ADU Construction in Boise, ID: Costs, Ideas & Options

ADUs in Boise — attached, detached, or converted — are limited to 900 square feet or 70 percent of the primary structure's total floor area, whichever is smaller, with no more than two bedrooms (Zoning Code Section 11-03-03.2B). Idaho's 2026 ADU law (Senate Bill 1354) raises the floor to 1,000 square feet or 75% of the primary home by February 1, 2027 — confirm the limit in effect with Planning & Development Services. The setback requirements of your specific zoning district (an ADU follows the underlying zone per Zoning Code 11-03-03.2.B) may further constrain the buildable ADU footprint. A preliminary zoning analysis — which Iron Crest provides at no cost as part of the ADU consultation — identifies the maximum buildable ADU footprint for your specific property before design investment is made.

No. Boise's Modern Zoning Code (effective December 2023) has no owner-occupancy requirement outside the Harris Ranch Specific Plan, and Idaho's 2026 ADU law (Senate Bill 1354) bars cities over 10,000 residents from requiring one (Idaho Code § 67-6541). Property owners who rent their primary dwelling can also build and rent an ADU on the same property. This significantly expanded the investment case for Boise ADU construction. Note that some ADU-adjacent programs — like certain housing finance programs that offer reduced-rate financing for ADU construction — may retain owner-occupancy requirements even where Boise zoning does not. Confirm the specific requirements for any financing program you are considering.

The total timeline from project initiation to certificate of occupancy typically runs 5 to 9 months for a new detached ADU in Boise — 6 to 10 weeks for design and permit preparation, 3 to 6 weeks for permit review, and 3 to 5 months for construction. Garage conversion ADUs are at the shorter end of the construction range (3 to 4 months) because the structure exists; new detached ADUs require longer construction timelines (4 to 5 months) for foundation, framing, and systems work. North End projects with Design Review add 4 to 6 weeks to the permit phase. Iron Crest provides a project-specific schedule at the proposal stage with milestone dates for permit submission, permit approval, construction start, and estimated completion.

For rental use in Boise, a new detached ADU or a garage conversion on the North End or Bench is usually the most workable approach. A detached unit with its own entrance, its own utilities, and no shared wall with the primary home is the closest thing to a standalone rental you can build on your own lot, and it draws from the widest tenant pool. The one-bedroom configuration at roughly 550 to 650 square feet is what we build most often for rental use — a studio costs less to build, a two-bedroom costs meaningfully more, and which one earns its cost back depends on rents we cannot responsibly predict for you. Before you settle on a bedroom count, get a written rent opinion from a property manager who works your neighborhood on all the configurations your lot will physically accommodate; that comparison, not a rule of thumb, is what should decide the scope.

ADU utility connections in Boise involve fees and construction costs for water service, sewer connection, and electrical service. Water service for most of Boise is supplied by Veolia Water Idaho and sewer by the City of Boise, and their charges depend on the ADU's size, distance from existing mains, and whether the connection is a separate meter or a shared connection through the primary dwelling's service. Electrical service for a rental ADU with an independent meter typically requires an Idaho Power service upgrade and meter installation. Total utility connection costs range from $5,000 to $18,000 depending on specific site conditions. Iron Crest obtains utility company cost estimates during the pre-construction planning phase and includes them in the project budget with appropriate contingency.

Yes. ADU projects require building permits, plan review, and multiple inspections. In most Boise-area jurisdictions, ADUs also require zoning compliance review to confirm lot size, setbacks, and parking requirements are met. We handle the entire permitting process.

Most Boise ADUs cost $95,000–$320,000, with a typical project near $165,000. A garage conversion typically runs $95,000–$145,000, and a new 600 sq ft one-bedroom detached ADU $145,000–$220,000; utility connection costs vary with distance to existing services and are one of the most variable line items. Our 600 sq ft detached North End ADU ran $120,000–$150,000.

From start of design to move-in, a typical ADU project takes 6 to 12 months. This includes design (4-8 weeks), permitting (4-8 weeks), and construction (3-5 months). Garage conversions are faster; detached new construction takes longer.

In most Boise-area jurisdictions, yes. ADUs can be rented as long-term rentals. Short-term rental rules (Airbnb, VRBO) vary by city and may have additional restrictions. Check local regulations before planning a short-term rental strategy.

It depends on location, size, finish level, and market conditions — check current listings for comparable units. Our 600 sq ft North End one-bedroom ADU has a recorded rental potential of $1,400–$1,600 per month.

Maximum ADU size varies by jurisdiction. In Boise, an ADU can be up to 900 square feet or 70% of the primary structure's floor area, whichever is smaller, with no more than two bedrooms. Idaho's 2026 ADU law (Senate Bill 1354) requires every city over 10,000 residents to allow at least 1,000 square feet or 75% of the primary home by February 1, 2027, so local limits are changing. We confirm the specific rules for your property during the feasibility phase.

No. Idaho's 2026 ADU law (Senate Bill 1354, Idaho Code § 67-6541, in force since July 1, 2026) prohibits every city over 10,000 residents from requiring owner-occupancy of either the primary home or the ADU, so you can live in one unit and rent the other, or rent both. Boise had already dropped its owner-occupancy rule in the 2023 Modern Zoning Code. Cities of 10,000 or fewer residents (such as Emmett, Fruitland, Payette, Homedale, Parma, or New Plymouth) still set their own rules, and lenders or HOA covenants that existed when you bought may have their own conditions — we confirm all of it during the feasibility review.