They’re called a number of different things, depending on who you ask Colloquially, you may know them as “granny units” or “mother-in-law suites.” But officially, a second, smaller dwelling on a property with a single-family home is known as an Accessory Dwelling Unit (ADU), and it’s becoming ever more popular in the Boise area.
What is an ADU?
Let’s start with the basics. An ADU is not the “tiny home” popularized by so-called hipsters and the socially conscious. It’s not a mobile home of any sort, but rather a second unit with specific requirements surrounding allowable space, ownership and living quarters. The City of Boise used to place more restrictions on ADUs, but the Boise City Council decided in 2019 to ease some of those restrictions after much debate and feedback from the community.
Amy Allgeyer, who has run her own Boise firm called Architect Inc. since 2005, designs many ADUs in the area, particularly in the North End. The ADUs she sees are typically separate from the main residence, although she’s done a few that are attached to the house or part of a basement conversion. She often sees ADU units converted from a garage space, especially in the North End.
What are people using them for?
Allgeyer said some people build an ADU as a long-term rental, some want an extra space for family that operates like a guest suite, and others want a short-term rental, such as an Airbnb or VRBO listing.
“I would say it’s an even mixture right now between those three uses,” she said.
Are they getting more popular?
Yes. Allgeyer said she has seen an increase in the past five years, and the City of Boise confirmed the number of permits for ADUs has slowly increased. Cody Riddle, the city of Boise’s deputy planning director, said 14 permits were issued for units in 2017, and in 2019 that number was up to 20. He said it’s too soon to know if any increase in 2019 could be attributed to the code amendment. Although the city elected a new mayor in December who took office in January, Riddle said he doesn’t expect the council to revisit the issue anytime soon.
What are the restrictions?
ADUs must be placed on a permanent foundation, and the materials and appearance of the unit must be “compatible” with other homes in the area. Riddle said the space of the unit is limited to 700 square feet, or 10 percent of the land parcel size, whichever is smaller. That’s up from 600 square feet prior to the code amendment. They can have up to two bedrooms, whereas it used to only be one bedroom, and if the ADU has two bedrooms, one dedicated parking space is required on the site or on an immediately adjacent street.
Allgeyer said many people considering ADUs don’t know the owner of the main home must retain that residence – they can’t build the ADU and then live elsewhere.
“I think that’s a caveat the city put in place to keep developers from buying up properties and turning them into rental plots,” she said.
Riddle said that’s correct – the limitations are in place because the city wants to maintain the “single-family character” of the neighborhoods where ADUs are built.
There are also specifications to the indoor features that qualify a space as an ADU, Allgeyer said. A shower or bathtub and a stove are the two distinguishing qualities that trigger the need for an ADU permit.
How much does it cost to build one?
Like anything else, it depends. Allgeyer said she hasn’t seen any ADUs in the area built for less than about $140,000, with the top range being about $200,000 for a unit with something like high-end finishes. There are also impact fees to use city power and plumbing, which usually range from $4,000 to $5,000.
“The biggest question people have when they go down this road is, ‘How much is it going to cost?’ And something they need to factor in is how much it will affect property taxes,” she said. “If you’re building something to make money, you have to consider that.”
Who builds them?
Allgeyer works on the design of ADUs, but she leaves it up to the client to choose a builder. Many small builders in the area take on the projects, but she said other residential contractors will often pick up the jobs because they are quick builds that can fill gaps in their schedules. But she sees more of an opening in the market for contractors who want to specialize in ADU builds.
“I get calls about ADUs all the time, so it’s definitely a topic people are interested in,” she said. “It’s not a bad way to go if you’re trying to make a spot for yourself.”
Kelcie Moseley-Morris is a former Idaho journalist whose work has been featured in the Idaho Press, Lewiston Tribune, Sun Valley Magazine and Electronics Point. She now works as a proposal writer for a national software company and continues to write freelance journalism pieces.