What Is an Accessory Dwelling Unit (ADU) and Is It Right for You?

An Accessory Dwelling Unit—commonly called an ADU—is a self-contained secondary living space located on the same lot as a primary single-family home. ADUs have their own entrance, kitchen, bathroom, and sleeping area, making them fully functional homes in a smaller footprint.

ADUs come in several forms: detached backyard cottages, garage conversions, basement suites, attached additions, and even prefabricated units installed on an existing foundation. Each type has different cost profiles, zoning considerations, and use cases depending on your property and goals.

Idaho's housing market has seen dramatic growth over the past decade, particularly in the Treasure Valley. As home prices have risen and rental demand has surged, ADUs have emerged as one of the smartest investments an Idaho property owner can make.

The math is simple: a well-built ADU in Boise can command $1,400–$2,500/month in long-term rent, or significantly more on short-term platforms. For investors, that rental income can fully offset a mortgage payment or return the construction cost within 6–8 years. For families, ADUs provide housing for aging parents, adult children, or guests without sacrificing privacy.

The good news: most cities across the Treasure Valley have updated their zoning codes to allow ADUs on single-family lots. Boise, Meridian, Eagle, Nampa, Star, and Caldwell all permit ADUs subject to specific setback, size, and utility requirements.

That said, rules vary significantly between cities and even between zones within the same city. Maximum square footage, required off-street parking, owner-occupancy requirements, and utility hookup policies all differ. Before investing in design or planning, TH Builds researches your specific parcel's zoning status—so you know exactly what you can build before spending a dollar.

ADU construction costs in Idaho generally fall into these ranges:

• Garage conversion: $80,000–$160,000
• Detached backyard cottage (under 600 sq ft): $180,000–$260,000
• Detached ADU (600–1,000 sq ft): $240,000–$380,000+
• Basement suite conversion: $60,000–$130,000

These ranges reflect full-service construction including design, permitting, all mechanical systems, and finishes. The wide range depends on size, site conditions, utility hookup complexity, and your finish selections. TH Builds provides detailed, itemized estimates before any commitment is made.

Not every contractor has experience navigating ADU permitting, which involves coordination across planning, building, and utility departments simultaneously. When evaluating builders, ask specifically about:

• How many ADUs they've permitted in your city
• Their familiarity with your city's ADU ordinance
• Whether they handle the full permit process or outsource it
• Their experience with utility service upgrades (often required for ADUs)

TH Builds has built ADUs across the Treasure Valley and handles every step in-house—from the first zoning check through the final certificate of occupancy.