If you own a lot in Boise and want to add a home, the ADU vs duplex Boise question usually comes up first. Both add rental income. Both are allowed in almost every residential zone. But they are built under very different rules, and the right pick depends on what’s already on your lot and what you want at the end. If you’re new to ADUs, our Ultimate Boise ADU Guide covers the basics. This article puts the two side by side.
The Short Answer
Keep your house and want to add rent? An ADU is usually the faster, simpler path. Starting with an empty lot, or tearing down an old house? A duplex gives you two full-size homes with no size cap. And here’s the twist most people miss: on the same lot, the ADU path can actually allow more homes than a duplex.
Bottom line: An ADU adds a small home next to a single-family house. A duplex replaces the single-family house with two full-size homes. You can’t do both on one lot — Boise only allows ADUs alongside a single-family home.
Side-by-Side: ADU vs. Duplex Under Boise’s 2026 Code
| ADU | Duplex | |
|---|---|---|
| What it sits with | A single-family home (required) | Stands on its own |
| Homes on one lot | House + up to 2 ADUs = 3 homes | 2 homes |
| Size limit | 900 sq ft or 70% of the main home, whichever is smaller | No unit size cap — zone setbacks and height only |
| Bedrooms | 2 max | No limit |
| Height | 2 stories max | Zone limit (35 ft in R-1A/R-1B, up to 4 stories in R-2/R-3) |
| Density cap | Does not apply | Does not apply in residential zones |
| Parking | See note below | 1 space per unit (1–3 BR), 0.75 per bedroom (4+ BR) |
| Open space | No ADU-specific rule | 200 sq ft per unit |
| Street upgrades | Exempt on a conforming lot | Required (curb, gutter, detached sidewalk) |
| Utilities | Can share sewer and power with the main home | Separate sewer, power, water, and gas for each unit |
Parking note: Idaho Code §67-6541 says that by February 1, 2027, cities can’t require off-street parking for an ADU. There are exceptions — for example, when the main home has no off-street parking, or when it sits within a quarter mile of transit, jobs, or shops. Ask us to check your address.
Why an ADU Can Get You More Homes
Boise lets a lot hold up to two accessory units — any mix of ADUs or tiny homes on wheels — next to a single-family home. Neither one counts toward the density cap. That means a house plus two ADUs is three homes on one lot. A duplex is two homes, and you can’t add an ADU to it. The trade-off is size. Each ADU tops out at 900 square feet (or 70% of the main home, if that’s smaller) and two bedrooms. Duplex units have no size cap, so they can be full family homes with three or four bedrooms.
The Hidden Cost: Street Upgrades
This is the line item that surprises people. An ADU on a conforming lot is exempt from Boise’s streetscape standards. A duplex is not. Every new duplex has to provide a paved driveway apron, curb, gutter, and a detached sidewalk at least 5 feet wide. In zones like R-1B and R-1C, that sidewalk sits behind an 8- to 10-foot planting strip with street trees. The Planning Director can grant waivers in some cases, like documented drainage problems. But on a street with no sidewalk today, plan for this cost from day one. This cost often makes building a duplex financially infeasible.
Utilities: Shared or Separate?
Utilities are the other big cost gap. A duplex needs a full set of utilities for each unit. That means a separate sewer line, power meter, water meter, and gas service for each side. Nothing can be shared.
An ADU has more options:
- Sewer: It can tie into the main home’s existing sewer line, if the line has the capacity.
- Power: It can run off the main home’s existing electrical meter.
- Water: In areas served by Veolia, a new water meter is installed for the ADU.
- Gas: Intermountain Gas lets you tie into the existing service or request a new one.
Sharing services can save real money on an ADU. But it isn’t automatic. We confirm sewer capacity early, before the budget counts on it.
Design Rules for Each
An ADU has to match the main house in materials, colors, and style. Only one entrance can face the front of the house unless more already exist. You also need a 4-foot sidewalk from the ADU’s front door to the street.
A duplex has a longer list. In plain terms:
- Open space: 200 square feet per unit, with one area at least 8 by 8 feet. At least a quarter of it has to be planted ground.
- Windows and doors: At least 15% of the street-facing wall.
- Garages: No more than half of the front wall (60% if set back an extra 5 feet).
- Entrances: Each unit’s main entrance needs a covered porch of at least 25 square feet.
- Sidewalks and trees: A 5-foot sidewalk from each front door to the street, and one tree out front per unit.
- Driveways: A shared driveway can be as narrow as 9 feet.
What It Costs
For a new detached ADU, Mode Homes’ current range is $275–$300 per square foot. An ADU built above an existing garage runs about $150,000 — more than most people expect, because the structure needs reinforcing and new utility lines. Garage and basement conversions vary too much by the existing building to quote a fair range. Duplex pricing depends heavily on unit size, finishes, and the street upgrades above, so we price those per lot. For financing options, see our guide to financing an ADU in Boise.
What Neither Option Does (Yet)
Neither path lets you sell one unit on its own lot by default. An ADU stays on the same parcel as the main home. In 2026, Idaho passed Idaho Code §50-1335, which lets cities and counties create a simple process to split an ADU onto its own parcel. But cities have to choose to adopt it, and as of this writing, no Treasure Valley city has. We’ll update this article when that changes.
One more catch for newer zones: in R-3 and the mixed-use zones, Boise only allows new single-family homes through its housing incentives. If you’re building new from scratch in those zones, a duplex is usually the cleaner path, since an ADU has to sit with a single-family home.
Which One Fits Your Lot?
Here’s how we usually see it play out:
- You love your house and want rental income: Build an ADU. You keep your home and skip the street upgrades on a conforming lot.
- You want the most homes on one lot: A house plus two ADUs beats a duplex on unit count.
- You want bigger units for families or long-term tenants: A duplex. No size cap, no bedroom cap.
- Your lot is empty, or the house is at the end of its life: Price both. A new house with an ADU and a new duplex can land closer than you’d think.
Talk to an ADU and Duplex Builder in Boise
Mode Homes builds both ADUs and duplexes across Boise, so we don’t have a reason to push one over the other. We start with your lot: the zone, the setbacks, the street frontage, and what you want to own when it’s done. Then we show you what each path looks like on paper before you spend a dollar on design. If the ADU vs duplex Boise decision is what’s holding you up, that’s the conversation to have first. You can also look up your zoning through the City of Boise Planning Division or check street standards with the Ada County Highway District.
Not sure if your lot is better for an ADU or a duplex? Send us your address. We’ll walk through both options, no cost, no pressure.
Frequently Asked Questions
Can I build an ADU and a duplex on the same lot in Boise?
No. Boise only allows ADUs as an accessory to a single-family home. A duplex lot can’t add an ADU.
Which gets me more units, an ADU or a duplex?
The ADU path. A lot can hold a single-family home plus up to two ADUs, for three homes total. A duplex is two homes.
How big can an ADU be in Boise?
Up to 900 square feet or 70% of the main home’s floor area, whichever is smaller, with no more than two bedrooms and two stories.
Does a duplex count against Boise’s density limit?
No. In Boise’s residential zones, the maximum density limit doesn’t apply to duplexes or ADUs.
Why does a duplex cost more in site work than an ADU?
A new duplex must add curb, gutter, a detached sidewalk, and a paved driveway apron. An ADU on a conforming lot is exempt from those streetscape rules.
Does an ADU need its own utilities in Boise?
Not always. An ADU can often tie into the main home’s sewer line (if it has capacity) and electrical meter. In Veolia service areas, it gets its own water meter. A duplex needs separate sewer, power, water, and gas for each unit.
Can I build an ADU and a duplex on the same lot in Boise? No. Boise only allows ADUs as an accessory to a single-family home. A duplex lot can’t add an ADU.
Which gets me more units, an ADU or a duplex? The ADU path. A lot can hold a single-family home plus up to two ADUs, for three homes total. A duplex is two homes.
How big can an ADU be in Boise? Up to 900 square feet or 70% of the main home’s floor area, whichever is smaller, with no more than two bedrooms and two stories.
Does a duplex count against Boise’s density limit? No. In Boise’s residential zones, the maximum density limit doesn’t apply to duplexes or ADUs.
Why does a duplex cost more in site work than an ADU? A new duplex must add curb, gutter, a detached sidewalk, and a paved driveway apron. An ADU on a conforming lot is exempt from those streetscape rules.
Does an ADU need its own utilities in Boise? Not always. An ADU can often tie into the main home’s sewer line (if it has capacity) and electrical meter. In Veolia service areas, it gets its own water meter. A duplex needs separate sewer, power, water, and gas for each unit.
