Accessory Dwelling Units & Junior Accessory Dwelling Units

Accessory Dwelling Units (ADUs) are also known as granny units, in-law units, second units, and backyard cottages. An ADU is a small dwelling unit that may be established in addition to single-family or multifamily dwellings in residential, mixed-use, or agricultural zones.

• Detached: A new construction unit separate from the primary dwelling.
• Attached: A newly constructed unit connected to a primary dwelling or another accessory structure.
• Conversion: A unit created entirely within an existing primary dwelling or accessory structure.
• Combination: A partially converted unit attached to an existing structure with newly constructed elements.

• Accessory dwelling units (ADUs) are allowed in conjunction with a primary single-family or multi-family dwelling in residential, mixed-use, or agricultural zones. 
• ADUs may be attached or detached from the main dwelling.
• Most residential zoning districts allow one ADU unless the property has the "Z" (Accessory Dwelling Unit Exclusion) overlay zoning district.
• ADUs can be rented for long-term occupancy, but they cannot be used as short-term rentals.

Sonoma County allows several types of residential accessory structures (see Policy 1-4-6, Residential Accessory Structures for a complete list). Below is a summary of the differences among a few of the most common residential accessory structures: Accessory Dwelling Units (ADUs), Junior Accessory Dwelling Units (JADUs), and guest houses.

Accessory dwelling units offer a host of benefits to owners, occupants, and the community. 

• Rental Income. Not only are ADUs relatively affordable to construct, but they can provide a source of income for homeowners. The ADU Center provides resources for estimating potential rental income and return on investment.  

• Increasing Housing Stock. Housing needs well exceed housing supply in California. ADUs add subtle yet valuable density to established areas that may not accommodate larger developments. ADUs may also be used to cater to people at a variety of income levels and can provide as much living space as many newly built apartments and condominiums. They are well suited for couples, small families, friends, young people, disabled people, and seniors. 

• Familial and Multigenerational Housing. ADUs allow extended families to live near one another while maintaining privacy and individual spaces. ADUs give homeowners the flexibility to share independent living areas with family members, friends, and others. Among many scenarios, this can mean providing seniors an accessible place to age as they require more care or allowing family members with certain disabilities to obtain a sense of independence while being close at hand. 

• Safety and Security. Because ADUs are often occupied by family or friends of the property owner, they can provide additional sets of trustworthy eyes and ears for the neighborhood. This can be especially appealing for homeowners who are concerned about their safety or the security of their property. Having additional people on a parcel may also mean more hands available to respond to crises such as fire or medical emergencies.  

• Environmental Benefits. ADUs are generally smaller in size, located on already-developed lots, rely on existing systems for utilities, and are relatively easy to construct for water and energy efficiency compared to a typical single-family home. ADUs are considered a sustainable way of increasing housing stock since they do not require outward urban expansion, take fewer materials to construct, and can help reduce per-housing unit urban impacts. For example, with the addition of an ADU, the same quantity of resource usage and pollutants commonly associated with landscape maintenance may cover the landscaping needs for two homes instead of one. 

The exact process and requirements for obtaining an ADU depend on the parcel's location. Check our ADU Map Viewer (coming soon) to determine whether your ADU is inland or in the coastal zone.

Inland ADUs

Building an ADU – Concept to Occupancy outlines the process for ADUs outside the coastal zone (parcels without CC zoning).

Coastal ADUs

ADUs in the coastal zone (parcels with CC zoning) have different standards than those outside the coastal zone. While the general seven-step process remains the same, the requirements for each step differ. See Coastal Zone ADUs for more information.

Consider Using ADU Ready! Pre-Reviewed ADU Plans

Sonoma County has developed a program to streamline the permitting process for detached Accessory Dwelling Units (ADUs) through pre-reviewed ADU plans. Plans are submitted by vendors, pre-reviewed by Building Division staff, and included in the ADU Ready Design Gallery.

Applicants can contact listed vendors to purchase pre-reviewed plans and integrate them into their projects. This program aims to reduce the costs, time, and effort associated with designing and permitting ADUs. For more details, visit the ADU Ready website.