Accessory Dwelling Units (ADUs)

ADU, Cottage, Granny Unit, carriage house

Accessory Dwelling Units are also known as a mother-in-law suites, carriage houses, or garage apartments. These are defined as “a second dwelling either within or added to an existing single-family home, or in a separate accessory structure on the same property as the main home, for use as a complete, independent living facility…”

General requirements for ADUs

Allowed as an accessory use to a single-family detached home
Permitted in residential or mixed-use zoning districts (check the City’s Zoning Map to verify zoning for a specific property)
The ADU must be subordinate in size to the main home (50% or less of the finished floor area of the principal dwelling)
One additional off-street parking space needs to be provided for the ADU
The property owner must occupy the principal or accessory dwelling unit
Accessory dwelling units cannot be sold separately from the principal dwelling
Accessory dwelling units cannot legally operate as short term rentals. The only situation where ADUs may operate as short term rentals is in Planned Unit Developments, such as Prospect and Riverside at Mill Village, where the site plan specifically allows for that type of use.
Mobile homes, recreational vehicles (RVs), tiny homes built on trailers, and travel trailers cannot be used as ADUs
Planning staff can help you determine the process to review and approve requests for ADUs
Building permits are required to construct ADUs
Fees for ADUs vary depending on the specific project and will be calculated with the building permit. Fees typically range from $5,000 – $10,000 and include plan review and permit fees, City and County taxes, and a number of community investment fees.

The complete, specific development standards are outlined in the Municipal Code’s §15.04.040.

For questions, contact us at (303) 651-8330 or longmont.planning@longmontcolorado.gov

_{Last modified: 10/15/2018, I. Colby}