Accessory Dwelling Units (ADUs)

According to the Colorado Department of Local Affairs, an “Accessory Dwelling Unit” or “ADU” means an internal, attached, or detached dwelling unit that:

A. Provides complete independent living facilities for one or more individuals;

B. Is located on the same lot as the main home (aka primary or principle residence) on the property; and

C. Includes facilities for living, sleeping, eating, cooking, and sanitation.

Other terms used include backyard cottages, in-law suite, secondary unit, and Fonzi’s apartment.

ADUs have many benefits toward housing affordability. ADUs add units to the City’s overall rental housing supply without heavily burdening the existing infrastructure like water, sewer and roads. Adding ADUs is often called “soft density” as it adds the supply that can be easily supported without adding a lot of people to an area. The increase in supply adds options for renters. This helps avoid rapidly rising or unstable rents associated when the number of available units are low and competition is high. An economic rule of thumb states that when supply goes up, prices come down.

ADUs can provide financial relief to property owners. The units can be rented out to provide additional income for the property owner that can offset a mortgage or other costs of living. It also allows for owners to downsize to a smaller unit and rent out the main house for income or convenience. With the rising cost of living, ADUs can provide an independent living situation for young adults until they can save the money needed to “move out” of their parents’ house. With the rising cost of tuition, it can also provide an alternative to paying for a dorm room.

ADUs help preserve neighborhood character as the units are much smaller in scale than the primary residence. Most communities, like the City of Longmont, have design guidelines for ADUs to ensure the structure is compatible with the surrounding structures and development pattern in the area. Compatibility is achieved through architectural features such as windows and doors, roof lines, colors, and building materials that match the front of the main home. Unit sizes are typically limited to between 500 and 800 square feet. However, if someone is converting an attic, basement or other portion of the existing house without increasing the overall footprint, the size limit does not apply. There are some other cases where there is leeway on the size of the ADU. Please contact Planning and Development Services for additional information.