Purpose

This guide is intended to help you design your accessory dwelling unit (ADU) and provide a list of materials which you are required to fill out and submit with your ADU application. To ensure your ADU meets the requirements, please read this page carefully before submitting a building permit application. Effective use of this guide can help you avoid costly revisions to your application later.

To obtain approval for the ADU, you must submit a building permit application. ADUs are subject to specific use standards in Section 9-6-3(n) of the Land Use Code. This guide is intended to help explain the various requirements, point you to relevant code language, and identify the additional application materials that will be required with your building permit application. Visit the City’s ADU webpage to view answers to frequently asked questions about ADUs.

Building an Accessory Dwelling Unit in Boulder

Determine Your Zoning District

  • Zoning District: ADUs are an allowed use by-right on a lot with a detached dwelling unit in all zoning districts except the RH-6, MH, BMS, BCS, IS-1 and IM zoning districts. First, check the City's zoning map to ensure your property is located in one of the districts where ADUs are allowed.

Design Your ADU

There are several requirements to consider when designing your ADU.

  1. Attached or Detached ADU

    There are two types of ADUs allowed in Boulder – attached and detached. Different rules apply to each type, so it is important to understand which type you will be constructing.

    A diagram illustrating the difference between attached and detached accessory dwelling units
  2. Affordable ADUs

    Some standards also differ based on whether the property is being rented at a market rate, or whether it is an affordable ADU. Rent for an affordable ADU may not exceed 75% of the Area Median Income. The city publishes a table each year of affordable ADU maximum rents. Affordable ADUs can take advantage of certain incentives such as larger size limits.

  3. Designated Historic Properties

    Properties with a building designated as an individual landmark or recognized as contributing to a designated historic district are permitted to have larger ADUs.

  4. Size Limit

    ADUs must comply with the following size limits, based on the type of ADU:

    ADU TypeAttachedDetached
    ADU

    1/2 of the total floor area of the principal structure or 1,000 square feet (whichever is less), except that if the principal structure has less than 1,500 square feet of floor area, the maximum is 750 square feet.

    		800 square feet maximum

    Affordable ADUs and

    Designated Historic Properites

    		2/3 of the total floor area of the principal structure or 1,200 square feet (whichever is less), except that if the principal structure has less than 1,125 square feet of floor area, the maximum is 750 square feet.1,000 square feet maximum
    • Compliance with the size limits are based on the definition of “floor area” in Chapter 9-16. Floor area is measured to the outside surface of the exterior framing or the exterior walls (if no exterior framing) and must also include:
      • Stairways, except the stairway opening at the top floor
      • Any egress path required by the building code (typically 3 feet) for exterior elevated above-grade corridors, balconies, and walkways
    • Some parts of buildings are not included in floor area. The most common exception applicable to ADUs is “uninhabitable space,” which includes:
      • Areas that are 6 feet or less in ceiling height
      • Rooms solely used for mechanical or electrical equipment for the building
      • Parking facilities that are completely below grade
  5. Attached ADU Design Standards
    • Interior connections and lockable separation: All attached ADUs must be physically separated from the principal unit by either a wall or a lockable door. In an existing building, if there is an interior connection between the ADU and the principal unit, the connection has to be maintained.
    • Rear Yard Setback: The minimum rear yard setback for attached ADUs is 5 feet, subject to standards related to height and building coverage in Section 9-6-3(n)(2)(B).
    • Height: Building height for attached ADUs must comply with the maximum height for principal buildings in Table 7-1 in Section 9-7-1. For all zoning districts that allow ADUs, the maximum height is 35 feet.
  6. Detached ADU Design Standards
    • Height: While the maximum height of accessory structures is 20 feet, detached ADUs with a roof pitch of 8:12 or steeper may be up to 25 feet. Detached ADUs being converted from an existing structure may exceed 20 feet, as long as no changes are proposed to the building’s height, floor area, or roof form for the conversion.
    • Building separation: Detached ADUs must have at least 6 feet of separation from any other building, measured between the two nearest points. See Table 7-1 in Section 9-7-1.

Confirm that the Design Meets Other Zoning Standards

  • Height: See height requirements for attached and detached ADUs in the section above.
  • Setbacks: ADUs must comply with required setbacks for the zoning district.
  • Floor area ratio: Properties with ADUs must comply with the maximum total floor area ratio.
  • Solar access: Solar access requirements apply to most properties with ADUs.
  • Building coverage: In most zoning districts, properties with ADUs must comply with maximum building coverage requirements.
  • Side yard bulk plane: ADUs in many zoning districts must comply with the side yard bulk plane requirements.
  • Side yard wall articulation: In many zoning districts, ADUs must comply with the side yard wall articulation requirements.
  • Landscaping: Depending on the scope of the project, landscaping improvements may be required.

Understand Important Requirements Related to ADUs

Aside from design standards, ADUs also have several additional requirements. Review the summary below, or go to Section 9-6-3(n) to learn more.

  1. Declaration of Use for Affordable ADUs

    Properties with approved affordable ADUs will have a Declaration of Use recorded with the Boulder County Clerk and Recorder to provide notice to all future owners of the affordable ADU approval, and as a record that the property will comply with all requirements related to affordable ADUs. The declaration shall include a sworn certification that the unit will meet the affordability standard and a statement of the number of bedrooms. This Declaration of Use must be signed by the property owner(s) and submitted with the building permit application. City staff will record the Declaration of Use upon building permit approval.

  2. Short Term Rental

    Short term rental (less than 30 days) of either the ADU or the main house is prohibited, unless the ADU and rental license were established prior to February 1, 2019.

  3. Rental License

    Long-term rental (30 days or more) of either the ADU and/or the principal unit on the property is permitted. If you intend to rent the ADU or principal dwelling unit on the property, a rental license is required. Learn more on the City’s Rental Licensing webpage.

  4. Independent Conveyance

    ADUs may not be conveyed, or sold, separately from the principal dwelling unit.

  5. Prior Approvals (Planned Unit Developments or Others)

    ADUs on properties with a prior approval that prohibits ADUs (a planned development, planned residential development, planned unit development, or site review) are permitted if an approved Minor Modification application is completed before a building permit application. ADUs in these areas must comply with other applicable requirements of that approval and the standards for ADUs in the land use code.

Application Submittal Requirements

Supplemental ADU Site or Floor Plan Information

In addition to the required site plan or floor plan information listed on the building permit application, the relevant site or floor plans for an ADU must also indicate:

For Attached ADUs:

  • Lockable separation between ADU and principal unit if connected internally
  • Floor plans at each level of the principal unit to confirm floor area

For Detached ADUs:

  • Roof pitch of the ADU

Affordable ADUs Only

Completed and signed Declaration of Use

Be sure to fill out the appropriate Affordable Accessory Dwelling Unit Declaration of Use (PDF) based on the affordability rate of your ADU. If you are amending a prior approval related to an ADU, fill out the Amended Accessory Dwelling Unit Declaration of Use (PDF).

  1. Minor Modification Approval

    If the property is located in a prior approval like a planned unit development, an approved minor modification application is required prior to building permit.

Accessory Dwelling Unit Application Form

Submit the Accessory Dwelling Unit Application Form as an additional application requirements form in the Customer Self Service Portal building permit online application.

Reference