Nyc Steps Up Adu Push With Pre-reviewed Plans And Funding

In his bid to improve New York City’s housing supply and affordability, Mayor Zohran Mamdani wants to speed construction of accessory dwelling units (ADUs) with pre-reviewed plans.

Launched last week, the city’s “ADU for You” package relies on a set of standard plans for small secondary homes. It also includes an online guidebook, feasibility tools and cost estimates to help owners build ADUs, which the city legalized in December 2024.

“If you own a home with a basement, attic, or backyard, chances are you’ve thought about using it to earn a little extra income or as space for a loved one,” Mamdani wrote in a social media post. “We want to make it as easy (and affordable) as possible for you to do that.”

The same basic approach to ADUs has taken root in several California cities, including Los Angeles. Boise, Idaho, officials launched preapproved ADU plans last week as well.

“ADUs support the city’s vision for thoughtful infill growth and for creating a variety of housing options for our residents, providing housing that is less water and energy consumptive, and enabling Boiseans to live where they work and play,” Maureen Brewer, the city’s Planning and Development Services director, said in a statement.

How NYC’s toolkit works

The ADU for You website walks one- and two-family homeowners through who qualifies, where units can go and which steps come first. A digital wizard checks basic zoning rules and helps owners decide whether a basement or attic conversion – or a freestanding backyard cottage – makes more sense for their lot.

City officials are working with private architects to assemble a growing library of standard plans that the New York City Department of Buildings has reviewed in advance for code compliance. The approach allows staff to focus on foundations, utilities and site conditions once an owner chooses a design.

In parallel, the Plus One ADU financing program can offer up to about $395,000 in low- or no-interest assistance for eligible homeowners who agree to keep the new units affordable. The aid is intended to bridge the gap between available financing and high construction costs.

California’s ADU head start

While New York City is just getting started, California has already shown what happens when a state clears the way for backyard units at scale. Beginning in 2016, lawmakers passed a series of bills that stripped away many local barriers, cut parking mandates and imposed 60-day permit deadlines for ADUs. The laws effectively legalized ADUs on most single-family lots by right.

Those changes turned what had been a niche product into a mass-market housing type in less than a decade. Permits for ADUs totaled about 1,400 in 2016. That number has climbed to more than 100,000.

Los Angeles built on the state laws with its ADU Standard Plan Program, a catalog of architect-designed models reviewed once for building, zoning and green codes. Homeowners who choose a standard plan can bypass much of the back-and-forth over code details, which city officials and designers say can cut weeks from the plan review process and reduce design fees.

Boise’s plan to address affordability

Idaho’s capital is experiencing a housing affordability crunch like many other cities. Population growth in Boise accelerated during the COVID-19 pandemic as residents moved from higher-cost states, including California, Washington and Oregon. Prices for rental and owner-occupied housing rose sharply.

Though the spike has cooled, Boise’s housing market remains under pressure. Median home values in the city are just under $490,000, well out of reach for many local workers. City analysis estimates Boise needs about 2,000 new or significantly renovated homes each year over the next decade to keep up with demand.

City leaders relaxed ADU rules in 2019 and then fully legalized them in 2023. Permitting increased soon after.

Like New York City, Boise officials want to make building ADUs easier and faster. The city’s planning department adopted a playbook with eight free, city-designed ADU plans sized for typical backyards.

Officials describe the catalog as a way to help more middle-income owners add density in established neighborhoods while maintaining design consistency.

Limits and criticisms

Even supporters caution that preapproved ADU plans are not a standalone solution to the housing shortage.

“Although not the entire solution, ADUs have the potential to meaningfully contribute to our city’s housing supply in such a way that is particularly mindful of the affordability challenges we are facing together,” Brewer said.

In California, ADUs have become one of the fastest-growing housing types. However, they still fall far short of closing the overall supply gap and tend to cluster in higher-income neighborhoods, where homeowners are more likely to tap savings or home equity.

Nationwide, the number of ADU permits has climbed as more cities and states allow them.​ And states such as Arkansas have turned to ADUs to boost housing supply and improve affordability.

“ADUs are not a silver bullet, but they are a scalable part of a balanced housing policy,” Morgan Friberg, vice president of market for permit data firm Shovels, wrote on the company’s blog. “The future of ADUs lies in integrating them into broader housing affordability strategies.”​

Critics argue that standard plans do little to address core obstacles such as high construction costs, aging infrastructure and lenders’ reluctance to finance small projects, especially for lower-income owners. Tenant advocates also argue that relying on backyard units can shift responsibility for production to individual homeowners rather than prioritizing large-scale rezoning or subsidized development.

New York’s rollout will test whether pairing legalization, pre-reviewed designs and public financing can overcome some of those limits in a dense, older city. For now, the move places Mamdani’s administration alongside Los Angeles and Boise, betting that a curated library of plans can add small units in underused space.