ADU rules in Longmont

Longmont allows one ADU per lot as an accessory use to a single-family detached dwelling, and the unit may be located in a detached accessory structure such as a garage or a new backyard cottage. Table 15.04.020 lists the use in the Residential Rural (R-RU), Residential Single-Family (R-SF), Residential Mixed Neighborhood (R-MN), Residential Multifamily (R-MF), Mixed-Use Downtown (MU-D) and Nonresidential Agricultural (N-AG) districts. A new detached ADU may not sit closer to the front lot line than the house (with a corner-lot exception), must keep 5-foot rear and side setbacks, and may not exceed the height of the principal structure. An ADU created inside an existing detached structure is exempt from the new setbacks unless the structure is expanded.

An ADU may be integrated into the principal dwelling, including a basement, or added onto it. Attached ADUs joined by a common wall follow the principal-structure front setback for the district plus 5-foot rear and side setbacks, and a basement ADU is exempt from the size cap entirely. Every ADU must have a separate entrance, although a common hallway inside the building may serve both units, and a 3-foot all-weather sidewalk connection to the public sidewalk is required.

Regardless of the principal dwelling's size, an ADU may be between 500 and 800 square feet. It may exceed 800 square feet only if it stays within 50 percent of the finished, above-ground floor area of the main house, and an ADU located in a basement of the principal dwelling has no size restriction at all. A detached ADU cannot be taller than the principal structure. Separately, the combined floor area of all accessory structures on a residentially zoned lot, ADU included, cannot exceed 75 percent of the principal structure's total floor area under Sec. 15.04.040.B.1.d.

The code imposes no requirement that the owner live in the main house or the ADU on an ongoing basis, and as a subject jurisdiction under HB24-1152 Longmont may not require owner occupancy of the ADU or any dwelling on the same lot. What Sec. 15.02.080.E.2 does require is proof of residency at application time: a valid Colorado driver's license or state ID plus at least two supporting documents (vehicle registration, voter registration, tax returns or other financial documentation, or a utility bill) showing City of Longmont residency. That proof is waived when the ADU is built simultaneously with a new primary dwelling. The city's ADU Guide also expects the applicant to own the legally permitted detached home the ADU will serve. Once built, the ADU may be occupied or rented without the owner living on site, but it can never be sold separately from the principal dwelling.

The codified standard is one sentence: an ADU is not required to provide any off-street parking and may not add more than two off-street spaces. The parking table in Sec. 15.05.080 likewise lists the ADU allowance as 2 spaces on top of the principal use's maximum. The city's ADU Guide (July 2025) adds that one new space is required where the lot has no existing off-street parking and has no public street frontage, takes access from an alley, or sits on a block where on-street parking is prohibited; that exception mirrors what state law permits (C.R.S. 29-35-103(3) as enrolled) but does not appear in the codified section text.

ADUs are approved administratively. The code lists them under review standards for specific administrative applications, and the only approval is a building permit issued by the chief building official after multi-department plan check; no ADU may be established before that permit issues. Site plan review is expressly waived for single-family detached dwellings and their accessory structures, including ADUs, under Sec. 15.02.070.C.2, and Table 15.04.020 shows the use as an accessory use (A) rather than a conditional use in every district where it is allowed, so no public hearing is involved. The city's ADU Guide estimates city fees of roughly 5,000 to 10,000 dollars through the building permit process.

Nothing in the code restricts renting an ADU for 30 days or longer, and there is no owner-occupancy condition attached to renting it. Short-term rental use is flatly prohibited: Sec. 15.02.080.D.2.c states that accessory dwelling units shall not be used as a short-term rental, where an STR is any rental under 30 consecutive days. Longmont otherwise licenses STRs annually for a resident's primary dwelling or one second or investment dwelling, but an ADU is not an eligible STR dwelling at all. The ADU also may not be sold separately from the principal dwelling.