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Boulder Ordinance 8256: Amending the Standards for Accessory Dwelling Units and Owner Accessory Units
Boulder, Colorado    
Publication Date
City of Boulder
On December 4, 2018, the Boulder city council adopted this ordinance amending several sections of the city's municipal code, including Title 9, the land use code. The city council's intent was to encourage the development of accessory dwelling units (ADUs).

ADUs are added as a conditional use in low-density, medium-density, and mixed residential zones and in public and agricultural zones. In the Residential Low districts, ADUs are allowed where less than 20 percent of the lots in the neighborhood area have ADUs. The ordinance reduces the minimum area for lots on which ADUs can be built to 5,000 square feet, and the minimum required floor area for ADUs is eliminated. For detached ADUs, the ordinance eliminates the maximum floor area restriction relative to the principal house and the absolute maximum is reduced to 550 square feet. Also under the ordinance, the required parking is exempt from setback requirements.

The ordinance permits ADUs to be licensed as affordable ADUs. Rents for this type of ADU must be affordable to households earning no more than 75 percent of the area median income; if a landlord rents an affordable ADU above that limit, they must repay any overcharges and 12 percent interest to the tenant. In exchange for agreeing to the rent restriction, the ordinance relaxes parking and some of the floor area requirements.

The ordinance provides similarly flexible parking and floor area requirements for ADUs on lots containing a historic landmark or a building that contributes to a historic district.

In addition, the ordinance allows a nonconforming duplex or two detached dwellings in certain low-density residential districts to be converted into limited accessory units. Such units may be expanded up to 20 percent of the floor area of the nonconforming structures and must provide at least 3 parking spaces.

Any principal dwelling or ADU constructed after the effective date of the ordinance cannot be used as a short-term rental.

Note: Guidance documents, except when based on statutory or regulatory authority or law, do not have the force and effect of law and are not meant to bind the public in any way. Guidance documents are intended only to provide clarity to the public regarding existing requirements under the law or agency policies.