Make room for mother-in-law: Boise’s free ADU plans take aim at housing crunch

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Boise this month made it easier — and cheaper — than ever to add an accessory-dwelling unit to homes in the city, releasing a slate of pre-approved plans aimed at spurring housing density in residential neighborhoods.

The catalogue of eight plans is part of Boise’s housing agenda, which already allows ADUs — smaller, secondary homes attached or adjacent to a primary residence that are often called in-law or mother-in-law units — in nearly all zoning districts. But those projects can be expensive to design and intimidating to apply for. That’s why the city released an eight-page pre-construction checklist along with the blueprints.

As of this week, the Boise spokeswoman Maria Ortega told the Idaho Statesman that she didn’t yet know how much the city-designed plans would cost to build.

“It depends on many variables, from size, location-based impact fees, and what the homeowner decides when it comes to contractors and fluctuating material costs,” she said.

Boise is working to issue “bid sets” — detailed construction documents laying out materials and amounts — to help homeowners and contractors estimate the all-in cost.

Using a pre-approved plan, though, cuts out one expensive step: “A custom design can cost upwards of $10,000,” Ortega said.

“What we can tell you is that by using the pre-approved plans, you will be saving time and money,” Ortega said, “The city also included in the designs cost-saving measures that lower construction and operating expenses.”

Is Idaho all in on ADUs?

Interest in ADUs is on the rise among Boise residents looking to leverage their properties, as well as lawmakers trying to ease Idaho’s housing crunch.

To landowners, they’re often pitched as options for multigenerational households, offering aging parents or adult children their own, self-contained space. They’re equally popular as rentals — short- or long-term — to defray the cost of homeownership.

For policy makers, ADUs are viewed as an unobtrusive way to get more people in residential neighborhoods without overhauling that neighborhood’s character. On a state level, ADUs are a small but important point of emphasis in the push for housing reform. A bill working through the Idaho Legislature this month hopes to allow the construction of ADUs by right statewide — something Boise’s modern zoning code already allows in most of the city.

Boise averaged 61 new ADUs per year between 2019 and 2023, Ortega said, but the number is on the rise. One-hundred thirty went up in 2024, and 82 were built or approved last year.

Typically, ADUs are required to be smaller than the main dwelling and can’t be sold separately.

“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,” said Maureen Brewer, Planning and Development Services director for the city. “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.”

Boise ADUs: What’s in the plans?

Boise selected architect Eamonn Parke and his team at Parke Architecture to design the city’s eight plans. The layouts range from 280 to 695 square feet and aim to fit a diversity of lot sizes and shapes, with both stand-alone and over-garage options. Studio, one-bed and two-bed layouts are included.

“These plans were made specifically for Boise, designed to fit the patterns of our existing neighborhoods while still offering room for customization, including options for Historic District compatibility, so the ADUs can adapt to the existing surrounding structures and the owners’ preferences,” Ortega said in a press release.

Each of the eight plans is named for a local bird featured in Boise Parks and Recreation’s “Field Guide to Boise Birds.”

“These plans are designed to balance efficiency, livability and neighborhood compatibility, giving homeowners a practical path to add housing while positively contributing to our community’s fabric,” said Parke said in a statement. “Accessory dwelling units are an important part of expanding housing choice in our neighborhoods while maintaining the character that makes Boise special.”