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Accessory Dwelling Units (ADUs)

Accessory Dwelling Units (ADUs)

News & Updates

Accessory Dwelling Unit Code Clarifications 

On Jan. 7, City Council approved the second reading of an ordinance with amendements to update the Accessory Dwelling Unit (ADU) regulations that would: 

  • Allow for an administrative modification process to the roof-pitch / height standards for legally existing accessory structures converting to detached ADUs. 
  • Remove the requirement for architectural consistency for all detached ADUs with existing and adjacent residences.
  • Allow for ADUs on properties with cooperative housing units.
  • Wording changes to clarify inconsistent terminology in the code.
  • Prohibit the independent sale of ADUs separate from the principal dwelling unit.

City Council will hear the third reading of the ordinance on Feb. 4. 

  • Feb. 4 at 6 p.m.: City Council Chambers at the Boulder Municipal Building (1777 Broadway). Meeting materials will be available on the City Council web page.


ADU Application Process

View a a training document pdf and on process and requirements for ADU applications.

A two-page summary of changes to ADU regulations is available here pdf.

What is an Accessory Dwelling Unit?​

An accessory dwelling unit, also known as an “accessory apartment”, “secondary suite”, or “mother-in-law apartment”, is an additional living unit that has separate kitchen, sleeping, and bathroom facilities, attached or detached from the principal residential unit on a single-family lot. An accessory unit is permitted only if it meets the requirements of Section 9.6.3(a) of the land use code. The owner must reside in one of the dwelling units.​

Why are ADUs Important?​

ADUs offer homeowners many benefits that can maintain and increase quality of life. Homeowners can either use the ADU for themselves, rent it, or move into it to free up their principal home for others to use as a rental. General benefits include: 

  • Income - Ability to grow equity by investing in income generating space and flexible housing options. 
  • Family Support - Ability to house multi-generational families and extended family for aging-in-place. This supports familial strength, savings on senior-living and/or childcare costs, and long-term household stability. 
  • Neighborhood Strength - Neighborhood stability is strengthened for homeowners and renters. Homeowners can gain income or services from their rental units and renters gain access to housing options that often have lower rents, more rental options, and housing choices across multiple neighborhoods.​

What are the types of ADUs?​

ADUs are smaller than existing single-family houses and can be either attached to or detached from the principal home. Below are the two types: 
  • Attached ADU-  An attached ADU is a unit functionally separate, but physically attached to the principal home. They are commonly built in basements or upper stories (see image A), or as adjoining wings/additions to principal homes (see image B):
  • Detached ADU-  A detached ADU is a unit both functionally and physically separate from the principal home. Detached ADUs typically are one story at ground level or above a garage or carport (see image C). 

Click for the full diagram.

Where are ADUs allowed?

Attached and Detached ADUs are allowed, with approval through an administrative review application, in the following zoning districts: Residential - Low 1 (RL-1), Residential - Low 2 (RL-2), Residential - Estate (RE), Residential - Rural 1 (RR-1), Residential - Rural 2 (RR-2), Residential - Mixed 1 (RMX-1), Residential - Mixed 2 (RMX-2), Agricultural (A), or Public (P). To learn what zoning district you live in, visit 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 of 5,000 square feet.

What are affordable accessory units? Are there additional allowances?

An Affordable ADU means a unit for which the rents meet the city’s affordability standard. An Affordable ADU can be either attached or detached. In exchange for agreeing to limit rents to  75% of the Area Median Income,  a homeowner may build a larger ADU (800 square feet for a detached ADU and 1,000 square feet or one half the size of the principal home (whichever is less) for an attached ADU) and is not required to provide an additional parking space.

Are there additional allowances for designated historic properties?

An Attached or Detached ADU that is located in a structure that is designated as an individual landmark, or recognized as contributing to a designated historic district, has a higher saturation limitation in the RL zoning districts (30% instead of 20%), can be larger than a standard ADU (1,000 square feet for a detached ADU and 1,000 square feet or one half the size of the principal home (whichever is less) for an attached ADU) and is not required to provide an additional parking space. Contact Marcy Cameron, Historic Preservation Planner, at 303-441-3209 or  [email protected]  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 utilize these additional allowances.

Can I rent my ADU? Is a rental license required?

A homeowner may rent either the ADU or the principal home on a property for more than 30 days at a time. A rental license is required if  either unit  is to be offered for rent for compensation.  Short term rentals (less than 30 days) are not allowed unless the ADU and rental license were established prior to January 3, 2019  (Section 10-3-19(k), B.R.C. 1981). For more information on the requirements for renting either the ADU or the principal home, visit the Rental Housing Licensing page at .

How long do I have to establish an ADU following approval of the administrative application?

Pursuant to Section 9-6-2(c) of the land use code, a conditional use must be established within one year of its approval. Therefore, building permits must be issued and construction commenced to establish the accessory unit within one year of approval. If not established within that time frame, the administrative approval will expire and you will need to reapply for an ADU.

How do I apply for an ADU within a proposed single-family dwelling unit (not yet constructed)? Or for a detached ADU on the same property as a new home?

The ADU standards require that accessory units are associated with a lot or parcel with an existing single-family dwelling unit. It is possible to submit an administrative review application for an accessory unit before a permit is issued for the principal unit. However, staff is unable to approve the ADU application until a permit is issued for the construction of a new home. The permit plans for the new home cannot show the proposed accessory unit. You will be required to submit approved administrative review plans with any permit submittals required to establish the ADU. If the accessory unit is proposed within a new home it may be possible to revise the scope of work for the active permit issued for the construction of a single family home to include the elements of the ADU.

What is a saturation limit and where does it apply?

The number of ADUs allowed is limited in the Residential Low (RL-1 and RL-2) zoning districts. Specifically, no more than 20% of the parcels within 300 feet of the property may have an ADU. In addition to ADUs, cooperative housing units and non-conforming structures count in the saturation limit. 

How is the saturation rate calculated?

A 300-foot buffer is created around the property with the proposed ADU. The saturation is the percentage of the number of parcels  within an RL zoning districts  that contain an ADU, cooperative housing unit, or non-conforming use divided by the total number of parcels. A project specialist in the City’s Development Services Center can calculate the saturation limit for your parcel in order to determine if you are eligible for an ADU.

What does it mean to owner occupy?

At least one owner of record of the lot or parcel is required to occupy either the ADU or the principal home. This means that the owner must reside in either the ADU or the principal home for more than one-half of the year and such unit is not eligible to rent. The definitions in  Section 9-16-1 , B.R.C. 1981 are below for your reference: 

  • Owner-occupied  means a dwelling unit or accessory 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 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.
  • 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.

Does Inclusionary Housing apply to ADUs?

No, the requirement for 25% of new development in the city to be permanently affordable to low- to middle-income households does NOT apply to ADUs. If a new principal dwelling is proposed, then Inclusionary Housing will apply.

Is a public notice required for an ADU application?

Yes, a public notice is sent to adjacent property owners a minimum of 10 days before final action by the city.  A public hearing before Planning Board or City Council is not required  for ADU applications. The Planning Department will review the application based on the criteria set forth in  Section 9-6-3(a), B.R.C. 1981.  The purpose of the public comment is for neighbors to learn about your proposal and raise any potential concerns prior to permitting. Although neighbors cannot stop you from creating an ADU, it allows you the opportunity to address any valid neighbor concerns.

How many occupants are allowed in an accessory dwelling unit?

The principal dwelling unit and accessory unit together are considered one dwelling unit for the purposes of determining occupancy requirements. Therefore, the total occupancy of both units shall not exceed the standards set forth in  Section 9-8-5, “Occupancy of Dwelling Units,” B.R.C. 1981 , for one dwelling unit. For the purposes of ADUs, any occupant and his or her dependents shall be counted as one person. For example, a family may live in the principal home and two unrelated persons may reside in the accessory unit (three unrelated persons in the RMX zoning districts). As mentioned above, any occupant and his or her dependents are considered one person. Therefore, in the example above, a parent with dependent children may occupy the accessory unit plus one unrelated person (two in the RMX zoning districts).

How do I legalize my illegal unit?

There are a number of single-family homes in Boulder that have illegal units. The ADU Update of 2018 was intended, in part, to encourage homeowners to comply with the ADU regulations. Many illegal units were created legally 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 not knowing that this was not allowed by city regulations. The city encourages homeowners with illegal units to meet with a project specialist in the City’s Development Services Center to review the new regulations and better understand if their unit may be eligible.

Which modifications to a previously established assessory dwelling unit require a new application?

Any modification to a legally established accessory unit that results in one or more of the following must be reviewed by the city and requires a new application to be filed with a Project Specialist at the Planning and Development Services Center:

  1. Change in the location of an attached ADU within the principal structure;
  2. Change from an Attached ADU to a Detached ADU, and vice versa;
  3. Change in affordability status;
  4. Change in total square footage of the accessory unit;
  5. Reduction in total square footage of the principal structure which contains an Attached ADU

What are the compatible development regulations? Do they apply to my property?

If you are proposing the construction of a building addition or a new building, and the property is located in the RR-1, RR-2, RE, RL-1, or RMX-1 zoning district, the following standards may apply. Some standards also apply to properties in RL-2 district within the boundaries of the Table Mesa 4 Planned Unit Development.

Compatible development is a set of four regulations that work in tandem to limit the scale and bulk of new construction in single family neighborhoods. The intent of these regulations is to protect the character of established single-family neighborhoods by assuring that new construction and additions are compatible in scale and bulk with the character of the neighborhood. More information can be found in the links below.

  • Floor Area Ratio (FAR): FAR is the ratio of the floor area of a building in relation to the area of the lot on which the building or buildings are situated. The purpose of a floor area ratio standard is to address the proportionality of building size to lot size and allow variation in building form within the established building envelope.
  • Side Yard Bulk Plane: The purpose of the side yard bulk plane is to ensure that buildings step down towards neighboring properties in order to enhance privacy, preserve some views, and allow visual access to the sky for lots adjacent to new development.
  • Side Yard Wall Articulation: The purpose of side yard wall articulation is to reduce the perceived mass of a building by dividing it into smaller components, or to step down (decrease) the wall height.
  • Building Coverage: The purposes of the building coverage standards are to establish the maximum percentage of lot surface that may be covered by principal and accessory buildings to preserve open space on the lot, and to preserve some views and visual access to the sky, and enhance privacy for residences that are adjacent to new development.

How does an ADU get a postal address?

Properties with approved accessory dwelling units shall maintain a single physical address with separate unit numbers associated with each of the units. At the time of administrative approval, new unit addresses will be assigned and established on the property. The principal structure will be Unit A and the accessory dwelling unit will be Unit B regardless of which unit the property owner occupies. A Notice of Address Assignment shall be sent to utility companies, USPS, emergency services, the County Assessor for review and approval.

What are other regulations that I need to consider?

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

Deed Restrictions and Home Owner 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 . Please see a project specialist in the City’s Development Services Center with questions about possible additional permitted requirements.

Building and Fire Codes

All ADUs must meet with city’s building code to insure 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.

Notice of Nondiscrimination

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The City of Boulder, Division of Housing does not discriminate on the basis of race, color, religion, gender, age, national origin, disability, marital status, familial status, sexual orientation or military status in any of its programs, services or activities.

Reasonable Accommodation

The Department of Housing and Human Services, upon request, will provide appropriate aids and services leading to effective communication for qualified persons with disabilities so they can participate equally in programs, services and activities. Anyone who requires an auxiliary aid, communication service, or a reasonable accommodation to participate in a program, service, or activity of the Department, should call the ADA/Section 504 Coordinator at 303-441-3231. For hearing or speech impaired residents, please dial 7-1-1 for Relay Colorado.

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