ADU Zoning

Accessory Dwelling Units are gaining traction as a form of gentle density zoning.

The longtime preference for single-family zoning in state laws and municipal ordinances will not disappear overnight. But the housing crunch, and especially the need for more affordable housing, is driving changes in several states, counties and cities. One area of particular interest is density zoning and ADUs — accessory dwelling units.

Photo collage featuring a woman smiling sitting in front of an accessory dwelling unit, and a white siding home with a blue door

(LEFT) Courtesy of Christina Supinski. (RIGHT) Courtesy of Amy Magyar.

Density zoning allows for more homes on a parcel of land. “Missing middle” housing refers to the housing between high-rise apartment buildings and freestanding single-family houses. Duplexes, triplexes, quadplexes and townhouses are examples, as are garden apartments. Those housing options are missing in many communities with zoning ordinances that make them difficult or even illegal to build.

Adding missing middle housing to a community can be controversial. When the Arlington, Va., county board and zoning commission voted to allow such housing in the liberal, wealthy suburb of Washington, D.C., last spring, they were hit with a lawsuit by a group of single-family homeowners. The group said that adding the housing would lead to overcrowded schools and traffic problems and take away parking and trees. Part of the reason for the vote to allow more density zoning in Arlington was affordability. The median price of a home in the county is $645,000, almost double the national average.

ADUs offer a single residence and come in a variety of forms, all accessory to the principal home on the property. An ADU can be a smaller home, detached from or attached to the main home. It can be a separate living space inside the home. Or it can be built above a garage or as a garage conversion. As an independent residence, it has at least a kitchen or kitchenette, bathroom and living/sleeping area.

“ADUs are a great form of gentle density,” said Christina Stacy, principal research associate at the Urban Institute. “They’re not going to solve our housing crisis, by any means. They can be one of the many tools to help us start addressing the crisis.”

Regina Gray, director of the Affordable Housing Research and Technology Division in HUD’s Office of Policy Development and Research, agreed that ADUs are a strategy to address housing supply.

“As a percentage, ADUs remain a very, very small part of the housing supply,” said Gray. “In 2019, 6.8 percent of active listings for sale were ADUs. They’re becoming much more popular. As more ordinances are adopted, about 5 million more are coming online in the next decade or so.”

Infographic: Examples of Attached Accessory Dwelling Units (ADUs) and Detached Accessory Dwelling Units (DADUs)

Opportunity for Older Residents

AARP has been involved with ADUs since at least 2000, said Rodney Harrell, AARP’s vice president of Family, Home and Community. The association believes ADUs can offer more housing options for people of all ages. Harrell gives three overarching reasons: affordability, location and accessibility.

“Affordability is a key driver,” said Harrell. ADUs can offer a way for seniors to age in place — the preference of most — in more expensive communities.

As for accessibility, many seniors prefer to live close to grocery stores, libraries and other places they can walk to, especially once they can no longer drive, Harrell said. But those communities tend to be expensive. Enter ADUs as an affordable solution. A senior who downsizes can live in an ADU. Or they may live in the main residence and rent out an ADU as a helpful income stream.

An ADU is often used to house a family member.

For Family Members or Rental for Income

An ADU is often used to house a family member — an elderly parent or an adult child who returns home and wants more independence. Even when the ADUs are not rented out, the family members who live there might have rented a different home, so the ADU frees up a little housing, said the Urban Institute’s Stacy. The other use of ADUs is as affordable rentals.

The Anne Arundel (Md.) County Council legalized ADUs in April 2023 after two years of lobbying — spearheaded by the Anne Arundel Association of REALTORS® (see sidebar). ADUs are very popular as an affordable option in a county where a one-bedroom apartment is $2,000 a month, said Marygrace Fitzhenry, the association’s government affairs director.

In Annapolis, the county seat and state capital, 40 percent of residents are renters, most of them younger, said Fitzhenry. Houses there average over $1 million to buy — out of range for that demographic.

Promoting Equity

Another benefit of ADUs touches on equity. “People who are Black are less likely to own their homes,” said Stacy of the Urban Institute. “ADUs can help people own a home who otherwise might not be able to.” If an individual wants to buy a home but can’t afford to maintain it, an ADU could provide an income stream to help with that.

Plus, ADUs, like other forms of gentle density, tend to be more supported by homeowners because they don’t change the character of the neighborhood, Stacy said.

Where ADUs Are More Prevalent

Before World War II, it was not unusual for a smaller home to share a lot with a main house. After the war, single-family zoning became the norm. That change was thanks in large part to loans guaranteed by the Federal Housing Administration and Veterans Administration, according to Alexander von Hoffman, senior research fellow at the Joint Center for Housing Studies of Harvard University.

Over time, suburban zoning ordinances imposed minimum lot sizes, and the Not in My Backyard (NIMBY) movement was born. But attitudes are starting to change as communities struggle with the housing crunch, especially affordable housing.

HUD’s research has found that over half of all ADUs in the United States today are in just four states: California (30 percent), Florida (12 percent), Texas (10 percent), and Georgia (5 percent), said Gray.

Jess Remington researched ADUs for a 2021 policy brief she co-wrote with Salim Furth for the Mercatus Center at George Mason University. She found that ADUs are more prevalent on the West coast and in southern college towns.

“The housing crisis has been particularly acute on the West coast, so more serious reform attempts are happening there,” Remington said. In the South, the zoning ordinances covering ADUs don’t have as many stipulations attached as those in the Northeast, making it easier to build them in the South.

Still, each community is different. Boston has a pilot program in which the city is offering zero-interest loans of up to $50,000 to homeowners to build an ADU, said HUD’s Gray. That can mean construction from the ground up or repurposing an existing garage.

Varying Demand

Von Hoffman, the Harvard historian, believes the problem isn’t so much zoning regulations as demand. Earlier in the twentieth century, there were a lot of smaller homes and multifamily housing for working-class and middle-class residents, he said. “There was a demand for them.” In older communities in Pennsylvania or Atlantic City, N.J., you can see a house behind a house.

“Now, in parts of the Southwest — the outer reaches of Las Vegas, greater Phoenix — there are a lot of small subdivisions with little houses,” said von Hoffman. “Developers can see a demand for low-cost housing.”

Changing zoning regulations is just the beginning, he said.

“Since the 1970s, there have been layer upon layer — open space requirements, environmental controls, requirements of town officials — of obstacles to new residential development,” von Hoffman said. “You can build an ADU now, but you have to jump through 12 hoops to do it.”

Importance of Zoning to ADUs

But zoning is an important first step. A helpful resource is the National Zoning Atlas (, which aims to depict key aspects of zoning codes across the country in an online, user-friendly map. It is produced by the Cornell University Legal Constructs Lab and founded by Sara Bronin, a planning and law professor at Cornell.

“It has already been used for research, advocacy and education,” said Bronin. “Connecticut, which documented restrictions on ADUs, helped encourage state legislators to adopt laws encouraging construction of ADUs.” And that’s a state, as documented by the Zoning Atlas, where 90.6 percent of the land is zoned for single-family homes.

“The model for statewide ADU legislation is California,” Bronin said. “Statewide rules that apply to most single-family housing enable property owners to build a second unit without restrictions that are common in many housing units across the country.”

As a result, she said, “ADU construction has boomed in California.” Twenty percent of all building permits issued in the state in 2022 were for ADUs, according to the Department of Housing and Community Development.

Obstacles that Prevent or Slow ADU Construction

A host of restrictions, whether in zoning codes or from community objections, can stand in the way of ADU construction:

Owner-occupancy requirements. A recent survey by Freddie Mac showed that 73 percent of respondents — once ADUs were explained to them — would consider using an ADU for short- or long-term rental. An owner-occupancy requirement makes rental difficult, particularly because of how it might affect resale value, said ADU researcher Remington.

“An owner-occupancy requirement is almost impossible to enforce,” said Stacy of the Urban Institute. “It’s going to impede development.”

Parking requirements. Requirements for off-street parking can be difficult to meet and can be used as a way for homeowners to object to ADUs, as in Arlington, Va.

“If you require off-street parking for an ADU, it cuts off a lot of people,” said Stacy. “It’s a lot of cost, it reduces green space, it looks worse. There’s very little evidence that we need as much parking as we require.” Many of the seniors or adult children who live in ADUs may not even have a car.

Public hearings requirement. A requirement to hold public hearings before an ADU is approved, which is part of many zoning ordinances, may slow a project down so much that it prevents the ADU from being built, said Bronin. If ADUs are permitted “by right,” that means they can be approved without the homeowner or developer having to go through public hearings or a discretionary review process from different boards.

Mandatory lot size. This could be a big obstacle to building more ADUs, said von Hoffman. “Historically, there were lots of shotgun houses, worker’s cottages — but if you have to put it on an acre of land,” they’re no longer practical. Before zoning reform was passed in Connecticut in January 2023, 80 percent of homes there were required to occupy an acre of land or more.

Size limits. “Some homeowners want strict size limits [on an ADU] so it’s not a big, towering unit that detracts from neighborhood quality,” said Stacy. “But you don’t want to block out families.”

Aesthetics. HUD’s Gray found that when ADUs were discussed in Montgomery County, Md., neighbors were worried about worsening traffic congestion and whether the ADUs would create an eyesore. Some were concerned about whether ADUs would meet subdivision design requirements or bring down property value.

Expense and financing. ADUs can be expensive to build and don’t always have the same financing options as single-family homes. HUD is looking at financing tools and mortgage support for ADUs, said Gray. “How can we spur production of ADUs, incentivize the housing industry to mass produce ADUs as they become more in demand?”

Some REALTORS® have already found demand. In 2016, Washington, D.C., passed zoning changes to make it easier to build ADUs. In June 2023, the D.C. Association of REALTORS® held an information session on ADUs for its members. Speakers were from the Coalition for Smarter Growth, the city’s Department of Buildings, the owners of a construction firm, a regulatory expert, and Ileana Schinder, a local architect.

An accessory dwelling unit built above a garage with an outdoor staircase.

Courtesy of Beyond DC

Panelists stayed an hour over the scheduled time to answer concerns about ADUs that REALTORS® brought from their clients. As a result of the meeting, the D.C. REALTORS® scheduled quarterly meetings with the Department of Buildings.

Schinder has designed ADUs for the greater Washington area for the past eight years.

“ADUs are one of those design features that starts slow,” said Schinder. “We are in the early stages of adoption. I anticipate they will become more popular in the future as more municipalities realize their zoning rules are not sustainable financially, environmentally — as politicians and voters realize that living in gentle density property is more secure, more pleasant to live in, without the need for super-density urbanism.”

REALTORS® Educating, Then Lobbying on ADUs

Until April 2023, ADUs were illegal in Anne Arundel County, a Maryland suburb on the Chesapeake Bay about an hour from Washington, D.C. But a lot of homeowners were renting one out anyway, said Marygrace Fitzhenry, government affairs director for the Anne Arundel Association of REALTORS®. She worked for two years on legislation to make ADUs legal.

Illustration featuring a home and an accessory dwelling unit (ADU) with a sign that reads: "Now Legal for ADUs in Maryland."

Courtesy of AACR

Parking was a concern for some council members and members of the public.

“We said, ‘when you build a house or buy a house, we don’t tell you how many cars you can have in your driveway,’” said Fitzhenry. In the new regulations, the off-street parking requirement went from two spaces to one to zero.

ADUs will still have to follow all setback requirements. People were worried an ADU would be like a trailer home. The final law says no ADU can be on wheels or be able to be moved. And the homeowner will have to live in either the main home or the ADU.

The Maryland Association of REALTORS® is still fighting for a law to allow ADUs statewide, said Lisa May, director of advocacy and public policy. After a poll showed that three-fourths of Maryland residents didn’t know what an ADU was, the REALTOR® association used an NAR housing opportunity grant to help produce a short video called “ADUs: A Solution in Our Own Backyards.” An NAR issues mobilization grant helped the state association incorporate the video into its annual State of the Housing Market press conference. Hundreds of millions of people viewed the video, said May.

The law the state association is pushing is based on the model ADU act put together by the AARP. Now, zoning ordinances are different for every county in the state.

“When you have a statewide standard, entire industries come forward,” said May. “They say, ‘We’ll be your one-stop shop, you come to us.’ That is really important because most of the people adding ADUs are regular homeowners.”

Maryland Gov. Wes Moore, elected at the end of 2022, has said he wants a statewide housing supply bill in 2024. So, May is working to grow the REALTORS® coalition with developers, housing advocates, AARP chapters, colleges and universities, and others that want affordable housing.


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