Accessory Dwelling Units

ADUs are allowed, subject to use standards and building permit approval, in the following zoning districts:

• RL-1 or RL-2 Residential - Low 1 or 2
• RE Residential - Estate
• RR-1 or RR-2 Residential - Rural 1 or 2
• RM-2 Residential - Medium 2
• RMX-1 or RMX-2 Residential - Mixed 1 or 2
• A Agricultural
• P Public

Review the ADU handout (PDF) to view information about important zoning standards. To learn what zoning district you live in, visit the city’s Map of Zoning Districts and enter your address. While checking on the zoning, also check to make sure the lot size meets the minimum requirement of 5,000 square feet.

YES. Properties with ADUs must have at least one owner living on site, either in the ADU or in the principal unit. The property must be the owner’s principal residence, which means the owner must live there for more than half the year. The definitions in Section 9-16-1, B.R.C. 1981 are below for your reference:

• Owner-occupied means a dwelling unit or accessory dwelling unit that is the principal residence of at least one owner of record of the lot or parcel upon which the dwelling unit or accessory dwelling unit is located, who possesses at least an estate for life, or a fifty percent fee simple ownership interest, or is the trustor of a revocable living trust, or is the member that owns at least fifty percent of a limited liability company, or is the partner that owns at least fifty percent of a partnership or limited liability partnership, or similar entity.
• Principal residence means the dwelling unit in which a person resides for more than one-half of the year. However, if (1) the person owns another dwelling unit that is not licensed for long term rental; (2) the person's spouse or domestic partner has a different principal residence; (3) the person's driver's license, voter registration or any dependent's school registration shows a different residence address; or (4) the Boulder County Assessor lists a mailing address different from the dwelling unit address, it shall be presumed that the dwelling unit in question is not a principal residence. Provided, however, no presumption shall apply in any criminal proceeding.

NO. Short term rental (less than 30 days) of either the ADU or the main house is prohibited, unless the ADU and short term rental license were established prior to Feb. 1, 2019.

YES. An Affordable ADU means a unit for which the rents meet the city’s affordability standard. In exchange for agreeing to limit rents to 75% of the area median income, a homeowner may build a larger ADU (1,000 square feet for a detached ADU and 1,200 square feet or 2/3 the size of the principal home, whichever is less, for an attached ADU) and is not required to provide an additional parking space.

YES. An ADU that is located on a property that is designated as an individual landmark, or recognized as contributing to a designated historic district, can be larger than a standard ADU (1,000 square feet for a detached ADU and 1,200 square feet or 2/3 the size of the principal home, whichever is less, for an attached ADU) and is not required to provide an additional parking space. Contact a Historic Preservation Planner at 303-441-1994 or historic@bouldercolorado.gov if you would like to pursue an ADU on a historic property. Additional documentation will be necessary from Historic Preservation staff at time of application to use these additional allowances.

There are single-family homes in Boulder that have unpermitted second units or ADUs. Many unpermitted units were originally created as additional space for living or an office. Over the years, these spaces may have been modified to create separate cooking/sleeping areas and rented separately, not knowing that this was not allowed by city regulations.

If you receive notification that your existing ADU has not been legally established through permitting, review the ADU handout (PDF) to learn about the land use code standards for ADUs. You will then need to follow the building permit application process to legalize your ADU. Note that modifications may be necessary to meet land use code standards or building permit requirements.

NO. As of Sept. 1, 2023, there are no longer saturation requirements for ADUs in the City of Boulder.

YES. Even if modifications to establish the ADU are located within previously permitted structures (studio, finished basement, etc.) a “change of use” permit is necessary for the following purposes:

1. To demonstrate that life safety standards (egress, minimum light, ventilation, smoke/carbon monoxide detectors, head height requirements, etc.) are met for the proposed dwelling unit and bedroom/sleeping areas, and that features like egress doors/windows, fixtures, etc. were permitted and meet code standards;
2. To formally establish the ADU in the city’s records.
3. To demonstrate that existing meters and services are adequate for the proposed use.

When applying for a building permit application, please include the request for a “change of use” in the permit description. Additional materials may be requested if it is determined that adequate permits have not been secured for existing improvements.

Any change to the size, affordability status, or other characteristics of an approved ADU must submit a new building permit application. This application will need to demonstrate compliance with the applicable ADU standards.

YES. Detached ADUs are required to have automatic sprinkler systems for fire suppression. The City Design and Construction Standards require sprinkler systems be connected to a dedicated sprinkler water service line that is connected to the City water system through a bypass water meter. “Mist type” fire suppression systems may be approved as alternate sprinkler designs when the systems can be demonstrated to be equivalent to recognized standards such as the National Fire Protection Association (NFPA) 13D system (IFC sec. 904). For example, the Plumis mist system was previously proposed and approved as an alternate type of fire extinguishing system for a detached ADU.

This example is not meant to endorse a proprietary system and is provided as an example of how the process of gaining approval of an alternate equivalent fire extinguishing system by Boulder Fire-Rescue can be attained. When mist type fire suppression systems have been demonstrated equivalent to sprinkler standards that require installation by approved contractors, then approved contractors are also required to install the “mist type” system proposed.

In addition to the requirements described above, additional regulations may prohibit or restrict ADU construction, including but not limited to the following:

Deed Restrictions and Homeowner Associations

If there is a deed restriction on the home for permanent affordability, it is not eligible for an ADU. Also, some homeowners’ associations (HOAs) prohibit ADU construction or use. The city has no authority to overrule an HOA since it is a contractual agreement between the homeowner and the HOA.

Permitting Requirements

Your property may be subject to additional development standards related to a zoning overlay, historic landmark designation, historic district, flood plain, or other special design standards or guidelines. In this case, your project will need to meet additional standards beyond the typical building, fire, and zoning codes. Zoning, historic, and flood information specific to your property can be found on the city’s online mapping site. Contact staff through Inquire Boulder with questions about possible additional permitted requirements.

Building and Fire Codes

All ADUs must meet with city’s building code to ensure the structure is safe and habitable. In addition, a detached ADU must follow additional fire code requirements (i.e., sprinkler system). Visit the Codes and Regulations web page for information on the required building and fire codes that regulate the construction of an ADU.