ADU Cost in Boise
Realistic construction costs for detached ADUs, garage conversions, and basement conversions — based on Boise-area project data and current material and labor pricing.
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Building an accessory dwelling unit is one of the most significant investments a Boise homeowner can make — and one of the most financially rewarding when planned correctly. Whether you are adding a detached new build in the backyard, converting an existing garage, or finishing a basement into a separate living space, understanding realistic costs before you commit prevents budget overruns and helps you make smarter design decisions.
The cost data below reflects actual ADU construction pricing in the Boise metro area — not national averages. Boise's specific soil conditions, impact fee structure, utility tap costs, and labor market all affect what an ADU actually costs to build in the Treasure Valley. We break down every major cost component so you can plan with confidence.
ADU costs vary dramatically based on the type of construction. A conversion project leverages existing structure, while a detached new build starts from the ground up. Here are the three primary ADU types and their typical price ranges in Boise.
| ADU Type | Typical Cost | Cost per Sq Ft | What's Included |
|---|---|---|---|
| Detached New Build | $150,000–$300,000+ | $250–$400 | Foundation, framing, roofing, full MEP systems, interior finishes, site work, utility connections |
| Garage Conversion | $60,000–$120,000 | $150–$250 | Insulation, interior framing, plumbing, electrical, HVAC, finishes, egress windows, entry modifications |
| Basement Conversion | $50,000–$100,000 | $120–$200 | Waterproofing, egress windows, plumbing, electrical, HVAC extension, finishes, separate entry |
Detached new builds command the highest price because every system — foundation, structure, roofing, plumbing, electrical, and HVAC — is built from scratch. Garage conversions save 40–60% by reusing the existing foundation, walls, and roof. Basement conversions are typically the most affordable option when the existing basement has adequate ceiling height (minimum 7 feet finished) and manageable moisture conditions.
Understanding where the money goes helps you make informed trade-offs. The following breakdown reflects a typical 600-square-foot detached ADU at mid-range finish level in the Boise area.
| Component | % of Budget | Cost Range |
|---|---|---|
| Site Work & Foundation | 12–18% | $25,000–$45,000 |
| Framing & Roofing | 15–20% | $30,000–$50,000 |
| Plumbing (Rough + Finish) | 8–12% | $15,000–$28,000 |
| Electrical (Rough + Finish) | 7–10% | $12,000–$22,000 |
| HVAC System | 5–8% | $8,000–$18,000 |
| Interior Finishes | 18–25% | $35,000–$55,000 |
| Exterior Finishes | 5–8% | $10,000–$18,000 |
| Permits, Fees & Design | 5–8% | $8,000–$18,000 |
| Utility Connections | 3–6% | $5,000–$15,000 |
Site work and foundation costs vary the most from project to project. A flat lot with stable soil and short utility runs can come in at the low end. A sloped lot requiring engineered retaining walls, extensive grading, and long utility trenches pushes toward the high end. Interior finishes are the area where homeowners have the most control — choosing standard-grade fixtures and finishes versus premium selections can swing total cost by $20,000–$40,000.
Cost per square foot is the most common way to compare ADU projects, but the number varies significantly based on size, type, and finish level. Smaller ADUs have a higher per-square-foot cost because fixed expenses (permits, utility connections, kitchen, bathroom) are spread across fewer square feet.
| ADU Size | Budget Finish | Mid-Range Finish | Premium Finish |
|---|---|---|---|
| 400 sq ft (Studio) | $300–$375/sq ft | $375–$450/sq ft | $450–$550/sq ft |
| 600 sq ft (1-Bed) | $250–$325/sq ft | $325–$400/sq ft | $400–$475/sq ft |
| 800 sq ft (1–2 Bed) | $225–$300/sq ft | $300–$375/sq ft | $375–$450/sq ft |
| 1,000 sq ft (2-Bed) | $200–$275/sq ft | $275–$350/sq ft | $350–$425/sq ft |
These ranges apply to detached new-build ADUs. Garage and basement conversions run 30–50% lower per square foot because the structural shell already exists. The "sweet spot" for most Boise homeowners is a 600–800-square-foot one-bedroom unit at mid-range finish — large enough to rent comfortably, small enough to keep construction costs manageable.
Factors That Increase Cost
- Larger footprint: every additional 100 square feet adds $25,000–$40,000 to a detached build
- Premium finishes: hardwood flooring, quartz countertops, custom tile work, high-end fixtures
- Difficult site conditions: slopes, rocky soil, limited access for equipment, mature trees to work around
- Long utility runs: extending water, sewer, gas, and electrical lines 50+ feet from the main house
- Two-story design: adds structural engineering, second-floor framing, stairs, and additional fire safety requirements
- Detached garage or covered parking included with the ADU structure
Strategies That Reduce Cost
- Convert existing space: garage or basement conversions cost 40–60% less than detached new builds
- Keep the footprint compact: a well-designed 600 sq ft ADU meets most needs at lower cost
- Standard-grade finishes: LVP flooring, laminate countertops, and builder-grade fixtures look good and perform well
- Locate near existing utilities: shorter runs for water, sewer, and electrical save $3,000–$8,000
- Simple roofline: shed or gable roof instead of complex hip or multi-ridge designs reduces framing cost
- Single-story slab-on-grade: eliminates crawlspace or basement foundation costs
Soil Conditions and Foundation Requirements
Boise's soil conditions vary significantly by neighborhood. The Bench area and parts of Southeast Boise have expansive clay soils that require engineered foundations — sometimes including deeper footings, post-tensioned slabs, or pier systems that add $5,000–$15,000 over standard slab-on-grade construction. The North End and areas near the Boise River may encounter high water tables that require additional drainage and waterproofing. A geotechnical report ($1,500–$3,000) is strongly recommended before finalizing foundation design.
Setback Requirements and Lot Coverage
City of Boise zoning code requires specific setbacks for ADUs — typically 5 feet from side and rear property lines, though this varies by zone. Maximum lot coverage rules limit how much of your lot can be occupied by structures. These constraints directly affect cost because they may dictate a smaller footprint (higher per-square-foot cost), a two-story design (added structural complexity), or significant site work to fit the building envelope. To fully understand these requirements before budgeting, review our guide to Boise ADU permits and zoning.
Utility Tap Fees and Connection Costs
Utility connection fees are one of the most commonly underestimated ADU costs in Boise. Water and sewer tap fees from Boise City Public Works or Suez (depending on your service area) range from $2,000–$5,000. If the ADU requires a separate water meter, add $1,500–$3,000 for meter installation. Electrical service — whether extending from the main panel or installing a new sub-panel — runs $2,000–$5,000 depending on distance and amperage requirements. Gas line extensions add $1,000–$3,000. These fees are non-negotiable and must be included in any realistic budget. See our detailed ADU utility hookup guide for complete cost breakdowns.
Impact Fees
Ada County Highway District (ACHD) charges impact fees for new dwelling units to fund road infrastructure. ADU impact fees in the Boise area currently range from $3,000–$6,000 depending on unit size and location. Parks and fire impact fees may add another $1,000–$2,000. These fees are paid at permit issuance and are a fixed cost regardless of finish level. Impact fee schedules are updated periodically, so we verify current rates during project planning.
Labor Market
Boise's construction labor market remains competitive due to sustained residential growth across the Treasure Valley. Skilled trades — framers, electricians, plumbers, and HVAC technicians — are in high demand, which keeps labor rates 10–15% above levels from five years ago. Working with an established general contractor who maintains long-term crew relationships ensures reliable scheduling and competitive labor pricing. Our multi-trade team approach reduces the subcontractor coordination overhead that drives up costs on many ADU projects.
Few homeowners pay cash for a full ADU build. Understanding financing options helps you evaluate the true cost of ownership including carrying costs.
Home Equity Loan or HELOC
Leverages existing equity in your primary home. HELOCs offer flexible draws during construction. Current rates in the Boise market typically range from 7–9% for HELOCs. Best for homeowners with substantial equity and good credit.
Construction-to-Permanent Loan
A single-close loan that funds construction draws and converts to a standard mortgage upon completion. Eliminates double closing costs. Requires detailed construction plans and contractor approval. Ideal for detached new-build ADUs over $150,000.
Cash-Out Refinance
Replaces your existing mortgage with a new, larger loan and uses the difference for construction. Makes sense when you can secure a lower rate than your current mortgage or when you have significant equity. Factor in closing costs of 2–3% of the new loan amount.
Personal Home Improvement Loan
Unsecured loans up to $100,000 from banks and credit unions. Higher interest rates (8–15%) but no home equity required and faster approval. Best suited for smaller conversion projects (garage or basement) under $100,000.
An ADU is both a lifestyle asset and a financial investment. In Boise's strong rental market, the numbers work in most homeowners' favor.
| ADU Configuration | Monthly Rent Range | Annual Gross Income | Estimated Payback |
|---|---|---|---|
| Studio (400 sq ft) | $900–$1,200 | $10,800–$14,400 | 12–18 years |
| 1-Bedroom (600 sq ft) | $1,200–$1,600 | $14,400–$19,200 | 11–16 years |
| 2-Bedroom (800+ sq ft) | $1,500–$1,800+ | $18,000–$21,600+ | 10–15 years |
Beyond Rental Income
- Property value increase: ADUs typically add 60–80% of their construction cost to assessed property value immediately, with potential for full recoupment over 5–10 years as Boise property values appreciate
- Multi-generational flexibility: housing aging parents or adult children reduces family housing costs while maintaining independence
- Home office or studio: a dedicated workspace with separation from the main home has measurable productivity and work-life balance benefits
- Short-term rental potential: furnished ADUs in desirable Boise neighborhoods can generate $2,000–$3,500 per month through platforms like Airbnb, subject to local short-term rental regulations
- Future resale advantage: homes with permitted, well-built ADUs sell faster and at higher prices in the Boise market — buyers value the income potential and flexibility
To understand total ADU costs including permitting and zoning requirements that affect your budget, read our Boise ADU permits and zoning guide.
How much does it cost to build a detached ADU in Boise?
A detached ADU in Boise typically costs $150,000–$300,000+ depending on size, finish level, and site conditions. A basic 400-square-foot studio unit starts around $150,000, while a fully finished 800–1,000-square-foot one-bedroom with mid-range finishes runs $220,000–$300,000. The largest cost variables are foundation type, utility connection distances, and whether the site requires significant grading or retaining walls.
Is a garage conversion or detached ADU cheaper in Boise?
A garage conversion is significantly cheaper — typically $60,000–$120,000 versus $150,000–$300,000+ for a detached new build. The savings come from using the existing foundation, framing, and roof structure. However, garage conversions are limited by the existing footprint (usually 400–500 square feet) and you lose covered parking. If your lot has space and your budget allows, a detached ADU offers more flexibility in layout and placement.
What are the ongoing costs of owning an ADU in Boise?
Ongoing ADU costs in Boise include property tax increases (typically $1,200–$3,000 per year based on added assessed value), insurance premium adjustments ($300–$800 per year), utilities ($100–$250 per month if owner-paid), and maintenance reserves (budget 1–2% of construction cost annually). If you rent the unit, add landlord insurance and potential property management fees (8–10% of rent). These costs are typically far offset by rental income of $1,000–$1,800 per month.
How much do ADU permits and fees cost in Boise?
ADU permit and fee costs in Boise total $8,000–$18,000 depending on the project. Building permits run $2,000–$5,000 based on project valuation. Impact fees — which fund infrastructure for the new dwelling — range from $4,000–$8,000. Utility tap fees for water and sewer connections add $2,000–$5,000. Plan review and inspection fees add another $500–$1,500. Iron Crest Remodel includes all permit coordination in our project management scope.
Can I finance an ADU construction project in Boise?
Yes. Common ADU financing options include home equity loans or HELOCs (leveraging existing home equity at competitive rates), construction-to-permanent loans (single close that converts to a mortgage after completion), cash-out refinancing, and personal home improvement loans. Some homeowners use a combination — for example, a HELOC for construction costs and savings for permits and fees. We can connect you with local lenders experienced in ADU financing.
What ROI can I expect from an ADU in Boise?
ADUs in Boise typically add 60–80% of construction cost to property value immediately and generate $1,000–$1,800 per month in rental income for a one-bedroom unit. At $1,400 per month average rent, a $200,000 ADU generates $16,800 per year in gross rental income — a payback period of roughly 12–15 years before accounting for property appreciation. Boise's strong rental demand and low vacancy rates make ADUs one of the highest-returning home investments in the market.
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