Longmont, CO Accessory Structures: ADU Rules (2026)

On June 3, 2025, the Longmont City Council adopted updated ADU regulations to comply with Colorado HB24-1152 (signed in 2024, effective for subject jurisdictions June 30, 2025). The ADU standards are codified in LMC § 15.02.080.E with related provisions in § 15.04.040. ADUs are permitted as an accessory use in the following zones: Rural Residential (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). Size: an ADU may exceed 800 sq ft only if it does not exceed 50% of the finished, above-ground floor area of the principal dwelling. Height: a detached ADU cannot exceed the height of the main house. Owner-occupancy: the property owner must live in either the main house or the ADU — this is retained under Longmont's local rules to the extent permitted by state law. Sales: an ADU cannot be sold separately from the main home. Parking: under HB24-1152, Longmont may not require additional parking for an ADU unless parking is already required for the principal dwelling and a usable parking space currently exists. Short-term rental of an ADU is prohibited except in certain Planned Unit Developments (such as Prospect and Riverside at Mill Village) whose site plans explicitly allow STR use. Utilities must connect to the primary home's water and sewer service — separate taps are not permitted. Approval is administrative (no public hearing or subjective design review per HB24-1152). A building permit is required from Building Inspection.