A(lready) D(esigned for) U

We have a severe affordable housing shortage in the United States — an urgent need for millions of additional homes. But exacerbating that housing shortage is a housing mismatch.

In much of the US, existing residential neighborhoods — the places where people already like to live, near their jobs, friends, and family members, and that are already served by utilities, transit, and other infrastructure — are overwhelmingly, and often exclusively, composed of single-family homes. While a four-bedroom Colonial can make good sense for a high-income family of five, it shouldn’t be the only housing option available in a community, given the kaleidoscopic variety of humanity and its households, from aging seniors to young adults to single parents.

“We’re going to have more people over the age of 65 than under 18 in the next decade,” says Rodney Harrell, AARP’s vice president of family, home, and community. The organization has a long history of advocating for better housing conditions and options for seniors. “People want to be near grocery stores, parks, libraries, transportation options — things that make them feel connected. But one of the challenges is that people want to stay in their existing neighborhoods, and there aren’t enough options there.”

Adding new housing options to existing communities, however, routinely elicits complaints about changes to the “neighborhood character.” This loaded phrase can contain exclusionary attitudes and bad-faith arguments within its ample ambiguity, but it can also be a response to dubious development decisions. A homeowner in a neighborhood of century-old Craftsman bungalows may understandably be put off by the idea of a sleek new seven-story steel and concrete building on the corner.

Therein lies the appeal of the humble accessory dwelling unit, or ADU — more colloquially known as an in-law apartment, carriage house, secondary suite, or casita, among other aliases. By converting a garage, attic, or basement to a separate apartment, or adding a small, detached cottage to a backyard, homeowners can create an extra space for family members or a small rental property that helps generate income. At the same time, they help increase the supply of affordable and accessible housing options in their neighborhood — without a dramatic impact to the local aesthetic. And making it easier for homeowners to do that can help communities everywhere address the local and national housing crunch.

Over the past decade, many cities and some states have relaxed decades-old restrictions on ADUs. California, for example, legalized ADUs on all single-family lots in 2017; a few years later, the nearly 27,000 ADUs permitted statewide in 2023 represented a 20-fold increase over 2016, and comprised more than 20 percent of all new housing permitted. In 2024, Los Angeles alone granted permits for more than 6,000 ADUs.

That’s not enough to singlehandedly solve California’s housing crisis — no one step is. But it’s certainly one piece of the puzzle, and a solution that many communities can get behind.

Still, making it legal to build an ADU at all is just the first hurdle. Making it easier for someone to accomplish is the next step — one that cities can assist in by removing unnecessary barriers.

For example, to encourage and accelerate the adoption of ADUs, many cities across the US and Canada have begun offering residents access to preapproved design plans for detached ADUs — complete technical schematics that have already been reviewed by building officials.

“The system can be a little bit stacked against the local homeowner who wants to be able to do this,” Harrell says. Between site reviews, utility plans, and architectural approvals, “there are so many things that you have to go through that you’re doing for the first time,” he adds. “Having these preapproved designs takes away one of those barriers. It says, ‘You don’t have to be a designer, or have enough money to hire one. Here are some designs that can work.’”

Preapproved ADU Plans in California

Los Angeles offers residents a growing catalog of preapproved ADU plans, including a standard one-bedroom architectural plan commissioned by the city, called the YOU-ADU (pictured), that any resident of Los Angeles can use for free.

Dozens of other plans are also preapproved, but require a modest licensing fee paid to their respective architects, most of whom can also be hired for site-specific consultations.

While a preapproved ADU plan already meets certain city codes (e.g. building, fire, and energy regulations), and thus can advance through the plan-check and permitting process more quickly than a custom design, it doesn’t mean a homeowner can just plop one in their backyard with no questions asked. There are still site-specific approvals required, such as land use or stormwater reviews.

But using a preapproved plan can shave weeks or even months off the process, and offers predictability for both homeowners and local officials. The efficiency of a standard design can also create cost savings.

“Custom plans not only take more time and money to design, they’re much more complex to deliver in the field,” says Whitney Hill, co-founder and chief executive of SnapADU in Southern California, whose standard design plans have been selected for preapproval in multiple cities around San Diego.

All of that drives up prices, she adds, noting that a fully custom ADU typically costs $30,000 to $50,000 more to build than a standard one of the same size and bed-and-bath count. “On the other hand, plans that we have built before have already been vetted for real-world constraints; we know we can build them efficiently.”

Hill says that faster permitting times on standard designs can also translate to lower costs. “Building an ADU in 12 months versus 18 months is far more economical from an overhead cost perspective for us,” she says. “We share that savings with the homeowner.”

Even when using a preapproved plan, homeowners should still be prepared for site-specific costs and work, she notes. “It’s critical to understand your site’s topography, existing utility locations, and existing utility loads,” she says. Some projects may require water service upgrades to accommodate an additional bathroom, for example, or an upgraded electrical panel—both of which can be costly.

But one of the biggest benefits to using a standard design, Hill says, is the predictability. “Build costs for an existing floor plan are available before you even kick off your own project,” Hill says, “[which] is great for homeowners who are trying to stick to a specific budget.”

Seattle’s ADUniverse

While Washington State recently passed legislation requiring cities to allow four homes on all residential lots (and six units near transit), Seattle began embracing ADUs over a decade ago, loosening some local restrictions that stood in the way of their adoption, such as minimum lot sizes. “That was an important first step, and a viable one, because land use regulations are what the city most directly controls,” says Nicolas Welch, senior planner in Seattle’s Office of Planning and Community Development.

Still, most homeowners have little if any experience with housing development, so the idea of hiring an architect and applying for permits to build a backyard cottage can feel overwhelming — even before the considerable cost involved. Seattle soon decided it should do more than simply improve its regulations, and developed a resource-rich website called ADUniverse.

“The site was meant to provide all the resources that a homeowner might need in one place with better, clearer information for folks who are basically trying to take on development for the first time without a background in it,” Welch says. “Offering some preapproved designs was one component of that, as well as letting them look up their property to see what’s actually feasible on their lot.”

The city invited architects to submit their ADU designs and then had a jury select 10 plans — out of about 150 submissions — to get preapproved by the building department. In the five years since, Welch says, “Some 350 permits have been issued for the preapproved designs,” or roughly 10 percent of all ADUs approved in that time; the city now permits an average of about 900 new ADUs per year.

“On the one hand, it’s a very small number in a city and county that has a shortage of hundreds of thousands of units, so I do think it’s important to right-size the expectations,” Welch says. “It’s very small and incremental. But it’s also hundreds of units that now exist, and that people are living in.”

Using a preapproved plan noticeably speeds up the early permitting process, Welch says: “If you don’t have something weird going on, like you’re on a very steep slope or you’re removing a gigantic tree or something, then you’ll get your permit in two to six weeks, rather than three or four months.”

While celebrating Washington’s statewide dissolution of single-family exclusive zoning, Seattle’s Director of Planning and Community Development, Rico Quirindongo, acknowledges that such a sea change in policy can also hasten gentrification pressure by opening up a new market.

“The challenge of gentrification in cities — and Seattle is no exception — is that an upzone happens, property values go up, property taxes go up, and then low- and middle-income families do not see the benefit of the upzone, they only feel the burden,” Quirindongo explains. “An opportunistic developer says, ‘I can buy you out for 10 percent over asking, and then you don’t have to worry about this anymore, you can go live somewhere else.’ That is how we have seen the Central District, a traditionally African American community here in Seattle, go from 75 percent Black families to 10 percent Black families over the last 20 to 25 years,” he says.

Easing the process, and cost, of building an ADU provides an “opportunity for homeowners to be a part of the development opportunity, where they’re building generational wealth,” he says. Whether a homeowner uses an ADU to generate long-term rental income or to house an aging relative or grown children, it can help them stay in their neighborhood and share in the benefits of local growth. “They are building a multi-generational campus that is their house and property. And you’re creating infill, missing middle housing, that is consistent with the context and feel of historic neighborhoods.”

Still, even if future rental income from an ADU might offset the cost of a construction or home equity loan, building one typically requires significant upfront investment. So Quirindongo helped devise a unique pilot program intended to open up the opportunity to more lower-income residents. Here’s how it works:

1. Selected homeowners (the pilot will begin with 10 parcels) will enter a partnership with the city and a developer, who will take out a 12-year ground lease on a portion of the homeowner’s lot.
2. In the first two years, the developer builds two detached ADUs in the homeowner’s backyard, at no cost to the homeowner.
3. The developer then rents out and manages both ADUs for 10 years. The developer keeps about half of the rental income, while the other half is split: a portion provides monthly revenue to the homeowner, while the rest is deposited in a set-aside account.
4. At the end of 10 years, the ground lease expires, and there’s enough money in that account to buy out the developer’s remaining interest and make them whole, so the homeowner ends up with two ADUs on their property, which they can continue to rent out or convey with the property should they sell their home. “Over that period of time, the homeowner builds up enough money in that account to buy out the partner, so they own those units outright after that 12-year period,” Quirindongo explains.

Preapproved ADU Plans in Oregon

Beyond creating unobtrusive infill housing, ADUs are, almost by definition, small — and thus inherently more affordable than most new single-family homes, which averaged 2,405 square feet in the third quarter of 2025.
In Oregon, Portland’s Residential Infill Project has yielded more than 1,400 new permits for ADUs and missing middle housing in single-family neighborhoods, comprising almost half of new development in the city from 2022-2024, even as other construction lagged. But as importantly, the project capped building sizes in an effort to encourage more small homes instead of fewer large houses — and that has demonstrably improved affordability. In 2023 and 2024, sale prices of new missing middle homes averaged $250,000 to $300,00 less than new single-family houses in the same Portland neighborhoods, largely due to their smaller sizes.

In a heartening example of municipal collaboration, Portland was able to borrow and tweak a preapproved plan from the city of Eugene, Oregon—the Joel, shown here—to offer its own residents a set of similar preapproved ADU plans.

Preapproved ADUs in Louisville, Kentucky

AARP published its first model ADU ordinance over two decades ago. Since then, the organization has helped a number of cities, including Louisville, Kentucky, to re-legalize ADUs by right locally, and helped communities hold contests to create free architectural plans for residents.

Louisville invited architects to submit their designs, and then purchased the rights to three preapproved ADU plans, which it offers for free to all residents.

Rodney Harrell, of AARP, says ADUs can enhance freedom for seniors by giving them more and better options in the places they already live. “What I love is that it’s a solution that gives more options to people who want to be in the communities that work best for them,” he says.

“I’ve talked to so many people who are stuck,” Harrell says. “They’ve got a house, and at some point it may have been their dream home, but now it’s become a nightmare. They’ve got too many stairs. Maybe it’s too big and their spouse passed, and they can’t afford it anymore.”

A senior who can no longer manage the stairs in their house can stay in the community they love by building a fully accessible, universally designed ADU in their backyard, he explains, and renting out the main house. “That gives you more freedom,” he says. “If you want to stay in your main house and have a caregiver stay in the ADU, that also gives you more freedom. Or maybe you just need a little bit of money to be able to afford to stay in your house, and maybe you’re able to rent out the ADU and stay in your main house.”

And Beyond

In Seattle, Welch says the city’s efforts to legalize ADUs in single-family neighborhoods helped pave the way for more middle housing (duplexes, triplexes, and fourplexes). “The sky didn’t fall, and so then state legislators felt more emboldened and empowered,” he says.

Many other cities and states across the US and Canada are now embracing ADUs as well, and providing design plans, guidance, and “ADU lookbooks” for residents interested in building one. Here’s a look at just a few preapproved designs offered in cities around North America.

There are dozens more examples across the country, and many cities continue to add new designs to their lists of approved plans. It’s merely one step in the right direction—but it’s a step nonetheless.

“People can be scared of things that are different,” AARP’s Harrell says. “But one thing that always gets me is that the ADU is really an old form of housing in a lot of the country. It’s just that we’re re-legalizing it. We’re making it able to be built again, and up to standards and codes of the modern day. So we shouldn’t put unnecessary barriers in place.”