The ADU cost in Idaho in 2026 ranges from $125,000 to $425,000. A detached ADU cost in Idaho is $250,000-$425,000. An attached ADU cost in Idaho is $175,000-$285,000. A garage conversion ADU cost in Idaho is $125,000-$215,000. This guide breaks down the ADU cost in Idaho by type, size, and location, plus the permitting rules you need to know.
What is an ADU and what is the ADU cost in Idaho?
An ADU (Accessory Dwelling Unit) is a secondary residence on the same lot as a primary home — sometimes called a granny flat, casita, or mother-in-law suite. The ADU cost in Idaho depends on size, type, and whether utilities and plumbing already exist nearby. The average ADU cost in Idaho in 2026 is around $235,000 for a 600 sq ft detached unit.
What is the ADU cost in Idaho per square foot?
The ADU cost in Idaho per square foot in 2026 is $275-$525. The ADU cost in Idaho per square foot is higher than a comparable new home because ADUs have a high ratio of kitchens, bathrooms, and mechanicals to total square footage. A 500 sq ft ADU in Idaho still needs a full kitchen and bath.
What is the detached ADU cost in Idaho?
The detached ADU cost in Idaho is $250,000-$425,000. A detached ADU in Idaho is a standalone structure with its own foundation, roof, mechanicals, and entrance. The detached ADU cost in Idaho is highest because everything from foundation to roof is new construction.
What is the attached ADU cost in Idaho?
The attached ADU cost in Idaho is $175,000-$285,000. An attached ADU in Idaho shares at least one wall with the primary home and often shares a foundation. The attached ADU cost in Idaho is lower because it shares structural elements and utility connections.
What is the garage conversion ADU cost in Idaho?
The garage conversion ADU cost in Idaho is $125,000-$215,000. A garage conversion ADU in Idaho turns an existing detached or attached garage into a livable unit. The garage conversion ADU cost in Idaho is lowest because the shell already exists — you’re insulating, finishing, and adding plumbing.
What’s included in the ADU cost in Idaho?
- Foundation (detached): $18K-$32K
- Framing and structural: $22K-$45K
- Mechanicals (HVAC, plumbing, electrical): $38K-$68K
- Kitchen (cabinets, counters, appliances): $18K-$48K
- Bathroom: $14K-$28K
- Insulation, drywall, paint: $12K-$22K
- Flooring and trim: $8K-$22K
- Sewer/septic connection: $4K-$28K
- Permits and design: $4K-$12K
Are ADUs legal in Idaho?
ADUs are legal in most of Idaho with local zoning approval. Idaho Falls, Rigby, and Rexburg allow ADUs with conditional use permits in most residential zones. Teton County Idaho (Driggs, Victor) has specific ADU rules. Always confirm ADU rules with your county planning department before budgeting the ADU cost in Idaho for your lot.
How long does it take to build an ADU in Idaho?
Building an ADU in Idaho takes 5-9 months for a detached ADU, 4-7 months for an attached ADU, and 3-5 months for a garage conversion ADU. The ADU cost in Idaho can rise 8-15% if construction extends across winter months.
Get a real ADU cost estimate for your Idaho lot
SwagerBuilds builds ADUs across Eastern Idaho and Teton Valley. Send us your lot info for a real ADU cost in Idaho quote →
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