What are the ADU setback requirements in Boise?
In most Boise residential zones, accessory structures need 5 feet from side property lines and 10 feet from the rear property line. Front-yard setbacks generally match the principal-structure setback for the zone. Corner lots have additional rules on the street-side yard. Lot-specific overlays (hillside, historic) can change these — we verify your zone before any drawings start.
Side and rear setbacks explained
| Setback | Typical distance | What changes it |
|---|---|---|
| Side-yard (interior) | 5 feet | Narrow infill overlays, fire-rated wall construction |
| Rear-yard | 10 feet | Single-story reductions in some districts, alley access |
| Street-facing setback (corner lot) | 10-20 feet | Zone-specific principal-structure rules |
| Distance to main house | 6-10 feet | Fire separation and egress requirements |
These are the standard rules for most R-1A, R-1B, R-1C, R-2, and R-3 residential zones. Your specific parcel may have an overlay that modifies them. The Boise zoning code's accessory-structure section is the authoritative source — the lot check pulls the rules that apply to your address.
Maximum height for a Boise ADU
ADU height is capped at the principal-structure height limit for your zone, typically 25-30 feet. In practice, most ADUs are single-story and well under the cap. Two-story accessory dwellings exist but require additional review, and at the 900 sq ft cap on living area they rarely make sense — the second-floor stair eats too much usable space.
Easements eat the buildable envelope
Utility, drainage, and access easements are subtracted from the buildable area in addition to setbacks. A 10-foot utility easement along the rear of your lot can eliminate 100+ square feet of otherwise-buildable area. The county assessor's plat shows recorded easements — pull yours before assuming the math works.
How setbacks shape your buildable envelope
Take your lot dimensions. Subtract front setback (typically 20-25 feet), side setbacks (5 feet each), and rear setback (10 feet). Subtract any recorded easements. Subtract the existing house footprint and the required distance between structures (6-10 feet). What's left is the area an ADU can occupy. For a typical 50' × 120' Boise lot with a 1,200 sq ft house at the front, the rear envelope is usually 35' × 50' minus easements — easily large enough for any pre-approved plan up to the 695 sq ft Kestrel.
How surrounding Treasure Valley cities differ
Each Treasure Valley city sets its own setbacks. Meridian under its Unified Development Code generally matches Boise (5 ft side, 10 ft rear) with some lot-size-specific variations. Eagle often has larger setbacks reflecting larger typical lot sizes. Garden City, Kuna, Star, Caldwell, and Nampa each have their own development codes. The lot check identifies which apply to your address.
Where to next
Setbacks define your buildable envelope; the next questions are which plan fits it and how the build actually runs. See Boise's six pre-approved plans for footprints that fit standard envelopes, the ADU feasibility framework for the full check, or backyard cottage options if you're orienting toward a detached unit. The free lot check runs the setback math against your specific parcel.
Sources & References
- City of Boise — Zoning code (Title 11)
- BoiseDev — Boise zoning rewrite passes (June 2023) — Modern Zoning Code raised the ADU size cap from 700 to 900 sq ft effective December 1, 2023
- Ada County Assessor — Parcel and plat lookup
- City of Meridian — Unified Development Code