ADU Cost in Idaho in 2026

ADU costs in Idaho range from $65,000 for a basic garage conversion in Pocatello to $328,000+ for a high-end detached unit in Boise or Eagle. The most common Boise build — a 600 sq ft detached mid-range ADU — costs $150,000 to $220,000. Impact fees ($6,000–$10,000 in Boise and Meridian) are commonly overlooked costs. Nampa, Caldwell, Twin Falls, and eastern Idaho markets run 30–40% less than Boise.

Boise launched a pre-approved ADU plan library in 2026. Homeowners who select a pre-approved design skip the architectural plan review, compressing permit turnaround to 3–4 weeks instead of 8–12 weeks. Maximum ADU size under the program is 900 sq ft or 70% of the primary home's square footage, whichever is smaller. Properties in Flood Protection Overlay or Hillside Development Overlay zones are excluded from the program.

No — and this is the most important Idaho ADU caveat. Idaho HB 166 (2023) only prevents HOAs from creating new restrictions on internal ADUs after July 1, 2023. It does not protect detached or attached ADUs, and it does not override pre-existing CC&Rs of any type. SB 1354 (effective July 2026) expands protections to all ADU types — but neither law wipes out CC&Rs that existed before the laws were passed. Always review your HOA documents before designing.

Boise charges development impact fees on new ADU construction — typically $6,000 to $10,000 depending on ADU square footage. These fees fund parks, transportation, and city services and are paid at permit issuance. They are separate from and in addition to building permit fees ($3,000–$8,000). Always confirm current impact fee schedules with the City of Boise Planning and Development Services before finalizing your project budget.

A mid-range 600 sq ft ADU in Boise costs $160,000 to $220,000 all-in (including impact fees) and generates $1,200 to $1,800 per month in long-term rental income. Cash-on-cash ROI runs 9–11%, with a payback period of 11–14 years. Nampa and Caldwell offer better ROI due to lower build costs. Coeur d'Alene commands higher rents ($1,400–$1,800) with lower build costs than Boise, producing strong north Idaho ROI.

Idaho's Treasure Valley (Boise-Meridian-Nampa) has been among the fastest-growing metros in the US since 2020, driven by California and Pacific Northwest in-migration. This creates sustained rental demand and low vacancy rates. Boise's ADU pipeline grew significantly following the 2026 zoning reforms removing the one-ADU-per-lot restriction. The combination of growth, new laws, and pre-approved plans makes 2026 the strongest ADU environment Idaho has ever offered.