Knowledgebase Collection
Accessory Dwelling Units
Adapted from the ABCs of ADUs
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.
ADUs in the San Francisco Bay Area
This APA Learn course explores the growth of ADUs in the San Francisco Bay Area and the successes and challenges in adopting the programs, plans, and strategies that have promoted this growth.
Seattle Fine-Tunes Backyard Cottages
This Commissioner article describes the Seattle Planning Commission's effort to encourage ADUs as a tool to address housing choice and affordability.
making room for mom and dad
This Planning magazine article tracks housing trends among multigenerational families, including increased demand for ADUs.
Overextended?
This Planning magazine article explores how ADUs can meet the housing needs of multigenerational families.
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
Accessory Dwelling Units: A Step by Step Guide to Design and Development
This guide is designed to help home owners, builders, and local stakeholders understand the process of designing and constructing an accessory dwelling unit (ADU).
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.
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.
The ABCs of ADUs: A Guide to Accessory Dwelling Units and How They Expand Housing Options for People of All Ages
This guide explains the multiple benefits of ADUs for property owners and communities and highlights opportunities to support ADUs through zoning.
Clearinghouses
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.
Reports
2018 Home and Community Preferences: A National Survey of Adults Ages 18-Plus
This report presents the results of a survey of adults, which included a series of questions about accessory dwelling units (ADUs).
Accessory Dwelling Unit (ADU) Zoning Code Audit Report
This report analyzes zoning regulations for accessory dwelling units (ADUs) across metropolitan Portland, Oregon.
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.
Briefing Papers
A Racial Equity Toolkit on Policies for Accessory Dwelling Units
This briefing paper uses Seattle's Racial Equity Toolkit model to evaluate the effects of the city's policy approach to accessory dwelling units.
ADU Update: Early Lessons and Impacts of California's State and Local Policy Changes
This briefing paper summarizes the early effects of state statute amendments in California related to accessory dwelling units (ADUs).
ADUs: Housing Options for a Growing Region
This briefing paper is designed to help local officials in Minnesota's Twin Cities region understand the benefits of accessory dwelling units (ADUs) as well as potential ADU policy approaches.
Accessory Dwelling Units: A Flexible Free-Market Housing Solution
This briefing paper provides a primer on accessory dwelling units (ADUs) and summarizes common regulatory barriers to ADU development.
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.
New Hampshire Says Yes to ADUs
This article discusses how New Hampshire's zoning enabling law establishes minimum regulatory requirements for accessory dwelling units (ADUs).
Blog Posts
ADU Parking Quotas Are Climate Killers
This blog post discusses the most common justifications for removing parking requirements for accessory dwelling units (ADUs).
ADUs and Don'ts
This blog post presents the results of analysis of ADU regulations across Cascadia in the early 2010s.
Could Your Backyard Help Ease the Affordable Housing Crisis?
This blog posts discusses how some communities are using accessory dwelling units (ADUs) to increase the supply of income-qualified affordable housing.
How Backyard Cottages Could Help Close the Affordable Housing Gap
This blog post discusses the potential role accessory dwelling units (ADUs) could play in addressing a shortage of affordable housing in Cascadia.
Impact Fees Are ADU Busters
This blog post discusses how waiving impact fees can boost accessory dwelling unit (ADU) production.
In-Law—and Out-Law—Apartments
This blog post summarizes common barriers to accessory dwelling units (ADUs) and holds up Vancouver, British Columbia, as an example of effective ADU regulatory reform.
Case Studies
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.
In-Law—and Out-Law—Apartments
This blog post summarizes common barriers to accessory dwelling units (ADUs) and holds up Vancouver, British Columbia, as an example of effective ADU regulatory reform.
Making Big Sense of Small Homes
This case study profiles efforts in Portland, Oregon, to remove regulatory and financial barriers to accessory dwelling unit (ADU) construction.
Seattle Says Yes to the Best Rules in America for Backyard Cottages
This blog post summarizes zoning reforms for accessory dwelling units (ADUs) in Seattle.
Tacoma Adopts Exemplary Reform for In-Law Apartments
This blog post profiles accessory dwelling unit (ADU) regulatory reforms in Tacoma, Washington.
Online Training
Big House, Little House, Back House...ADU?
This online training provides a primer on zoning issues related to accessory dwelling units (ADUs).
Webinar: Accessory Dwelling Units
This online training presents an overview of accessory dwelling unit (ADU) policies and practices across the U.S.
Videos
ADUs Help Families Address a Variety of Housing Needs
This video highlights the individual and community benefits of accessory dwelling units (ADUs).
Big House, Little House, Back House...ADU?
This online training provides a primer on zoning issues related to accessory dwelling units (ADUs).
Video: Eliminating Barriers and Increasing Opportunities for Building ADUs
This video, recorded at the 2017 Build Small Live Large Summit in Portland, Oregon, highlight common barriers to accessory dwelling units (ADUs).
Webinar: Accessory Dwelling Units
This online training presents an overview of accessory dwelling unit (ADU) policies and practices across the U.S.
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 Guidebook
This guide is designed to help local officials, planners, and affordable housing advocates in Florida plan and implement strategies to support accessory dwelling unit (ADU) construction and occupancy.
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 in New Hampshire: A Guide for Municipalities
This guide is designed to help municipalities in New Hampshire develop and implement accessory dwelling unit (ADU) regulations that comply with state law.
Model Ordinances
Accessory Dwelling Unit Guidebook
This guide is designed to help local officials, planners, and affordable housing advocates in Florida plan and implement strategies to support accessory dwelling unit (ADU) construction and occupancy.
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.
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
This city's comprehensive plan belongs to the Accessory Dwelling Units, Affordable Housing Programs, Comprehensive Planning, Environmentally Sensitive Areas, Inclusionary Housing, Social Equity, Solar Energy, and Tree Preservation and the Urban Forest collections.
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.
Temple Terrace, FL, Imagine 2040 Comprehensive Plan
This city's comprehensive plan belongs to the Accessory Dwelling Units, Capital Improvements Programming, and Comprehensive Planning collections.
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.
Beaufort, SC, The Beaufort Code
These regulations belong to the Accessory Dwelling Units and Form-Based Zoning collections.
Bentonville, AR, Zoning Code
This regulation belongs to the Accessory Dwelling Units, Content-Neutral Sign Regulation, Downtown Revitalization, and Solar Energy collections.
Bloomington, MN, Code of Ordinances
This regulation belongs to the Accessory Dwelling Units, Active Transportation, Affordable Housing Programs, Creative Placemaking, and Green Building collections.
Boulder, CO, Revised Code
This regulation belongs to the Accessory Dwelling Units, Active Transportation, Environmentally Sensitive Areas, Food Trucks, Inclusionary Housing, Marijuana-Related Uses, Rethinking Off-Street Parking Requirements, Social Service Uses, Solar Energy, and Student Housing collections.
Web Pages
ADUniverse: Creating a Tool to Support Homeowners Interested in Building an ADU
This website describes an effort to build a tool to help Seattle residents evaluate opportunities to build accessory dwelling units (ADUs).
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