Accessory Dwelling Units in Adams County

Yes. If an existing ADU was not created through a building permit, a building permit would be required to permit the ADU and make sure that it meets current standards.

It depends! ADUs generally follow accessory structure standards in the zone district they are in. The ADU-specific standards that replace general zoning regulations are the following:

• ADUs can be up to 25 feet in height, measured to the roof mid-pitch.
• Detached ADUs must be set back 10 feet behind the front structure line.
• ADU floor areas count towards principal structure coverage, not accessory structure coverage.
• Minimum rear setbacks shall not be greater than those that apply to other accessory structures in the same zone district or five feet, whichever is greater.

ADUs must be designed consistently with the color, façade, roof pitch, siding, lighting, and windows of the existing home. An aesthetic goal of ADUs is to reflect the existing development on a property as well as surrounding context.

An ADU can be a part of the existing home, a new structure, or part of a new structure. A new structure could be modular construction or prefabricated (manufactured homes) as long as the ADU is placed on a permanent foundation and connected to metered utility services.

Yes, a building permit is required to ensure that the modular ADU complies with zoning regulations, building codes, and minimum design criteria.

Yes, with conditions. Barndominiums are popular topics of conversation, where an accessory structure like a barn also includes a residential living space. An ADU may take up part of an accessory building and will still have to meet all the relevant performance standards for approval. For example, an ADU for a home less than 1,875 square feet in floor area may take up 500-750 square feet of a larger accessory building that is used for other purposes. It cannot take up more space than the regulations would allow. A similar example of this type of space usage would be allowing for an ADU on the top floor of a garage; the ADU is in an accessory structure and takes up part of the building, but not the entire building.

One off-street parking space needs to be designated for an ADU in addition to whatever is required the primary home. A new parking space would need to be constructed if:

• The property is in a zone district that requires parking for the primary home.
• There is not an existing parking space on the property.
• The property is on a block where on-street parking is not allowed.

If you are in a water district, you will need to provide a will-serve letter from that water district that specifically acknowledges they will provide water service to the additional unit on your property, as well as sewer if applicable.

• It is important to connect with a water district before putting together plans for an ADU. Tap or impact fees for adding more use to infrastructure may be a large expense in your budget and knowing how much it will cost before you embark on cost estimates will be important for your planning.
• You will also want to contact your relevant fire district to make sure you are aware of all requirements that they may have for a new residential space. A sprinkler system may be required.

If your property uses a residential water well, you will need to provide a well permit that shows the well can support multiple units and demonstrate that those units are not already being served by that well.

If your property uses a septic system, you will need to provide a septic permit specific to the ADU or an updated septic permit from the existing one to demonstrate the additional bedroom(s) is accounted for.

No. There can only be one ADU for a primary single-family home on a property. Accessory building sizes may also be further restricted by zone district depending on other buildings on the property.

Contact the E-Permit Center at 720.523.6800 or epermitcenter@adamscountyco.gov to talk to the Planner of the Day. You can also contact CEDD-Plan@adamscountyco.gov or the Building Safety team for questions about ADU regulations and requirements and how an ADU might work for your property.

Your site plan should show all setbacks from all structures (existing and proposed) to all property lines, other structures, wells, septic tanks, and leach fields, if applicable. Having a detailed site plan is important for moving through the building permit process smoothly. The site plan can be completed by anyone as long as it is detailed, clear, and accurate.

ADU permit applications generally require Colorado-registered design professional stamped plans. The Adams County Building Safety team is available to answer questions about the required items. A submittal checklist of requirements can be found on the Adams County website. The Engineering team can also answer questions about requirements if you need to acquire an access permit.

ADU structural and architectural plans will need to comply with the international building codes and minimum design criteria adopted by Adams County. The plans should demonstrate that the ADU will match the style of the principal dwelling.

Please refer to our development and building fee schedules on the E-Permit Center. Building Permits fees are based on job valuation, which includes labor and materials. For questions, please contact the E-Permit Center at 720.523.6800 or epermitcenter@adamscountyco.gov.

It depends! There are 10 fire districts that serve unincorporated Adams County. Each district is responsible for enforcing the International Fire Code, plan review, and inspections. For more information, please reference Adams County ordinance 12 and contact your local fire district. Adams County will review P2904 systems.

Yes, an ADU can act as a short-term rental unit. As of October 2025, the county does not have regulations related to short-term rentals unless any unit proposes to host 12 or more people. While the county does not currently have those regulations, it is anticipated to adopt regulations at some point, and if/when those regulations are adopted, any existing short-term rental property would be required to come into compliance with any new rules and standards.

No, a property owner does not have to live on-site in order to build an ADU.

No, a lot cannot be subdivided to sell an ADU independently of the primary home. The Adams County Development Standards and Regulations state the purpose of an ADU is to be an accessory use/structure to a principal dwelling unit and an ADU shall not be platted for individual sale through the condominium platting process or otherwise.