Boulder regulates accessory dwelling units under Boulder Revised Code (BRC) Title 9 Land Use Code, primarily Section 9-6-3(n). City Council adopted sweeping ADU reforms in February 2025 that apply to any ADU proposed on or after March 8, 2025: owner-occupancy is no longer required (per Colorado HB24-1152), parking minimums were eliminated, and the prior administrative review step was removed. ADUs are now reviewed through the standard building permit process at Planning & Development Services, 1739 Broadway. Both attached and detached configurations are permitted, with unit-concentration caps tracked by neighborhood area.
Boulder's ADU framework lives at BRC 9-6-3(n), updated by City Council in February 2025 and effective for permits submitted on or after March 8, 2025. The new standards address unit concentration by neighborhood area, lot size, unit size, utilities, internal connection, and the conversion of existing nonconforming detached structures to permanently conforming ADU status. Detached ADUs are capped at 750 square feet by default, but a homeowner who records a deed restriction limiting rents to households at or below 75% AMI may build up to 1,000 square feet detached or up to 1,200 square feet attached (capped at 2/3 the size of the principal dwelling, whichever is less). Internal connection between an attached ADU and the main residence is now permitted. Since September 1, 2023, ADUs are reviewed and approved through the typical building permit process — no separate administrative review filing is required. Plans go to Planning & Development Services at 1739 Broadway through the Customer Self-Service portal. Standard residential plan review runs 4-8 weeks. The package needs site plan, floor plans, elevations, energy compliance to the 2021 IECC adopted in BRC Title 10, and confirmation of Boulder water and sewer service. For properties inside the expanded Wildland-Urban Interface (effective Aug. 1, 2025, covering ~16,000 homes), additional Ignition Resistant Type 1-3 construction standards apply.
Building an ADU without a permit is a violation of BRC Title 10 (Building Code) and Title 9 (Land Use Code), prosecuted in Boulder Municipal Court. Under BRC 5-2-4, fire code violations carry penalties up to $1,000 per day and/or 90 days in jail; general municipal code violations carry similar exposure. Code Enforcement typically issues a Stop Work order, requires after-the-fact permitting at doubled fees, and may require demolition of nonconforming work. WUI-zone violations can require additional retrofits before a Certificate of Occupancy.
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