Accessory dwelling units are self-contained residential units that are either:
— Set aside within a larger single-family home, such as a separate basement or attic apartment,
— Attached to a primary residence, such as an apartment above an attached garage, or
— Smaller separate units built on the same lot as single-family homes.
Accessory dwelling units (ADUs) – also called secondary units, granny flats, carriage houses and in-law apartments – can be an important source of low-cost rental housing for small households in many communities. ADUs may also be a boon to owners of the primary home, who collect extra income from the rental unit that can be used to help cover mortgage payments, property taxes and other costs.
Capitol Heights Cityhomes, St. Paul MN — Photo courtesy of LHB, Inc.
In some cases, the lease may include an agreement that renters help with household chores and basic maintenance, an arrangement that can help aging homeowners remain in their homes when they are no longer able to perform these tasks. Seniors and empty nesters may eventually choose to move into the ADU and rent out the main house, an adaptation that further helps them age in place. ADUs are generally built and financed entirely by the homeowner, so no public subsidy is required to make them available. Communities interested in using ADUs as an affordable housing option should comprehensively review their zoning policies to ensure they facilitate, rather than hinder, development of ADUs.
Solutions in Action
Addressing community concerns: Objections to ADUs are most often based on concerns about overcrowding, traffic congestion and parking shortages, and a loss of neighborhood character. However, the experience of many communities has been that when ADUs are approved, units tend to “trickle in” rather than overwhelm entire neighborhoods overnight.  Because they are often incorporated into or next to existing homes, ADUs have minimal impact on neighborhood density and can blend in seamlessly with the community.
Obstacles to development: Some communities prohibit ADUs entirely, while others have regulatory requirements (often unintentional) that severely limit opportunities for this development or make the development process so long and costly that private homeowners opt out. These include zoning policies that:
— Allow only one residential unit per lot
— Increase the minimum lot size for units with ADUs to twice the minimum lot size specified in the underlying zoning code
— Limit the proximity of ADUs to property lines, making it difficult to develop above-garage units adjacent to back alleys
— Require an excessive number of parking spaces per unit
— Impose excessive permit fees or impact fees that make development of ADUs too costly
In 1982 the state of California passed the Second Unit Law – legislation intended to promote the development of accessory dwelling units by codifying a state standard for ADU approval. Under the law, communities that had not already adopted a local ADU ordinance were given considerable discretion in setting the terms of their own approvals procedure and development standards. However, those that chose not to adopt an ADU ordinance were required to approve all proposals that met state standards.
While the purpose of this legislation was to facilitate creation of additional units, implementation hit a snag when all of the jurisdictions that adopted an ordinance approved ADUs as a conditional use only. Rather than allowing development as of right, applicants had to undergo a lengthy and potentially contentious review and public hearing process that made creation of new units difficult at best. With development falling far short of expectations, in 2003 the state legislature passed Assembly Bill 1866, which amends the original law by requiring local communities to allow ADUs as of right (provided they are otherwise in compliance with the zoning code) by prohibiting local ordinances that mandate discretionary review of ADU applications.
Some critics of the law point out that it only facilitates the administrative approvals process, and does not address structural conditions that localities may choose to apply, such as size and design requirements, parking regulations and other restrictive standards that can constrain development of ADUs or make them cost-prohibitive.
More at Accessory Dwelling Units. 2004. Instant Advocate. Oakland, CA: Transportation and Land Use Coalition, Section 2: Is this the Right Tool for You?
Click on the links below to learn more about other types of housing that can help communities meet the needs of households with a range of preferences and budgets:
Multifamily/attached homes, which may include apartment buildings, condominiums, town homes, row houses and duplexes.
“Factory-built” homes, from manufactured homes built entirely in production facilities to modular housing that is assembled on-site.
Accessory dwelling units within or attached to a larger single-family home, or on the same lot.
Mixed-use housing, where residential units co-exist with commercial and retail enterprises.
Single-room occupancies, also called efficiency apartments and residential studio units.
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Greater housing diversity and affordability may be achieved by revising zoning policies to eliminate both direct and “back door” prohibitions and explicitly allow a range of housing types, rather than requiring a special review process or disallowing certain types of structures entirely.
Other pages in this section:
Local officials can implement an array of land use tools to create a regulatory environment that is hospitable to the development of homes affordable to working families.
Click here to view other resources on zoning policies that allow housing diversity.
 Accessory Units: An Increasing Source of Affordable Housing. 1991. By Patrick H. Hare and John Danbury. Public Management 73, pp. 5-8, cited in Accessory Dwelling Units. 1995. Report No. 33. Seattle, WA: Municipal Research and Services Center of Washington; and Accessory Dwelling Units. 2004. Instant Advocate. Oakland, CA: Transportation and Land Use Coalition, Section 2: Is this the Right Tool for You?