Elisabeth Haub Law School of Law
Pace University
Land Use Law Center
Supervisor: John R. Nolon, Distinguished Professor
Blog No. 21 of the Land Use, Human Health, and Equity Project
Editor: Brooke Mercaldi
Contributing Author: Colt Watkiss [*]


ADU Introduction

This blog begins a series of reports on local land use strategies that mitigate the alarming social, economic, and health impacts of our nation’s housing insecurity crisis. Recently, state and local governments have focused on law reform that permits the addition of dwelling units in neighborhoods zoned exclusively for single-family housing. Since most communities have zoned most of their residential land as single-family, this trend has the potential to greatly increase the limited supply of housing that is at the root of the problem.

Accessory Dwelling Units (ADUs) are typically defined as “additional living quarters on single-family lots that are independent of the primary dwelling unit. The separate living spaces are equipped with kitchen and bathroom facilities.” The “Accessory” in ADU refers to the traditional zoning definition of accessory uses: a use that is subordinate, incidental to, and customarily found in connection with the principal use allowed on a lot by the zoning law. ADUs come in three basic forms: detached – separate, freestanding structures on the same lot as the principal dwelling; attached – attached additions to the main home like a converted garage unit; and interior or junior ADUs (JADUs) – units built inside the principal unit.

ADUs were not a foreign concept for much of American history. Thomas Jefferson lived in essentially an ADU while Monticello was being constructed. Many ADUs were built as “carriage” or “coach” houses.” “Originally built for horse-drawn carriages, the structures…were frequently large enough to double as living quarters for workers and stable hands.” “In-law units” or “granny flats” “grew at a time when multigeneration living was more common.” However, “the rise of the nuclear family resulted in fewer extended family living arrangements and smaller households.” Almost 70% of people aged 65 and older lived with their adult children in 1850. Fewer than 15% did in 2000. But a recent rise in multigenerational households is helping revitalize interest in ADUs as an effective way to house multigenerational families while providing privacy.

ADUs “began to disappear after World War II when several trends led to urban sprawl and low-density suburban developments.” The rise of restrictive single-family zoning practically stopped ADU development. “Then as now, residential zoning codes typically allowed only one home per lot, regardless of the acreage and with no exceptions.” At the same time, the size of single-family homes grew. “In 1950 the average single-family home was 983 square feet…[I]n 2019 [it] was 2,301 square feet”; however, the size of households has dropped from over 3.5 people per household in 1947 to around 2.5 today.

In spite of zoning restrictions, illegal construction of ADUs continued in communities where the existing housing stock was not meeting demand.” San Francisco had between 20,000 and 30,000 ADUs by 1960, and “90 percent…were built illegally.” In fact, “[i]n some densely occupied East and West Coast communities, illegal ADUs might compose 2% – 10% of housing stock.” “Some cities, including Chicago, grandfathered in pre-existing ADUs — but only if the residences remained consistently occupied.Barnstable, MA, “recogniz[ing] that although unpermitted and unlawfully occupied, these dwelling units are filling a market demand for housing at rental costs typically below that of units which are and have been lawfully constructed and occupied,” created an amnesty program for its illegal ADUs. Owners are allowed to convert them to legal units by complying with certain criteria, including agreeing to rent the unit “to a person or family whose income is 80% or less of the area median income[,] agree[ing] that rent (including utilities) shall not exceed the rents established by the Department of Housing and Urban Development (HUD) for a household whose income is 80% or less,” and executing a deed restriction which binds future owners to the same agreements.

Recently, ADUs have begun to come back as many states and municipalities see their benefits, including: providing more affordable houses, fighting housing insecurity, subsidizing homeowners’ income, aiding communities with infill and transit-oriented development by gently densifying neighborhoods, being better for the environment, more seamlessly complementing neighborhood character, and providing more housing options to fit different ages and needs, especially older folks as a group one report predicts will be the biggest growth demographic for households in the next 20 years.

Despite America’s history with ADUs and the litany of benefits they provide, ADUs are often a highly controversial topic. They run into numerous barriers and face Not In My Backyard (NIMBY) objections that range from fears of parking issues to concerns over losses in property value. Even in jurisdictions that do allow for ADUs, many ordinances are embedded with “poison pills” that discourage the building and occupation of accessory units.

The next blog in this series will discuss types of “poison pills” and how some states and municipalities have removed them to make ADUs more accessible.


For additional resources, the Gaining Ground Information Database is free and features best practice models used by governments to control the use of land in the public interest. Please direct your search toward the Healthy Communities topic.

[*] Colt Watkiss is a second-year student at the Elisabeth Haub School of Law and Student Associate at the Land Use Law Center.

Brooke Mercaldi is a second-year student at the Elisabeth Haub School of Law and Research Assistant to Professor Nolon.


The previous blogs in the series are listed here:

  1. Reframing Sustainability: Introducing the Land Use, Human Health, and Equity Project
  2. Planning for Public Health: A New Beginning for Land Use Law
  3. The Role of Density in Combatting Climate Change and COVID-19
  4. Novel Coronavirus Claims Implicate Age-Old Property Rights Questions
  5. State & Local COVID-related Emergency Powers: Individual Rights
  6. COVID-Related Land Use Regulations and Judicial Deference
  7. Mediation of Eviction Disputes May Hold the Key to the Survival of Small Businesses
  8. Using Zoning to Help Eliminate Food Deserts: A Few Steps Forward
  9. Urban Heat Islands and Equity
  10. Urban Heat Island and Equity: What Can Local Governments Do?
  11. The Recovery Lease: Preventing Evictions of Commercial Tenants During the Pandemic
  12. The Role of Hazard Mitigation Planning in Promoting Public Health and Resilience
  13. Hazard Mitigation Planning: A Case Study
  14. Complete Streets: Protecting Public Health
  15. Zoning and Lease Mediation as a Way to Retain Critical Small Businesses
  16. Segregation by Law and the Racial Inequity Pandemic
  17. Combating Food Swamps to Improve Equity and Public Health
  18. The Pandemic Plan for Healthy Buildings
  19. Remediating Distressed Properties to Improve Public Health
  20. Housing, a Crucial Determinant of Health

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