Accessory dwelling units (ADUs), or self-contained residential units within a larger single-family home, can be an important source of low-cost rental housing for small households. ADUs are most valuable in communities with limited developable land, as well as communities where there are significant concerns about the physical character of their neighborhoods, as they only minimally change the appearance of single-family areas. ADUs can also improve the affordability of single-family homes in the community by providing homeowners with additional income from the ADU rental.

While ADUs are often allowed in communities by conditional use permit, allowing them by right within the zoning code can promote their development on a much larger scale. Eliminating regulations that restrict development to only one residential unit per lot, reducing the minimum lot size for homes with ADUs, allowing ADUs both as part of the primary residence as well as above garages and other accessory uses, and relaxing parking requirements or allowing tandem parking can promote ADU development by easing the design and approval process. Informing the public on how to develop an ADU on their property, bringing illegal ADUs into compliance and incentivizing ADUs in newly developed single-family homes may also be useful strategies for encouraging the development of a substantial number of ADUs.

Case Studies

Accessory Dwelling Unit Ordinance (Santa Cruz, Calif.)

The city of Santa Cruz has struggled with affordability and growth for years. In 2002, the city adopted an Accessory Dwelling Unit Ordinance in order to preserve the local greenbelt while accommodating new growth, transit and affordable housing. The ordinance permits ADUs by-right in designated residential zones and waives development fees for ADUs made available for low- and very low-income households. To support the goals of the ordinance, the city also established an ADU development program that provided technical assistance, a wage-subsidized apprentice construction program and an ADU loan program. In addition, the city published an ADU Manual, and an ADU Plan Sets book, which allows the homeowner to choose a pre-approved ADU design and receive permits in an expedited manner.

Zoning changes have been particularly important to facilitating ADUs in Santa Cruz. Most significantly, the city exempted homes with ADUs from the covered parking requirement, thus allowing homeowners to convert their garages to ADUs. Additionally, the city allowed tandem parking and parking in the front yard setback. In 2014, the city further modified setback requirements, decreased the minimum lot size from 5,000 square feet to 4,500 square feet and allowed exemptions for illegal ADUs in order to bring them into compliance.

For More Info:

Alex Khoury, City of Santa Cruz Planning and Community Development
Email: [email protected]

Accessory Dwelling Unit Regulations (Mercer Island, Wash.)

Mercer Island promotes accessory dwelling units through its zoning code. Faced with high housing costs and limited developable land, ADUs provided an opportunity for Mercer Island to expand its housing stock and increase affordability without changing the character of existing neighborhoods. Similar to Santa Cruz, ADUs on Mercer Island are allowed by-right in all single-family residential zones and are subject to a series of restrictions. The property owner is required to live in either the principal unit or the ADU. Units must be a minimum of 220 square feet and a maximum of 900 square feet. Most single-family homes are required to have at least three parking spaces, including two covered spaces, one of which must be available for the tenant of the ADU.

Within the first five years of its adoption, Mercer Island’s ADU ordinance had encouraged the development of over 150 ADUs on an island of 24,000 residents. While there has not been a recent study of ADU affordability on the island, a 2000 study by King County, Wash., found that the average rent of a Mercer Island ADU was affordable to households earning 60 percent of median income.

For More Info:

Shannon Restall, City of Mercer Island Development Services – Building & Planning
Email: [email protected]