Boise ADU Rules

Boise's Table of Allowed Uses permits an ADU 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's not allowed in the higher-intensity mixed-use districts (MX-4, MX-5, MX-H) or 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. By February 1, 2027, it requires each city's own code to guarantee, at minimum:

• One ADU per lot — either an internal unit or a detached one, not necessarily both
• Administrative approval, not discretionary
• No ADU-specific off-street or guest parking requirement (a few statutory exceptions apply)
• A minimum allowed size of 1,000 sq ft or 75% of the primary structure's size

As of this writing, we haven't confirmed whether Boise's own code below has been updated to match yet. Where the two differ, the state standard is a floor the city can't regulate below.

Under Boise's own code, an ADU can't exceed 900 square feet or 70 percent of the primary structure's floor area, whichever is smaller — capped at two bedrooms and two stories, whether it's new construction, an addition, or a conversion.

State law (SB 1354) will eventually require a higher floor — 1,000 sq ft or 75% of the primary structure — once Boise's code is updated (required by February 1, 2027). The statute doesn't say how those two figures resolve against each other, so we don't guess either. Until that update is confirmed, the 900 sq ft / 70 percent figures above are what's published today.

Only one ADU is permitted per lot under Boise's own code today. Minimum lot area and density rules don't apply to ADUs specifically, but that doesn't open the door to a second one.

SB 1354 doesn't change that count — it guarantees at least one ADU per lot (internal or detached, not both), so it doesn't require Boise to allow two. Separately, Boise's code allows up to two Tiny Homes on Wheels (THOWs) per parcel (Zoning Code § 11-03-03.2P) — a different use category with its own size limit and standards, not a second ADU. A property could have one ADU and up to two THOWs; that's not the same thing as "two ADUs."

Every ADU has to meet the dimensional standards of its underlying zoning district — there's no separate, looser setback table just for ADUs. In R-1C, Boise's most common single-family district:

• Front: 15 ft
• Street-side: 15 ft
• Interior side: 5 ft
• Rear: 15 ft
• Max height: 40 ft
• Min lot: 3,500 sq ft

Other districts (R-1A, R-1B, R-2, R-3, and the mixed-use zones) have their own numbers. A rear setback reduction can also apply near a public alley, and small detached accessory structures can qualify for their own reduced setback — but the exact figure depends on the lot, so we don't treat it as confirmed until it's checked against the actual property.

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

Under Boise's own code, converting a garage or accessory structure into an ADU can't reduce the lot's required off-street parking. Separately, SB 1354 requires Boise, by February 1, 2027, to stop requiring parking specifically for ADUs — except where the primary home itself lacks off-street parking, the street isn't built for on-street parking, or the property is within a quarter mile of transit, employment, or commercial services. Until that update is confirmed, the garage-conversion rule above is what's published.

An ADU can come from an internal conversion (basement, attic, existing living area), an addition to the primary or an accessory structure, new detached construction, or converting an existing garage or accessory building. That last route is the one bound by the parking rule above.

Meeting the zoning standards above doesn't confirm your water, sewer, or electrical service can support a second unit. A septic system, an old sewer lateral, or a limited electrical panel can turn a zoning-compliant concept into one that needs real utility work first — one of the most common gaps between "zoning allows it" and "this lot is ready for it," and it has to be checked property by property.

A property inside a historic district, floodplain, the hillside/foothills overlay, the Boise Airport influence area, or the wildland-urban interface overlay faces extra review beyond base zoning — design review, a floodplain permit, grading standards, or wildfire mitigation, depending on the overlay. None of this shows up in the base zoning code; it has to be checked against the parcel's actual location. Historic-district properties are also excluded from SB 1354's statewide protections, so local review still governs there.

A legally nonconforming property — built under an older code, not fully compliant with the current one — isn't automatically unbuildable, but needs its own case-by-case review.

• 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