FAQ
ADU Questions From Treasure Valley Homeowners
Rules, cost, timeline and renting. Don't see yours? Ask us.
Rules
What's allowed
What counts as an ADU in Idaho?
Idaho Code § 67-6541 defines an ADU as a self-contained living unit with its own cooking, sleeping and sanitation facilities on the same lot as a single-family home. RVs and tiny homes on wheels don't count.
What does Idaho's 2026 ADU law require?
By February 1, 2027, cities over 10,000 people must allow one ADU per single-family lot by right, without owner-occupancy rules, most parking requirements, or size caps below 1,000 square feet or 75 percent of the main home. Historic districts are exempt.
How big can an ADU be in Boise today?
Boise's 2026 applicant packet lists 70 percent of the house's floor area or 900 square feet, whichever is smaller. The state law will raise the minimum cap in 2027.
Can my HOA prohibit an ADU?
Private covenants are separate from city zoning. Review your CC&Rs before design.
Cost & timeline
Budget and schedule
How much does an ADU cost?
Angi's 2026 data puts the national average around $180,000, with conversions at $60,000 to $150,000 and detached units at $110,000 to $285,000. See the cost guide.
How long does an ADU take?
Conversions often take a few months including permits; detached units take longer because of design, permitting, foundation and utilities.
Do impact fees apply to ADUs?
Yes, but under Idaho's 2026 law, cities over 10,000 people can't charge more for an ADU than for other single-family homes after February 1, 2027.
Living & renting
Using your ADU
Can I rent my ADU?
Many owners rent long term. Check city rules and any HOA covenants before planning short-term rentals.
Can an ADU have its own address and meter?
Often it can, depending on the city and utility. We sort out addressing and metering during permitting.
Is an ADU a good fit for aging parents?
Very often. A single-level design with a curbless shower, wide doors and a short, level path to the main house makes it work for years.