Boise ADU Rules

Boise's Table of Allowed Uses permits an ADU as an allowed use in the R-1A, R-1B, R-1C, R-2, and R-3 residential districts, the MX-1, MX-2, MX-3, and MX-U mixed-use districts, and the A-1 and A-2 districts. It is not an allowed use in the higher-intensity mixed-use districts (MX-4, MX-5, MX-H) or the industrial districts.

A separate state law, Idaho Senate Bill 1354 (Idaho Code § 67-6541), took effect July 1, 2026 and applies to Idaho cities above 10,000 residents — Boise included. It requires each city to update its own comprehensive plan and zoning code by February 1, 2027 to guarantee, at minimum, one ADU per lot — either an internal ADU within the primary home, or a detached ADU sited in the rear yard or within the primary dwelling's own setbacks — approved administratively rather than discretionarily, with no ADU-specific off-street or guest parking requirement (subject to a few statutory exceptions), and a minimum allowed size of 1,000 square feet or 75% of the primary structure's size. That deadline is still several months out, and as of this writing we haven't confirmed whether Boise's own published code below has been updated to match it. Where the two differ, the state standard is a floor the city cannot regulate below.

Under Boise's own published code, an ADU may not exceed 900 square feet or 70 percent of the primary structure's total floor area, whichever is smaller. It may not have more than two bedrooms, and it may not exceed two stories. These three limits apply regardless of whether the ADU is a new detached structure, an addition, or a conversion of existing space.

State law (SB 1354) separately prohibits Boise from limiting an ADU's size to less than 1,000 square feet or less than 75 percent of the primary structure's size once the city's code is updated — the statute doesn't spell out how those two figures resolve against each other, so we don't either. Until that update is confirmed (required by February 1, 2027), the 900 sq ft / 70 percent figures above are what's currently published — check current status for an active project.

Only one ADU is permitted on each lot or parcel where an ADU is allowed, under Boise's own published code today. Minimum lot area and maximum density standards — the rules that would otherwise cap how many units a lot can hold — do not apply to ADUs specifically, but they still don't open the door to a second one.

SB 1354 doesn't increase that count at the state level — it guarantees at least one ADU per lot, delivered as either an internal unit within the primary home or a detached unit sited in the rear yard or within the primary dwelling's own setbacks, not both. It doesn't require a city to allow two ADUs on the same lot.

Every ADU has to meet the dimensional standards of its underlying zoning district — there is no separate, looser setback table just for ADUs. In R-1C, Boise's most common single-family district, the base standards are a 15 ft front setback, 15 ft street-side setback, 5 ft interior side setback, and 15 ft rear setback, with a 40 ft maximum height and a 3,500 sq ft minimum lot area. Other districts (R-1A, R-1B, R-2, R-3, and the mixed-use zones) each have their own numbers.

A rear setback reduction can apply where the rear lot line abuts a public alley, and a smaller detached accessory structure can qualify for its own reduced rear setback under a separate size threshold. Both of these are real exceptions, but the exact figure and whether it applies at all depends on the specific lot and structure — we don't treat either as confirmed until it's checked against the actual property.

State law doesn't set its own numeric setback distance for ADUs. Instead, once Boise's code is updated, SB 1354 requires that ADU setback, lot-size, coverage, and building-frontage rules be no stricter than what already applies to single-family homes in the same zoning district, and that a detached ADU be sited either in the rear yard or within the primary dwelling's own setbacks. The R-1C figures above are still what governs a specific Boise project today.

Under Boise's own published code, an ADU created by converting an existing attached garage or detached accessory structure can't drop the lot's off-street parking below whatever minimum already applies to it. SB 1354 separately requires Boise, by February 1, 2027, to stop requiring off-street or guest parking specifically for an ADU — except where the primary dwelling itself lacks off-street parking, the street isn't built for on-street parking, or the property sits within a quarter mile of transit, an employment area, or commercial services. Until that update is confirmed, the garage-conversion rule above is the published standard — worth confirming directly for a specific project.

An ADU can be created through internal conversion of existing living area, a basement, or an attic; an addition to the primary structure or a detached accessory structure; new construction of a detached ADU (including alongside a new primary home); or conversion of an existing detached accessory structure or attached garage. The garage/accessory-structure route is the one bound by the parking-preservation rule above.

Meeting the zoning standards above doesn't by itself confirm that a property's water, sewer, or electrical service can actually support a second dwelling unit. A property on a septic system, an older or undersized sewer lateral, or a limited electrical panel can all turn a zoning-compliant concept into one that needs real utility work before it's buildable. This is one of the most common gaps between "zoning allows it" and "this specific lot is ready for it," and it has to be checked property by property.

A property inside a historic district, a mapped floodplain, the hillside/foothills overlay, the Boise Airport influence area, or the wildland-urban interface overlay faces additional review on top of the base zoning rules above — extra design review, a floodplain development permit, grading/drainage standards, or wildfire-mitigation requirements, depending on which overlay applies. None of these overlays are visible from the base zoning code alone; they have to be checked against the parcel's actual location. Historic-district properties are also carved out of SB 1354's statewide protections, so local review still governs there.

A property that's legally nonconforming — legally built under an older code but not fully compliant with the current one — isn't automatically unbuildable, but the nature of the nonconformity needs its own case-by-case review before an ADU concept on it can be confirmed.

• Which zoning district actually applies, and its exact dimensional standards
• Whether the property sits in a historic district or any of the overlays above
• Real setbacks and buildable area given the lot's specific shape and existing structures
• Sewer, water, and electrical capacity for a second unit
• Any HOA covenant — SB 1354 limits an HOA's ability to ban ADUs outright, but reasonable design and size rules can still apply
• Whether Boise's own code has been updated yet to match SB 1354's February 1, 2027 compliance deadline — minimum ADU size and parking requirements are the two most likely to change once it has