How Much Does an ADU Cost in Idaho? 2026 Price Breakdown

TL;DR: A detached ADU in Twin Falls costs $100,000 to $200,000 to build. A garage conversion runs $50,000 to $120,000. Statewide, detached ADUs range from $140,000 to $260,000, with Boise and the Treasure Valley running 10 to 20% above the state average. The per-square-foot cost in Twin Falls falls between $150 and $250, depending on size, finishes, and site conditions. But the build cost alone doesn't tell the full story. Impact fees, utility connections, permits, design, and site prep can add $15,000 to $40,000 on top of your construction estimate. This guide breaks down every cost category, shows what drives the number up or down, and gives you a realistic total project budget for the Magic Valley market.

"How much does an ADU cost?" is the first question every homeowner asks and the hardest one to answer honestly. The real answer is always "it depends," but that's not useful to someone trying to figure out whether they can afford to move forward.

So let's make it useful. This guide covers every cost category in an Idaho ADU project, with specific ranges for Twin Falls and the Magic Valley. The numbers come from industry data, Idaho-specific construction reporting, and what we see on the ground in this market. They're planning estimates, not quotes. Your actual number depends on your lot, your design, and what the feasibility check reveals about your property.

Total Project Cost by ADU Type

The type of ADU you build is the single biggest factor in your total cost. Here's how the three main types compare in the Twin Falls market.

Why the ranges are so wide: Two homeowners building the same 600 sq ft detached ADU on different lots can easily see a $40,000 to $60,000 difference in total cost based on utility distance, soil conditions, site access, and finish level. The low end assumes a flat lot with short utility runs and mid-range finishes. The high end reflects slope, longer utility trenching, complex site prep, or higher-end materials.

How Twin Falls compares to the rest of Idaho: Boise and the Treasure Valley run 10 to 20% above the state average due to higher labor demand. Twin Falls benefits from lower labor costs and generally simpler permitting. A detached ADU that costs $200,000 in Meridian might cost $150,000 to $170,000 in Twin Falls for comparable size and finishes.

Where the Money Goes: Cost Breakdown by Category

Understanding how costs distribute across a project helps you see where you have control and where you don't.

Hard costs (70 to 80% of total)

These are the physical construction expenses: everything the builder touches.

A note on finishes: Mid-range finishes (LVP flooring, laminate countertops, standard cabinets, builder-grade fixtures) are the sweet spot for rental ADUs. Upgrading to quartz countertops and custom tile adds $3,000 to $6,000 without meaningfully increasing rent. Our guide to what tenants actually want covers which upgrades pay for themselves and which don't.

Soft costs (10 to 15% of total)

These are the design, engineering, and professional fees that happen before and during construction.

Regulatory costs (5 to 10% of total)

These are the fees charged by the city, county, and utility providers.

Site costs (highly variable)

This is the category that produces the most budget surprises, because it's entirely dependent on your specific lot.

For a detailed breakdown of the costs that catch homeowners off guard, our hidden costs guide covers the six most common budget surprises.

Total Project Cost: Putting It All Together

Here's what a complete ADU project looks like when you add every cost category together, not just the construction line.

Example 1: 500 sq ft garage conversion (mid-range)

Example 2: 600 sq ft detached ADU (mid-range)

Example 3: 800 sq ft detached ADU (higher finishes)

These examples are planning estimates for the Twin Falls market in 2026. Your actual costs will vary based on site conditions, design decisions, and material selections.

Cost Per Square Foot: How to Use It (and How Not To)

Cost per square foot is a useful rough comparison tool, but it can be misleading if you rely on it too heavily.

Why it's misleading: A 400 sq ft ADU and an 800 sq ft ADU both need one kitchen, one bathroom, one HVAC system, one electrical panel connection, and one set of permit fees. Those fixed costs get spread across more square footage in the larger unit, which makes the per-square-foot number go down. A tiny 400 sq ft ADU might cost $250 per square foot. An 800 sq ft unit might come in at $190 per square foot, even though the total cost is much higher.

How to use it correctly: Per-square-foot cost is best for comparing bids on the same project (apples to apples) or for quick mental math during early planning. It's not useful for comparing your project to a project in a different city, a different size, or a different lot condition.

Twin Falls ranges:

What Drives the Cost Up

Understanding what makes ADUs more expensive helps you make smarter design decisions early.

Long utility runs. Every foot between your existing services and the ADU adds trenching cost. A 30-foot run is routine. A 100-foot run across the backyard can add $5,000 to $10,000.

Sloped or problem lots. Grading, retaining walls, engineered foundations, and drainage solutions all add cost. Flat lots with good soil are the cheapest to build on.

Design matching requirements. The Twin Falls code requires ADUs to match the primary home's roof pitch, siding, and windows. If your house has cedar siding or a steep roofline, replicating that on the ADU costs more than standard lap siding and a simple gable.

Premium finishes. Quartz countertops, custom tile, hardwood flooring, and designer fixtures can add $10,000 to $20,000 to a project without proportionally increasing rental income.

2025 tariffs. Tariffs on imported building materials introduced in 2025 have added an estimated $5,000 to $10,000 to a typical residential build in Idaho. Locking in material selections and pricing early in the design phase protects against mid-project increases.

What Keeps the Cost Down

Garage conversions. The existing foundation, walls, and roof eliminate the most expensive phases of construction. Our garage vs. detached comparison covers the full tradeoff analysis.

Simple, rectangular footprints. Every corner, angle, and bump-out adds framing complexity and cost. A simple rectangle is the most cost-efficient shape per square foot.

Standard materials. Fiber cement siding, asphalt shingles, LVP flooring, and laminate countertops are durable, attractive, and significantly cheaper than premium alternatives.

Short utility runs. Building close to existing service connections (within 30 to 50 feet) minimizes trenching costs.

Right-sizing the unit. A 500 to 600 sq ft unit hits the sweet spot for rental income relative to construction cost in the Twin Falls market. Going to 800+ sq ft doesn't proportionally increase rent but significantly increases the build price.

The Total Cost of Ownership: Year One and Beyond

The build cost is the biggest number, but it's not the only one. Here's what the full first-year cost of ADU ownership looks like in Twin Falls.

Year one is typically negative if you're financing the full amount. But several things change in your favor over time: rent increases annually (2 to 4% per year in Twin Falls), the loan balance declines, and eventually the loan is repaid entirely. Once the construction loan is paid off, the same unit generates roughly $7,500 to $9,000 per year in net income after expenses.

For a deeper dive into ongoing costs, see our guides on property taxes, insurance, and maintenance.

Frequently Asked Questions

How much does an ADU cost per square foot in Idaho?

In Twin Falls, expect $150 to $250 per square foot for a detached new build and $100 to $200 for a garage conversion. Boise and the Treasure Valley run 10 to 20% higher due to increased labor demand. Per-square-foot costs decrease as the unit gets larger because fixed costs (kitchen, bathroom, HVAC, permits) are spread across more area.

Is a garage conversion always cheaper than a detached ADU?

Almost always, because the foundation, walls, and roof already exist. However, if the garage needs structural reinforcement, a new foundation, or significant utility upgrades, the gap can narrow. In rare cases where the existing garage is in very poor condition, building new on a clean pad can be more cost-effective.

What's the cheapest ADU you can build in Twin Falls?

A basic garage conversion with minimal utility work, mid-range finishes, and a straightforward lot can come in around $50,000 to $70,000. Below that, you're cutting corners that will cost more in maintenance and reduced rental income over time. The cheapest build isn't always the cheapest project.

Do prefab ADUs cost less than site-built?

The unit itself can cost less, but site prep, foundation, utility connections, and permits apply regardless of build type. The total installed cost of a prefab ADU in Twin Falls (including delivery, crane, foundation, and connections) typically ranges from $80,000 to $160,000, which overlaps with site-built pricing. Prefab's main advantage is timeline (faster) rather than cost (similar).

How much should I budget for contingency?

5 to 10% of total project cost. For a $140,000 project, that's $7,000 to $14,000. Every project encounters something unexpected, whether it's a soil condition, a utility surprise, or a material delay. A contingency fund prevents these from derailing your budget.

Can I build an ADU for under $100,000 in Twin Falls?

Yes, through a garage conversion on a straightforward lot with short utility runs and mid-range finishes. A detached new build under $100,000 is very difficult in 2026 due to foundation, framing, and regulatory costs. If your budget is firm at $100,000, a garage conversion is your most realistic path.

If you want to know exactly what an ADU would cost on your specific property in Twin Falls, reach out to Twin Falls ADU Guys for a feasibility check. We'll evaluate your lot, assess utility access and site conditions, and give you a realistic cost projection before you commit to design. The number that matters isn't the average. It's yours.