What is an ADU?

    ADU stands for Accessory Dwelling Unit. An ADU is a small interior apartment or separate backyard house that shares a parcel with the primary residence. ADUs are also known as mother-in-law apartments, or backyard cottages.

    Title 20, Missoula’s zoning ordinance, defines an Accessory Dwelling Unit as: A separate dwelling unit within a primary residence or a separate dwelling unit that occupies an accessory building that shares a parcel with a primary residence. As the name implies, accessory dwelling units are an accessory use to the principal use of the property.

    Why does the City encourage building ADUs?

    In A Place to Call Home: Meeting Missoula’s Housing Needs (accessible through the Important Links tab), the city adopted recommendations to “update ADU regulations to lower the cost of construction, incentivize development of rent-controlled homes, and promote accessible rental infill in all areas of the city.” This is because ADUs are one of the most ecologically sound ways to develop new homes. They help reduce carbon dioxide and other pollutants that cause climate change by increasing urban density and requiring fewer natural resources to build and operate than a standard-sized home. And because ADUs are generally smaller than typical single-family homes, they use fewer natural resources to build and less energy to heat and cool. Using ADUs as a strategy for housing is also more resource efficient for local governments. By utilizing existing city infrastructure, it combines an ecological and financial benefit when compared to costly infrastructure expansion at the urban fringe. The deployment of ADUs in existing neighborhoods also reduces the need for automotive transportation—a source of about 25% of greenhouse gases—and supports alternative modes of transportation such as walking, biking, and public transit. The wider adoption of ADU development represents a housing approach strongly aligned with the City’s Focus Inward growth policy

    What types of ADUs are there?

    There are three types of ADUs:

    • Internal ADU – conversion of existing floor area in a house or planned into a new house. For example, a basement apartment.
    • Internal Addition ADU - added to the floor area of an existing house. For example, adding new square footage above or alongside the existing house.
    • Detached ADU – a dwelling unit in a structure that is separate from the primary dwelling residence. This may be a new accessory unit or a conversion of existing accessory structure space. For Example, an alley house or garage apartment.

    Where are ADUs allowed?

    Missoula’s zoning ordinance allows ADUs in all Title 20 (R, B, C and M1R) zoning districts, though development covenants may restrict ADUs in some places. ADUs are not allowed in special zoning districts or planned unit developments (PUDs) unless explicitly regulated within the special district or PUD. ADUs are also not allowed in Townhome Exemption Developments.

    Does building an ADU require a permit?

    Yes. A building permit must be issued by Development Services before construction of a new ADU, and renovation of existing structures to include an ADU. For information about the permitting process, contact the Development Services zoning information hotline, 552-6625.

    What are the steps I should be taking to build an ADU?

    The first thing to do if you are thinking of building an ADU is do your research. Figure out what zoning district you are located in (see the What’s My Zoning map in the Important Links tab). Check your covenants to make sure ADUs are not prohibited. Contact your utility agencies for information about utility requirements (the link to Development Services is included in the Important Links tab for questions about water and sewer connections). Locate easements, setbacks, and other no-build areas on your property. Determine what type of ADU works for you (internal, internal attached, or detached) and how that will fit onto your site. Consider hiring a design professional and/or professional contractor to design and draw up plans for your building permit review. Consider notifying and discussing your plans for an ADU with your neighbors, especially if your zoning district requires notification. Contact the Zoning Desk (552-6625) or Development Services permit coordinators (552-6630) with questions about requirements or permitting processes.

    Once you have done your research and designed your ADU, you are ready to submit for permits.

    Where can I find zoning information on ADUs?

    ADUs are addressed in Missoula City zoning code in section 20.45.060.

    Can an ADU be on a parcel with two or more primary residences?

    Missoula’s zoning ordinance only allows one ADU on parcels that are otherwise occupied by a single house.

    Can an ADU share utility infrastructure with a primary residence?

    Yes. An ADU can share utility infrastructure with a primary residence. The infrastructure must have the capacity to handle an additional dwelling unit. The property owner should contact utility agencies (natural gas, electricity, water, sewer) for information about utility requirements.

    Are there design standards for ADUs?

    Yes. ADU design standards are in Missoula’s zoning ordinance, Title 20, Chapter 20.45.060.

    Do ADUs require additional parking?

    No. Based on the zoning code updates in 2020, ADUs no longer require an additional parking space.

    Who can live in an ADU?

    The property owner, a family member, or a renter may live in either the primary residence and/or the ADU.

    Are neighbors notified when an ADU is built?

    Applicants are required to notify all property owners and residents of surrounding properties when they apply for a building permit to build an ADU in R5.4, RT10, R20, R40 or R80 zoning district. After the ADU permit has been issued, Development Services mails a notice to adjacent residents and property owners confirming that the ADU has been permitted.

    Do covenants restrict ADU’s?

    If the property is located in an area with development covenants or a homeowners association, it is the responsibility of the property owner to determine if an ADU is allowed. Development or protective covenants may restrict your ability to build an ADU.

    What is the difference between an ADU and a second dwelling unit?

    A second dwelling unit is considered an additional primary dwelling unit on a parcel, which means that it is only permitted if the density that is regulated by zoning allows for an additional unit. In comparison, an ADU is defined as being accessory to the primary residence, so for applicable parcels, even if zoning would not otherwise allow additional units, that property is still allowed to have an ADU.

    How is a primary dwelling with an attached ADU different from a duplex (two-unit house)?

    ADUs have specific design standards, cannot be larger than 600 square feet, are not included in the land area calculation that determines housing density. Duplexes are not subject to ADU regulations.

    What are the differences between conforming, non-conforming, and illegal second dwelling units?

    A second dwelling unit can be detached or attached to another unit (i.e. a duplex or two-unit house), and may be conforming, non-conforming, or illegal. Buildings that look like ADUs but were built prior to 2009 are considered second dwelling units because the term ADU did not exist in Missoula’s zoning ordinance until 2009.

    Conforming second units - A conforming second unit complies with the current regulations of the zoning district. A second unit can be on a parcel that is large enough to meet the zoning district’s density standard for two units of housing.

    Non-conforming second units – A non-conforming second unit was lawfully established but no longer complies with some of the requirements of the zoning district in which it is located. This occurs when zoning did not exist when the unit was built or through changes to zoning over time.

    Illegal second units – A second unit may be illegal if it was constructed without appropriate permits and does not meet zoning requirements.

    What if I have an existing second dwelling unit not approved through a permit process?

    The Development Services Office will determine if the dwelling is considered a second unit or an accessory dwelling unit.  If it is considered a second unit (as described above) the general standards of the zoning district or the nonconforming uses chapter (Ch.20.80 Nonconformities) may apply. Appropriate permits must be obtained and documentation filed to make the unit legal.

    The City Building Division currently has a voluntary housing inspection program. The purpose of the program is to improve the quality of residential structures in the City of Missoula. The inspection will cover a checklist of items that constitute a substantial health and safety risk to the occupants and owners, and can be requested by landowners, agents, or tenants.  To obtain an inspection, fill out a request form provided by the City Building Division (552-6044) and pay an original inspection fee of $15.00.