Real ADU pricing across Boise, the Treasure Valley, and Idaho's other major markets, shaped by rapid Treasure Valley growth pressure, the state's one-ADU 'pick a lane' rule, and parking exceptions near transit.
Governor Little signed SB 1354 on March 31, 2026, after the Senate raised its population threshold from an original 5,000 residents up to 10,000 before passage. It took effect statewide on July 1, 2026, and covers incorporated cities over 10,000 residents — including Boise, Meridian, Nampa, Coeur d'Alene, and Post Falls — while smaller cities and unincorporated counties remain free to set their own rules. Covered cities have until February 1, 2027 to update their local codes to comply. Properties inside designated historic districts are exempt from SB 1354's protections. The detailed figures below are representative planning ranges, not a statewide fee schedule or review-time guarantee.
Signed March 31, 2026; effective statewide July 1, 2026. Cities over 10,000 residents must update their local codes to comply by February 1, 2027 — that window is still open as of this review.
These are planning benchmarks, not a permit quote, eligibility ruling, or legal opinion. Confirm zoning, building, utility, site, private-covenant, fee, and timing requirements with the current local authorities. See the 51-jurisdiction law directory and national permit guide.
A standalone structure in the rear yard. SB 1354 protects this as one of two mutually exclusive options — choosing detached means you're not separately protected for an internal unit as well.
Detached ADU Guide →An addition sharing a wall with the main house. Falls under Idaho's "internal ADU" protection alongside basement and garage conversions — the other of the two mutually exclusive protected options.
Attached ADU Guide →A conversion of an attached or detached garage. Idaho Code defines internal ADUs to include units within the primary home or its attached or detached garage — check with your city on which category applies.
Garage Conversion Guide →A converted lower level with an independent entrance, within the existing home's footprint. The classic "internal ADU" — protected statewide, no owner-occupancy requirement as of July 1, 2026.
Basement ADU Guide →A factory-built unit installed on a permanent foundation in the rear yard — treated as the "detached" option under SB 1354, not the internal option.
Prefab ADU Overview →A small private suite within the existing home footprint — Idaho's cheapest internal ADU option, carrying the same statewide protection as a basement conversion.
Junior ADU Overview →
*Total-cost ranges outside Boise are derived from regional construction-cost data at a representative ~600 sq ft build; verify against local bids before publishing final figures. Idaho Falls and Twin Falls are notable additional markets not pictured here — each qualifies under SB 1354's 10,000+ population threshold and is worth checking with local contractors directly.
Implementation transition: SB 1354 (2026). Effective July 1, 2026 for covered cities; cities above 10,000 residents have an implementation deadline of February 1, 2027. Permit fees and review timing remain planning estimates until confirmed for the specific property.
Calculate My ROI →SB 1354 is in implementation transition for covered cities. These steps help homeowners select an appropriate path while confirming the current local compliance status.
Calculate My ADU ROI →
Illustrative 1-bedroom layout. Actual floor plans vary by ADU type, size, local code, and homeowner needs.
Start with the ROI calculator. Free, no signup, no contractor pitch. Just the data.