Accessory Dwelling Units (ADUs)

Following multiple years of consideration and public outreach, Centennial adopted ADU regulations on May 7, 2024. The ADU Ordinance is in effect as of June 15, 2024.

The ADU regulations implement the City’s comprehensive plan, Centennial NEXT, which identifies the community’s desire to:

Provide opportunities for new housing options that meet market trends and the evolving needs of current residents.
Explore the allowance for Accessory Dwelling Units (ADUs) within residential areas, subject to operation and design guidelines that respect the character and context of the City’s various neighborhoods and zone districts.

Offering ADUs can address a portion of the City’s housing demand with minimal impact on neighboring properties. ADUs can house family members or provide a rental home for a small household.

ADUs are not mandatory. This change to the Land Development Code (LDC) only creates an opportunity for a property owner to establish an ADU on residentially-zoned property. The ADU will need to meet the regulations in Ordinance No. 2024-O-03(PDF, 279KB) and the property owner is responsible for paying for construction.

What are ADUs?

Description

An Accessory Dwelling Unit (ADU) is a second smaller dwelling unit located on the same lot as the main house. The unit is designed for independent living with space for sleeping, cooking and a bathroom.

Types of ADUs

Attached ADUs

Attached ADUs are new additions attached to the principal dwelling. This might include space such as a converted attached garage or structural addition. Attached ADUs are required to have a separate entrance from the principal dwelling unit, which could be provided through a shared stairwell or foyer.

Attached accessory dwelling unit for an addition to Primary Home

Detached ADUs

Detached ADUs are separated from the principal dwelling unit, typically at ground level or above a detached garage or other similar structure.

Detached accessory dwelling unit for Accessory Building Garage

Detached accessory dwelling unit for an Above Detached Garage

Interior ADUs

Interior ADUs are located within (internal) the principal dwelling, such as a finished basement or attic. An Interior ADU would not add additional square footage to the primary home. Interior ADUs are required to have a separate entrance from the principal dwelling unit, which could be provided through a shared stairwell or foyer.

Attached accessory dwelling unit for Interior Conversions

Where are ADUs Allowed?

ADUs are allowed in the following zoning districts. To find out the zoning of your property, use the City’s online Zoning Map.

AG: Agriculture
RS: Suburban Residential
RA: Auto-Urban Residential
RU: Urban Residential
NC: Neighborhood Conservation
NI: Neighborhood Infill
CG*: General Commercial
AC*: Activity Center
UC*: Urban Center

*ADUs in these districts are limited to single-family lots with single-family homes.

Do ADUs Have Health and Safety Requirements?

Yes. ADUs shall:

Be permanent, habitable structures (mobile homes, recreational vehicles and travel-trailers are prohibited);
Receive a will serve letter from all applicable utility districts and connect to utilities as required by the individual utility provider; and
Comply with the Centennial Municipal Code Chapter 18, Building Regulations.

How does an ADU fit into the Neighborhood?

The new ADU regulations include the following requirements to integrate into the surrounding neighborhood.

Dimensions

Minimum Setbacks: The same setbacks that apply to principal structures apply to an ADU.

See Section 12-3-301 of the Land Development Code for minimum setback requirements by zone district.

Maximum Building Coverage: The same maximum building coverage that applies to principal dwelling units applies to an ADU.

See Section 12-3-205 of the Land Development Code for information about how to calculate building coverage. Table 12-3-301 details the allowed building coverage by zone district.

Maximum Building Height:

See Section 12-3-204 of the Land Development Code for information about how to measure height.

For attached ADUs: The same maximum height that applies to primary homes apply to an ADU. Table 12-3-301 details the allowed building height by zone district.
For detached ADUs: 17 feet.

Size

Must be less than or equal to 50% of the principal dwelling footprint. A building footprint is the border of a building drawn along the exterior walls, to create a polygon, representing the total area of the building on the ground.

Be the total size of an attic or basement area (including any internal stairwell providing access) as long as no additional construction would expand those spaces.

Design

ADUs require their own separate entrance. This can be provided through an external door or from inside the entrance to the main house (for example, through a shared foyer or mud room).

Detached ADUs and attached ADUs that are constructed as an addition to the main house, need to be designed consistently with the main house in at least three of the following design elements:

Roof pitch and form
Soffit and fascia style
Exterior wall cladding color, style and/or material
Window style, trim and orientation
Exterior door style
Architectural features such as but not exclusive of, building recessions or projections, dormers, balconies, porches, columns and prominent entry features
Color palette

Parking

The ADU shall provide 1 off-street parking space in addition to the spaces required for the principal dwelling. This space may be provided in a driveway if space is available.

Trash and Recycling

The ADU may share trash and recycling facilities with the principal dwelling unit or trash and recycling facilities may be provided separately. Where applicable, external screening of trash bins shall be provided for the lot but is not required between the principal dwelling unit and the ADU.

What Other Standards Should be Considered?

Homeowners Associations (HOAs)

HOAs and similar private groups such as community associations may have additional rules about uses and design of ADUs that may be different from the City. Some HOAs may prohibit ADUs. It is the property owner’s responsibility to comply with private homeowner association requirements, covenants, conditions and restrictions related to ADUs.

Water Districts

Each water district that serves the City of Centennial maintains individual policies and regulations related to ADUs. This includes metering and service line requirements, tap fees and additional construction permits. Checking early with a water provider about their requirements for ADUs can help save time and additional costs later in the project.

Rentals and Home-Based Businesses

Short-term rentals are not allowed within ADUs.

If the property owner does not live on the property, the owner must register the ADU with the City.

Home-based businesses are allowed within ADUs subject to the standards in Section 12-2-404 of the Land Development Code.

How do I Apply for an ADU?

Pre-Submittal Application

A pre-submittal meeting pursuant to Section 12-14-303 is required prior to submittal of a building permit application for an ADU. Visit the Centennial Customer Service Portal. Create a new account or log in to an existing account. Apply for a Pre-Submittal Plan.

Address Assignment

All ADUs are required to maintain a separate address from the principal dwelling. Visit the Centennial Customer Service Portal. Create a new account or log in to an existing account. Apply for an Address Assignment Plan. The key information required is the address of the main home and contact information for the applicant.

Building Permit

ADUs are reviewed and approved through a building permit application. Additional information and applications are available on the City’s Building Division website. To better understand the material that need to be submitted, see the following ADU guides - Attached ADU Addition Guide(PDF, 2MB), Detached ADU Guide(PDF, 1MB) and Interior ADU Guide(PDF, 5MB).

Note: A will serve letter from all applicable utility districts will be required as part of the permit application.

Fees

Building permit fees are based on the total valuation of the project. View the building permit fee schedule.

ADU Registration

If the property owner does not live on the property, the owner must register the ADU with the City using the ADU Registration process. The steps and required documents are included in the ADU Rental Registration PDF(PDF, 297KB). To submit the Registration, visit the Centennial Customer Service Portal. Create a new account or log in to an existing account. Apply for an ADU Rental Registration.

Other Resources

Attached ADU Addition Guide(PDF, 2MB)

Detached ADU Guide(PDF, 1MB)

Interior ADU Guide(PDF, 5MB)

Land Development Code (LDC)

ADU FAQs

Q: What is an Accessory Dwelling Unit (ADU)?

A: An Accessory Dwelling Unit is a second, subordinate dwelling unit located on the same lot as a principal dwelling unit. The unit includes its own independent living facilities with provisions for sleeping, cooking, and sanitation, and is designed for residential occupancy independent of the principal dwelling unit.

Q: Can I have more than one ADU on my lot?

A: No. Only one ADU is allowed per single-family detached dwelling.

Q: Can my lot be subdivided to sell my ADU independently?

A: No, lots containing both a principal dwelling unit and an ADU may not be subdivided so that the ADU occupies a different platted lot than the principal dwelling unit. The ADU cannot be sold independently of the principal dwelling on the lot.

Q: Can I build an ADU and a single-family dwelling at the same time?

A: Yes, as long as the principal dwelling has been issued a building permit for which construction has commenced within 180 days of permit issuance or is making progress toward completion every 180 days.

Q: Can I use my recreational vehicle as an ADU?

A: No, ADUs must be permanent, habitable structures. Mobile homes, recreational vehicles and travel-trailers are prohibited.