There is no one-size-fits-all approach to allowing ADUs. Municipalities regulate ADUs through building and zoning codes. Communities typically tailor how they permit ADUs to meet local goals and fit with the local character. Zoning codes may address the following ADU standards:
Neighbors are frequently concerned that an ADU will not be cared for, or that an absentee landlord will rent out both the home and the ADU. To ensure the property is maintained, some municipalities require that the owner live in either the primary residence or the ADU. In other communities, only family members can live in ADUs.
Communities may specify the form and appearance of ADUs to fit in with the character of existing homes. Zoning standards may require that the exterior materials of the ADU are compatible with the primary home, including siding material, window design, and roof shape.
Communities with larger homes may allow attached ADUs with a separate entrance, while communities with larger lots may allow detached ADUs by adding a new freestanding unit or converting a detached garage. Some communities allow both attached and detached ADUs.
Communities can limit the size and height of an ADU to ensure that they are clearly secondary to the principal housing unit on the site. When communities allow detached ADUs, they can limit the total square footage or its coverage of the lot area. For an attached unit, communities can limit the total square footage or percentage of interior space.
Communities can also set the maximum number of bedrooms within an ADU. In municipalities with many housing types, these square footage and bedroom limits can vary based on the size of the lot or the zoning district of the home.
The potential impact on parking is a frequent concern, especially in communities where parking is scarce. Research suggests that ADU tenants tend to own fewer cars than other households do and may walk, bike, or use transit if those options are convenient.
Communities may require one parking space per ADU or per bedroom in an ADU. Not including parking requirements encourages people to develop ADUs.
The way in which ADUs are administered can impact their feasibility within a community. Some are permitted by right and only require building code and zoning compliance. Some communities categorize ADUs as a special or conditional use requiring a public hearing. Although the public hearing process allows for more in-depth review of projects, it can be time consuming and generally has additional requirements that can be a significant financial burden for homeowners. Allowing ADUs as a permitted use encourages people to develop them.