Knowledgebase Collection
Accessory Dwelling Units
An accessory dwelling unit (ADU) is a smaller, independent residential dwelling unit located on the same lot as a stand-alone (i.e., detached) single-family home. ADUs go by many different names throughout the U.S., including accessory apartments, secondary suites, and granny flats. ADUs can be converted portions of existing homes (i.e., internal ADUs), additions to new or existing homes (i.e., attached ADUs), or new stand-alone accessory structures or converted portions of existing stand-alone accessory structures (i.e., detached ADUs).
Internal, attached, and detached ADUs all have the potential to increase housing affordability (both for homeowners and tenants), create a wider range of housing options within the community, enable seniors to stay near family as they age, and facilitate better use of the existing housing fabric in established neighborhoods. Consequently, many cities and counties have signaled support for ADUs in their plans and adopted zoning regulations that permit ADUs in low-density residential areas.
From this page you can search for resources that provide background, policy guidance, and examples of local plan recommendations and zoning standards for ADUs from across the country. And you can filter these search results by various geographic and demographic characteristics.
APA Resources
Accessory Dwelling Units
This edition of PAS QuickNotes provides a primer on different types of accessory dwelling units (ADUs) and explains the benefits of making space for these units in single-family residential areas.
Zoning to Promote Garage Apartments
This edition of Zoning Practice details a proposal for zoning reforms to facilitate garage-to-apartment conversions.
Zoning for Accessory Housing
This edition of Zoning Practice article outlines a strategy for incorporating accessory housing standards into local zoning codes and highlights a few examples of communities where this approach has been effective.
On Demand: Accessory Dwelling Units (ADUs), The Durango Experience
This session from the 2016 National Planning Conference highlighted how planners in Durango, Colorado, worked through various challenges associated with planning for and regulating accessory dwelling units.
Planning for Accessory Dwelling Units
While many communities are interested in expanding housing choices by allowing ADUs in single-family areas, some residents of these areas may be concerned about ADUs changing the character of their neighborhoods or overburdening existing infrastructure. The research to date does not support fears about lower property values or parking shortages. Conversely, there are some indications that ADUs do increase the supply of affordable housing and do make significant economic contributions to their host communities, through construction activity and property taxes.
Conducting a housing needs assessment before drafting zoning regulations for ADUs can highlight demographic and spatial mismatches between the existing housing supply and the current and projected housing demand. Consequently, it may provide an indication of the total number of ADUs likely to be created in a given time period under a permissible regulatory scheme. Meanwhile, a residential design study can help proactively identify challenges associated with integrating ADUs into established single-family neighborhoods.
When cities and counties address ADUs in their comprehensive plans, they often include policy recommendations related to updating zoning regulations or providing public information about existing regulations. Some communities also explicitly identify land-use categories or place types where ADUs are appropriate.
Zoning for Accessory Dwelling Units
Many cities and counties permit ADUs in one or more single-family zoning districts by right, subject to use-specific standards. Common provisions include an owner-occupancy requirement (for one of the two dwellings), dimensional and design standards to ensure neighborhood compatibility, and off-street parking requirements. Other relatively common provisions include minimum lot sizes and limits on the number of occupants or bedrooms. While some codes also include occupancy restrictions that stipulate that ADUs can only house family members or domestic employees, this type of restriction can severely limit the potential for ADUs to address a shortage of rental housing.
In some states, such as California and Vermont, localities must permit ADUs by right, under certain conditions. In some others, state laws pre-empt some aspects of local zoning for ADUs or actively encourage cities and counties to adopt permissive zoning regulations for ADUs.
Many older communities have an existing supply of illegally created ADUs. Some of these communities offer, or have offered, some form of limited amnesty to owners of illegal ADUs. These amnesty programs may waive permitting and inspection fees in exchange for owners registering their units, and they typically expire within a year or two of adoption.
Background Resources
A Room of One’s Own: Accessory Dwelling Unit Reforms and Local Parochialism
This article discusses the national trend in state and local regulations toward more permissive regulations for accessory dwelling units (ADUs) and presents the findings of a study of local regulations in California.
Accessory Apartments for Today’s Communities
This short article highlights the potential of accessory dwelling units (ADUs) as an affordable housing strategy and briefly discusses common barriers to developing ADUs.
Accessory Dwelling Units: A Smart Growth Tool for Providing Affordable Housing
This article provides a general overview of accessory dwelling units (ADUs), discusses potential regulatory barriers to ADUs and state efforts to promote ADUs in Florida, and includes considerations for local zoning ordinances.
Accessory Dwelling Units: Case Study
This short report documents how six different localities regulate accessory dwelling units (ADUs) and discusses the potential benefits of removing regulatory barriers to ADUs.
Accessory Dwellings
This website contains numerous pages with information about accessory dwelling units (ADUs) as a concept, ADU design and construction, state and local policy for ADUs, and research about ADUs.
Accessory Housing Is Part of the Solution
This short article summarizes a number of common regulatory barriers to affordable housing. These barriers include process requirements, design standards, occupancy standards, and lot size and parking standards.
Responding to Changing Households: Regulatory Challenges for Micro-Units and Accessory Dwelling Units
This report summarizes the research to date about the potential benefits of accessory dwelling units (ADUs) and micro-apartments, analyzes the regulatory barriers to developing these dwellings, and identifies questions for future research.
Reports
Accessory Dwelling Unit Survey for Portland, Eugene, and Ashland, Oregon
This report presents findings from a survey of accessory dwelling unit (ADU) property owners in three Oregon cities.
Accessory Dwelling Units in Portland, Oregon: Evaluation and Interpretation of a Survey of ADU Owners
This report analyzes findings from a survey of accessory dwelling unit (ADU) property owners in Portland, Oregon.
Accessory Dwelling Units: Case Study
This short report documents how six different localities regulate accessory dwelling units (ADUs) and discusses the potential benefits of removing regulatory barriers to ADUs.
Home Remedies: Accessory Apartments on Long Island
This report analyzes local regulations for accessory dwelling units (ADUs) on New York's Long Island.
Jumpstarting the Market for Accessory Dwelling Units: Lessons Learned from Portland, Seattle, and Vancouver
This report presents the results of a survey of accessory dwelling unit property owners in Portland, Oregon; Seattle; and Vancouver, British Columbia.
Promoting Workforce Housing: Expanding Locations and Development Potential
This report includes a model ordinance template to help cities in Montgomery County, Pennsylvania, draft regulations that encourage the development of accessory dwelling units.
Articles
A Room of One’s Own: Accessory Dwelling Unit Reforms and Local Parochialism
This article discusses the national trend in state and local regulations toward more permissive regulations for accessory dwelling units (ADUs) and presents the findings of a study of local regulations in California.
Accessory Apartments for Today’s Communities
This short article highlights the potential of accessory dwelling units (ADUs) as an affordable housing strategy and briefly discusses common barriers to developing ADUs.
Accessory Dwelling Units as Low-Income Housing: California’s Faustian Bargain
This article evaluates the efficacy of a California statute that permits cities and counties to count accessory dwelling units (ADUs) toward affordable housing production goals.
Accessory Dwelling Units: A Smart Growth Tool for Providing Affordable Housing
This article provides a general overview of accessory dwelling units (ADUs), discusses potential regulatory barriers to ADUs and state efforts to promote ADUs in Florida, and includes considerations for local zoning ordinances.
Accessory Housing Is Part of the Solution
This short article summarizes a number of common regulatory barriers to affordable housing. These barriers include process requirements, design standards, occupancy standards, and lot size and parking standards.
Understanding and Appraising Properties with Accessory Dwelling Units
This article describes challenges associated with appraising properties with accessory dwelling units (ADUs) and proposes an appraisal method based on income capitalization.
Statutes
California Planning and Zoning Law
These statutes belong to the Accessory Dwelling Units, Capital Improvements Programming, Environmentally Sensitive Areas, and Scenic View Protection collections.
Florida Accessory Dwelling Units Statute
Florida state law authorizes all localities with a shortage of affordable rental housing to adopt an ordinance that permits accessory dwelling units (ADUs) in all areas zoned for single-family residential use.
Hawaii County Zoning Statute
Hawaii state law authorizes all counties to adopt zoning standards that permit two single-family dwellings on any lot where a residential dwelling unit is permitted.
New Hampshire Planning and Zoning Statutes
These statutes belong to the Accessory Dwelling Units, Capital Improvements Programming, and Transfer of Development Rights collections.
Oregon Comprehensive Land Use Planning Statutes
Oregon requires most localities to permit one interior, attached, or detached accessory dwelling unit (ADU) by right for reach existing or newly constructed single-family detached dwelling.
Rhode Island Zoning Enabling Act
Rhode Island state law authorizes cities and towns to protect scenic views, establishes a transfer of development rights program, and specifies conditions for localities to permit accessory dwelling units.
Guides
Accessory Dwelling Unit Homeowners’ Handbook: A Guide for Homeowners on Oahu Interested in Building an Accessory Dwelling Unit
This local guide explains property owner options for developing accessory dwelling units in Oahu County, Hawaii.
Accessory Dwelling Unit Manual
This local guide explains property owner options for developing accessory dwelling units in Santa Cruz, California.
Accessory Dwelling Unit Memorandum
This guide is designed to help cities and counties in California update local zoning regulations for accessory dwelling units (ADUs) to comply with state statutes.
Accessory Dwelling Units Program Guide
This local guide explains the application and review process for developing accessory dwelling units in Portland, Oregon.
Accessory Dwelling Units: A Guide to Accessory Apartments in Charlottesville, VA
This local guide explains property owner options for developing accessory dwelling units in Charlottesville, Virginia.
Accessory Dwelling Units: A Practical Option to Promote Affordability
This concise guide provides an overview of local policy issues related to accessory dwelling units (ADUs) and includes specific recommendations for cities and counties in Delaware to help them develop zoning regulations for ADUs.
Accessory Dwelling Unit Memorandum
This guide is designed to help cities and counties in California update local zoning regulations for accessory dwelling units (ADUs) to comply with state statutes.
Accessory Dwelling Unit Model Ordinance
This model ordinance provides a template to help cities in Southwest Idaho draft regulations that encourage the development of accessory dwelling units.
Accessory Dwelling Unit Model Ordinance
This model ordinance provides a template to help cities and towns in Rockingham County, New Hampshire, draft regulations that encourage the development of accessory dwelling units.
Accessory Dwelling Units: Issues & Opportunities
This guide provides a comprehensive overview of local policy issues related to accessory dwelling units (ADUs) and includes specific recommendations for cities and counties in Washington to help them develop zoning regulations for ADUs.
Accessory Dwelling Units: Model State Act and Local Ordinance
This guide includes model state and local legislation to encourage the development of accessory dwelling units.
Massachusetts Model Bylaw for Accessory Dwelling Units
This model bylaw provides a template to help municipalities in Massachusetts draft regulations that encourage the development of accessory dwelling units.
Comprehensive Plans
Auburn, WA, Imagine Auburn Comprehensive Plan
The city’s comprehensive plan includes several policy recommendations that support the development of accessory dwelling units.
Moscow, ID, Comprehensive Plan
The city’s comprehensive plan discusses the benefits and challenges associated with expanding the supply of accessory dwelling units (ADUs) and includes a policy recommendation that supports the development of ADUs.
Orono, ME, Comprehensive Plan
The town’s comprehensive plan includes two specific policy recommendations that support the development of accessory dwelling units.
Portland, OR, Comprehensive Plan
The city's comprehensive plan includes policy recommendations that support accessory dwelling units, comprehensive planning, environmentally sensitive area protection, inclusionary housing, solar energy use, and urban forestry.
Prince George’s County, MD, Plan 2035 General Plan
The county’s comprehensive plan, which achieved Bronze-level recognition in APA's Comprehensive Plan Standards for Sustaining Places Recognition Program Pilot, includes several policy recommendations that support the development of accessory dwelling units.
Saint Paul, MN, Comprehensive Plan
The city’s comprehensive plan includes two policy recommendations that support the development of accessory dwelling units.
Regulations
Auburn, AL, Zoning Ordinance
The city’s zoning code addresses accessory dwelling units (ADUs) and student housing.
Auburn, WA, Municipal Code
The city’s zoning code permits internal, attached, and detached accessory dwelling units by right in all districts that permit single-family homes, subject to use-specific standards.
Bentonville, AR, Zoning Code
The city's zoning code belongs to the Accessory Dwelling Units, Content-Neutral Sign Regulation, Downtown Revitalization, and Solar Energy collections.
Bloomington, MN, Code of Ordinances
The city’s zoning code permits internal and attached accessory dwelling units by right in multiple residential districts, subject to use-specific standards, as well as sets requirements for sidewalk connectivity.
Boulder, CO, Revised Code
The city's codified ordinances address accessory dwelling units, active transportation, environmentally sensitive areas, food trucks, inclusionary housing, marijuana-related uses, off-street parking requirements, social service uses, solar energy, and student housing.
Chapel Hill, NC, Code of Ordinances
The town's land-use management code addresses accessory dwelling units, inclusionary housing, residential infill development, solar energy use, student housing, and tree preservation and urban forestry.
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