One-Story vs. Two-Story ADU: Which Fits Your Boise Lot?

Every ADU project starts with the same question: build out, or build up? The right answer depends on your lot, your budget, and what you want the space to do. If you haven’t read it yet, our Ultimate Boise ADU Guide covers the basics of ADUs in Boise — this article goes deeper on one specific decision: one story or two.

What Boise Code Allows

Boise’s zoning code sets the outer limits either way. An ADU can’t be bigger than 900 square feet, or 70 percent of your main home’s floor area — whichever number is smaller. It can have up to two bedrooms. And no matter how big your lot is, an ADU can’t exceed two stories in height. That’s the ceiling — literally. There’s no version of a three-story ADU in Boise. Once you know that, the real question is which of those two options — one story or two — actually fits your property.

Why Choose a One-Story ADU

A one-story ADU is the simpler build. Everything lives on one level — no stairs, no second-floor framing, no structural work to carry weight overhead. That makes it a strong choice if the ADU is for aging parents, a family member with mobility needs, or anyone who wants single-level living long-term. It’s also usually the faster build, since the crew isn’t framing, wiring, and plumbing a second floor on top of the first.

The trade-off is footprint. A one-story ADU uses more of your buildable lot to hit the same square footage, since everything is spread across one level. If you’ve got a wide, deep backyard, that’s not a problem. If your lot is tight, it can eat into the yard space you were hoping to keep.

Why Choose a Two-Story ADU

A two-story ADU trades construction simplicity for lot efficiency. By stacking square footage instead of spreading it out, you can hit the same 900-square-foot cap on a much smaller footprint — which matters a lot on narrow infill lots or properties where the primary home already takes up most of the buildable area. It also tends to create more separation and privacy between the ADU and the main house, since the two structures aren’t sitting shoulder to shoulder.

Bottom line: One story is simpler and better for accessibility. Two story fits more square footage onto a smaller lot but costs more to build. Most Boise homeowners land on one or the other based on lot size first, budget second.

Cost: What Actually Changes Between the Two

For a detached ADU, Mode Homes builds in the $275–$300 per square foot range, and that range holds whether the unit is one story or two — it’s not that a two-story ADU costs more per square foot of living space. What changes is everything that comes with adding a second level: stairs, a second-floor structural system, additional framing labor, and more complex plumbing and electrical runs between floors. A one-story ADU with a larger footprint puts more of its budget into site work and foundation. A two-story ADU with a smaller footprint puts more of its budget into the building envelope and structure itself.

What This Doesn’t Cover

This article is about story count, not ADU type. Whether you’re doing an internal conversion, an addition to your existing home, or a fully detached new build, the one-story-vs-two-story decision still applies — but the cost and process details differ by build type. For a full cost breakdown by ADU type, see our ADU cost guide. For the underlying zoning rules, see our Boise ADU zoning guide.

What This Means for Boise Homeowners

There’s no universally “right” choice — it’s a lot-and-goals question. On a wide lot with room to spare, one story often makes more sense: simpler build, easier long-term living, especially for family members who don’t want stairs. On a narrow or tight lot, or when you’re trying to preserve yard space, two story usually wins. And if you’re eyeing the space above your garage, plan for the real cost of a second story from the start, not after the framing bid comes in.

We walk every Boise homeowner through this trade-off using their actual lot — setbacks, existing structures, and budget — before recommending one story or two.

Not sure which layout fits your property? Send us your address and we’ll walk your lot with you — no pressure, just a straight answer on what one story or two would actually look like on your land.

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