Building an ADU in Boulder

What you can build here

Boulder has simplified its ADU rules several times. Since September 1, 2023, ADUs are approved through the regular building permit process with no separate administrative review, and there is no longer a "saturation limit" on ADUs in a neighborhood. City Council approved more changes in February 2025 (Ordinance 8650), which apply to ADUs proposed on or after March 8, 2025. Boulder is a subject jurisdiction under Colorado's ADU law, HB24-1152. See our Colorado ADU law page.

An ADU is allowed by right on a lot with a detached house in every zoning district except RH-6, MH, BMS, BCS, IS-1 and IM. It must have separate kitchen, sleeping and bathroom facilities, and it can be attached to the house (including a basement unit) or detached. An attached ADU must be separated from the house by a wall or a lockable door.

Modular ADUs approved by the state still need a Boulder building permit and must meet local building and zoning codes. See our prefab ADU guide. [1][2][3]

Size, height and setbacks

Size.

• Attached ADU: half of the house's floor area or 1,000 square feet, whichever is less. If the house has under 1,500 square feet, the limit is 750 square feet.
• Detached ADU: 800 square feet.
• Affordable ADUs and designated historic properties: attached, two-thirds of the house or 1,200 square feet, whichever is less (750 square feet if the house is under 1,125 square feet); detached, 1,000 square feet.

Floor area is measured to the outside of the exterior walls and includes stairways. Uninhabitable space, such as areas with ceilings 6 feet or lower and mechanical rooms, doesn't count.

Height. A detached ADU can be 20 feet tall, or up to 25 feet with a roof pitch of 8:12 or steeper; dormers are allowed within limits. A building converted to a detached ADU can stay above 20 feet if you don't change its height, floor area or roof form. An attached ADU follows the main building height limit, which is 35 feet in all districts that allow ADUs.

Setbacks and other zoning rules. ADUs must meet the district's setbacks. An attached ADU needs at least a 5-foot rear setback. A detached ADU must be at least 6 feet from any other building. Floor area ratio, building coverage, solar access, side yard bulk plane and wall articulation rules apply in many districts, and landscaping may be required. [1][2]

Fire sprinklers, utilities and building code

Every ADU must meet the city's building code. Detached ADUs must have automatic fire sprinklers. The sprinkler system must connect to a dedicated sprinkler water line through a bypass water meter. Boulder Fire-Rescue can approve "mist type" systems as alternates when shown to be equivalent to standards such as NFPA 13D. HB24-1152 lets cities keep applying their building and fire codes to ADUs.

A detached ADU also needs an In-Property Wet Utility Permit for the water and sewer lines from the house. Even if you create an ADU inside an existing permitted space with no construction, you need a "change of use" building permit to show life safety standards are met and existing meters and services are adequate.

Detached ADUs must meet the city's SmartRegs energy rules when rented, and so must the main house of any property with an ADU. Attached ADUs don't need to. [1][2][4]

Parking, owner occupancy and HOAs

Boulder's ADU page says that as of March 8, 2025, ADUs are "no longer required to be owner occupied, in accordance with recent state legislation."

Boulder's ADU pages don't list a parking requirement. Under HB24-1152, a subject jurisdiction can't make you build a new parking space for an ADU, except in narrow cases, though it may ask you to designate an existing driveway or garage space.

Since March 8, 2023, ADU bans in prior planned unit developments and other prior approvals are void, but you need an approved minor modification before you apply for the building permit, and the ADU must still meet the PUD's other requirements. The city says it can't overrule an HOA, but notes that state law addresses HOA restrictions on ADUs. Under HB24-1152, HOA rules that restrict ADUs in ways the law prohibits are void in subject jurisdictions, though reasonable restrictions are allowed. [1][3][4]

Garage conversions and legalizing existing units

You can convert an existing structure, such as a garage or studio, into a detached ADU. If you keep its height, floor area and roof form, it can exceed the usual 20-foot height limit. It still needs a building permit, sprinklers and the other detached ADU standards. See our garage conversion guide.

Boulder says unpermitted ADUs are common and can now be legalized. You need stamped plans from a licensed design professional, and finishes like drywall may have to come off so inspectors can check framing, insulation, plumbing, mechanical and electrical work. Existing detached ADUs need fire suppression to legalize. [1][2]

Fees and timeline

Boulder's ADU pages don't list ADU-specific fees. Building permit fees are paid as part of the permit, and the permit is closed only once inspections, conditions and fees are complete. Ask Planning & Development Services at 303-441-1880 about fees for your project, and see our ADU cost guide.

Review times vary with the scope of the project and how complete your application is. The city posts current processing times. Boulder doesn't offer pre-review of building permit applications; once your application is in review, staff tell you what needs correcting. See our ADU permit process guide. [1][5]

Renting it out and selling

Long-term rental (30 days or more) of the ADU, the main house or both is allowed. Every rented ADU needs a city rental license, and staff may inspect the ADU for its approval criteria as part of licensing. Contact Rental Housing Licensing at 303-441-3152.

Short-term rental (under 30 days) of either the ADU or the main house is prohibited, unless the ADU and its rental license were set up before February 1, 2019. HB24-1152 lets cities regulate short-term rental of ADUs.

Affordable ADUs. If you agree to keep rent at or below the city's affordability standard (75% of area median income), you can build a larger ADU. A Declaration of Use is recorded with the Boulder County Clerk and Recorder.

You can't sell an ADU separately from the main house. [1][2][4][6]

Financing and state programs

Boulder is on DOLA's list of certified ADU Supportive Jurisdictions as of October 9, 2026. That means homeowners may qualify for the Colorado Housing and Finance Authority's ADU interest rate buydown or credit enhancement programs, applied for through a lender of your choice. CHFA describes the programs as supporting ADUs for low- and moderate-income borrowers and/or tenants. See CHFA's ADU finance programs and our ADU financing guide. To compare nearby rules, see our Denver and Fort Collins pages. [7][8]

Permit steps in Boulder

1. Check your zoning district

   Look up your district on the city's zoning map. The same mapping shows historic, floodplain and overlay rules that can add requirements. [1][2]

2. Clear any prior approvals

   If your property is in a PUD, planned development or site review approval, complete a minor modification before you apply for the building permit. A home with a permanent affordability deed restriction isn't eligible for an ADU. [1][2]

3. Design to the ADU standards

   Follow the size, height, separation and zoning rules in the city's ADU guide. The city has no pre-review service, so it suggests a licensed architect or design professional who knows Boulder codes for design questions. [1][2]

4. Prepare the ADU materials

   Along with the normal permit materials, show lockable separation and floor plans of each level of the house (attached ADUs) or the roof pitch (detached ADUs). Add the ADU Application Form, and a signed Declaration of Use for an affordable ADU. [2]

5. Apply in the Customer Self-Service Portal

   Apply for a building permit through the city's Customer Self-Service Portal. A detached ADU also needs an In-Property Wet Utility Permit for the water and sewer connections from the house. [2][5]

6. Respond to plan review

   Staff send correction requests once the permit is in review. Review times depend on scope and completeness; the city posts current processing times. [1][5]

7. Inspections and rental license

   Schedule inspections in the portal. The permit closes once inspections, conditions and fees are complete. If you will rent the ADU or the house, get a rental license. [5][6]

Who to call

• Planning & Development Services

  Building permits, ADU zoning questions and inspections

  303-441-1880

  1101 Arapahoe Avenue, Boulder, CO 80302 (Tuesday to Friday, 10 a.m. to 4 p.m.)

  [5]

• Historic Preservation

  ADUs on landmarks or contributing properties in historic districts: historic@bouldercolorado.gov

  303-441-1994

  [1]

• Rental Housing Licensing

  Rental licenses for ADUs and main houses: RHL@bouldercolorado.gov

  303-441-3152

  [6]

Questions people ask