What is an ADU?

ADU stands for Accessory Dwelling Unit. In normal language, it’s a smaller, second home located on the same lot as your primary home.

A plain-English guide to accessory dwelling units (and “secondary residences” in Ada and Canyon County)

Designed Structures & Construction (DSC) | February 14, 2026

If you’ve ever looked at a side yard, a garage, or that unused corner of the lot and thought, “We could build something there,” you’re in good company. Across the Treasure Valley, homeowners are using ADUs to create space for family, add flexibility for the future, and (sometimes) offset a mortgage with short-term or long-term rental income.

But one quick warning before we go too far: the same idea can be called different things depending on where you live. In Boise, you’ll hear “ADU.” In unincorporated Ada County, the code often uses “secondary dwelling.” In Canyon County, you’ll see “secondary residence.” The concept is similar, but the rules are not identical.

First, what does ADU actually mean?

ADU stands for Accessory Dwelling Unit. In normal language, it’s a smaller, second home located on the same lot as your primary home. A true ADU functions like a real residence: it has its own place to sleep, its own bathroom, and typically its own kitchen. It’s “accessory” because it is meant to be secondary to the primary home on the property (not a separate standalone lot).

People also call ADUs by other names: backyard cottages, carriage houses, in-law suites, garage apartments, or basement apartments. The label changes, but the key idea stays the same: it’s a second living space that can operate independently.

Common ADU layouts we see in real projects

Detached ADU: a separate small structure in the backyard (often the best for privacy).
Attached ADU: an addition connected to the main home, with a separate entrance.
Internal ADU: a basement or interior conversion that becomes its own unit (great when the existing layout works).
Accessory structure conversion: converting a shop or garage into a dwelling unit (sometimes simple, sometimes a parking and code puzzle).

Why homeowners build ADUs

In our conversations with clients, ADUs usually start with one of these goals:

Family proximity: a place for parents to age in place, or a safe landing spot for an adult child.
Long-term flexibility: guest space now that can become a caregiver suite later.
Income: a long-term rental that helps the property carry itself over time.
Work-from-home space: an office or studio that can convert to a unit in the future.

ADU vs. “secondary residence”: local language matters

Here’s the practical takeaway: when you start researching rules, don’t get hung up on the acronym. Look for the term your jurisdiction actually uses. In Ada County’s zoning code, the ADU concept is typically regulated as a “secondary attached or detached dwelling.” In Canyon County’s zoning code, you’ll see “secondary residence.”

Unincorporated Ada County: “secondary dwelling” (attached or detached)

Ada County’s definitions describe a secondary dwelling as a subordinate dwelling unit on the same property as the principal home. It can be attached (sharing a wall with the main home but with a separate outside entrance) or detached (in a separate structure).

Ada County also spells out a detailed standards list. A few highlights homeowners should know early:

Owner-occupancy is required: either the main home or the secondary dwelling must be continuously occupied by someone with an ownership interest in the property.
Lot size matters: the property must be at least 6,000 square feet.
Size is capped: the secondary dwelling can be 350 to 1,200 square feet, or 60% of the above-ground living area of the primary home (whichever is less).
Bedrooms are limited (up to three), and the unit must have its own kitchen and bathroom facilities.
A recorded deed restriction is required. Among other things, it prevents the secondary dwelling from being sold separately, and it states that if no owner lives on the property, the secondary dwelling becomes non-habitable and may not be rented or leased.

Unincorporated Canyon County: “secondary residence”

Canyon County’s zoning definitions describe a secondary residence as a dwelling (other than a temporary residence) that is detached from the primary residence. Canyon County’s standards section for secondary residences is short but important: the owner of the lot must live on-site, and the secondary residence must have its own address for emergency purposes.

Canyon County also lists where a secondary residence is allowed by zoning. In the County’s land use matrix, secondary residences are allowed in some residential and rural/ag zones and require a conditional use permit in others. That’s a reminder that “it’s called a secondary residence” doesn’t automatically mean “it’s allowed on my specific lot.”

So what’s different between Ada and Canyon County?

Both counties treat these units as “secondary” to a primary home and both require owner-occupancy. The difference is how much is spelled out in the code and how the project is constrained:

Ada County is very prescriptive. It sets specific minimum lot size, maximum unit size, bedroom limits, parking requirements, and a required deed restriction tied to owner occupancy.
Canyon County’s “secondary residence” language is simpler in the standards section, but the zoning layer is a bigger gatekeeper: you need to confirm the zoning district and whether the use is allowed outright or requires a conditional use permit.
If your property is in an Area of City Impact, city rules may apply instead of (or in addition to) county rules. This is one of the most common reasons two nearby lots can have very different outcomes.

The three questions that decide whether an ADU is feasible

No matter what your jurisdiction calls it, most projects come down to three feasibility questions:

1) Is a secondary unit allowed in your zoning district (and is it by-right or discretionary)?

2) Can you physically fit it on the lot while meeting setbacks, height limits, parking, access, and utility requirements?

3) Can you build it in a way that keeps the project aligned with your budget and the neighborhood context?

That’s why we recommend starting with a quick feasibility review before you pay for full design drawings. A short up-front review can surface deal-breakers early (or show you the smartest path forward) and save weeks of redesign later.

If you’re considering an ADU or secondary residence in the Treasure Valley, DSC can help you translate the rulebook to your specific property and build a plan that’s realistic, compliant, and cost-aware from day one. Reach out and ask about our FREE ADU feasibility review.

References

Fannie Mae — Accessory Dwelling Units (ADUs). https://singlefamily.fanniemae.com/originating-underwriting/mortgage-products/accessory-dwelling-units
Ada County Code of Ordinances — 8-1A-1 Definitions (Dwelling, Secondary Attached / Detached). https://codelibrary.amlegal.com/codes/adacountyid/latest/adacounty_id/0-0-0-3396
Ada County Code of Ordinances — 8-5-3-40 Dwelling, Secondary Attached or Detached (standards, owner-occupancy, size caps, deed restriction). https://codelibrary.amlegal.com/codes/adacountyid/latest/adacounty_id/0-0-0-6980
Canyon County Code of Ordinances — 07-02-03 Definitions Enumerated (Secondary Residence definition). https://codelibrary.amlegal.com/codes/canyoncountyid/latest/canyoncounty_id/0-0-0-2371
Canyon County Code of Ordinances — 07-14-25 Secondary Residence (owner-occupancy, addressing). https://codelibrary.amlegal.com/codes/canyoncountyid/latest/canyoncounty_id/0-0-0-3645
Canyon County Code of Ordinances — 07-10-27 Land Use Regulations (Matrix) (zoning districts where secondary residence is allowed/conditional). https://codelibrary.amlegal.com/codes/canyoncountyid/latest/canyoncounty_id/0-0-0-3051