria when that building permit is submitted. The county's own FAQ answers the question directly: no hearings are required as long as the ADU criteria are met, and only a building permit application is needed. Notice is mailed to adjoining lots, and to any property owners' association or similar entity registered with the county, once a complete application is submitted, but that notice does not create a hearing or a discretionary approval. A hearing only enters the picture if the applicant needs relief from a Lot and Building Standard such as the 7,500 square foot detached minimum, the size caps, owner occupancy or parking, and pursues a variance before the Board of Adjustment rather than an administrative exception.

The county's ADU FAQ answers 'May I rent my Accessory Dwelling Unit?' with 'Yes', and allows the reverse arrangement, owner in the ADU with the house rented, so long as the rental runs 30 consecutive days or more; renting both units is blocked by the owner occupancy rule. No more than three people may occupy the ADU, and the ADU may not be sold separately from the primary dwelling unless the owner goes through the county subdivision process. For stays under 30 days, Section 46 of the Zoning Resolution, adopted December 16, 2025 and effective February 2, 2026, allows a short-term rental in an accessory dwelling unit under an annual, non-transferable STR License issued by Planning and Zoning; applications are now taken through the Jefferson County Citizen Portal. The main STR conditions are: if the ADU is the rental, the single family dwelling on the same property must be the owner's primary dwelling; only one STR license per property; occupancy in an ADU short-term rental capped at three people; one off-street parking space per bedroom; the property must lie within a Fire Protection District and satisfy defensible space requirements; a 750 foot separation from any other permitted STR; and a cap of 1 percent of the dwelling units in each Fire Protection District, with primary residence licenses exempt from both the separation and the 1 percent cap.