ADU Calculator
Run the numbers on your ADU project. Adjust rates, terms, and rental income to see what your investment really looks like — then explore strategies to pay it off faster.
Quick Answer
An ADU in Twin Falls costs $80,000–$350,000+ depending on type. Garage conversion is cheapest ($80k–$150k), detached new build is most flexible ($150k–$350k+), and pre-fab lands in between all-in ($150k–$280k). Add the $250 Zoning Use Permit, building + trade permits, and ~$6,000–$10,000 in Idaho impact fees. Most ADUs pay back through $800–$1,400/month rent in 5–10 years, plus a 20–30% property value increase per FHFA research.
What Does an ADU Cost in Twin Falls?
ADU costs vary widely based on type, size, finishes, and site conditions. Here are the typical ranges for Twin Falls and the Magic Valley area:
Garage Conversion
Converting existing structure. Fastest and most affordable option.
Detached New Build
Custom-built from the ground up. Most design flexibility.
Pre-Fab / Modular
Factory-built and installed on-site. Faster timeline.
* Monthly payments estimated at 10% APR over 25 years, principal + interest. Actual rates depend on your credit, lender, and loan type. Ranges based on national averages of $150-$300/sq ft adjusted for Idaho market conditions.
Get Your Rough Estimate
Fill out the form below with your project details. This gives you a starting point — for a precise budget range, book our Feasibility Check.
Get Your ADU Cost Estimate
What Drives ADU Costs?
Understanding where the money goes helps you make smarter decisions. Here's how a typical ADU budget breaks down:
Hard Costs (85-90% of total)
Direct construction expenses:
- • Labor — approximately 40% of total project cost
- • Materials — lumber, concrete, roofing, siding
- • Mechanical — plumbing, HVAC, electrical
- • Finishes — flooring, cabinets, countertops, fixtures
- • Foundation — concrete work and site prep
- • Utility connections — water, sewer, electric, gas
Soft Costs (10-15% of total)
Planning and preparation expenses:
- • Architectural design — plans and engineering
- • Permit fees — building, electrical, plumbing
- • Impact fees — approximately $6,000-$10,000 in Idaho
- • Inspections — multiple required stages
- • Survey / site assessment — property measurements
- • Contingency — 10-15% recommended buffer
Property-Specific Factors
- • Lot size, shape, and slope
- • Distance from existing utilities
- • Soil conditions and drainage
- • Access for construction equipment
- • Existing structures to work around
- • Setback requirements in your jurisdiction
Design Choices That Affect Cost
- • Square footage (biggest cost driver)
- • Number of bedrooms and bathrooms
- • Kitchen complexity (full vs kitchenette)
- • Finish level (standard vs premium)
- • Accessibility features (ADA compliance)
- • Energy efficiency upgrades
ADU Investment Calculator
Plug in your numbers and see how an ADU pays for itself
Ready to run the numbers?
Enter your home value, ADU cost, and expected rent above to see your investment breakdown.
* P&I at selected rate and term. Property value based on FHFA 20-30% avg for ADU properties. Does not include taxes, insurance, maintenance, or vacancy.
Want a Precise Budget Range?
This estimator gives you a rough starting point. For a detailed, property-specific budget range with good/better/best options, book our ADU Feasibility + Budget Range service. It includes a professional site visit, detailed measurements, and a comprehensive report — and the fee ($250-$750) is credited toward your build.
Detailed Cost Breakdown — Where the Money Actually Goes
ADU cost estimators on the internet tend to give a single dollar figure and call it a day. The reality is that an ADU is 10+ separate cost categories, and which ones dominate depends heavily on your build type, lot conditions, and existing home. Here is what actually shows up on a Twin Falls ADU invoice.
| Cost category | Garage Conversion (600 sq ft) | Detached New Build (800 sq ft) | Notes |
|---|---|---|---|
| Design + engineering | $4,000–$8,000 | $8,000–$15,000 | Stamped plans required for permit. Custom designs cost more than re-using an existing floor plan. |
| Permits + fees | $1,500–$4,000 | $2,500–$8,000 | $250 Zoning Use Permit + building permit (valuation-based) + trade permits + impact fees ($6k–$10k in Idaho) |
| Foundation | $0 (existing) | $12,000–$25,000 | Garage already has slab. New builds need full footings + slab or crawlspace. |
| Framing + structural | $5,000–$10,000 | $25,000–$50,000 | Garage frame is largely re-usable; new build is fresh framing + sheathing + roof. |
| Roofing + siding | $3,000–$8,000 | $15,000–$28,000 | Must match primary residence per Title 10-6-10-B design consistency rule. |
| Plumbing rough + finish | $8,000–$15,000 | $12,000–$22,000 | Tie-in to primary dwelling per code (separate meters prohibited). Capacity check often required. |
| Electrical rough + finish | $6,000–$12,000 | $10,000–$20,000 | May need primary panel upgrade if existing service can't handle ADU load. |
| HVAC | $5,000–$10,000 | $8,000–$16,000 | Mini-split is common for ADUs — efficient + simple install. |
| Insulation + drywall | $5,000–$10,000 | $10,000–$18,000 | Garage conversions almost always need insulation upgrades to meet residential code. |
| Kitchen + bath fixtures | $10,000–$25,000 | $15,000–$40,000 | Biggest finish-level variable. Basic IKEA cabinets vs. custom millwork can swing this 3-4x. |
| Flooring + paint + trim | $4,000–$10,000 | $8,000–$18,000 | LVP flooring is the budget standard; hardwood / tile push higher. |
| Site work + utility tie-ins | $3,000–$8,000 | $10,000–$25,000 | Detached units often need trenching for water/sewer/electrical from primary dwelling. |
| Project management + overhead | $10,000–$20,000 | $15,000–$35,000 | Licensed GC's margin + insurance + warranty + inspection coordination. |
| TOTAL (typical range) | $80,000–$150,000 | $150,000–$300,000+ | Pre-fab modular lands $150k–$280k all-in. |
These are real ranges from Magic Valley ADU projects in 2025-2026. The high end of each range tends to apply when: the lot has access constraints (narrow side yard, slope, existing trees to preserve), the primary home needs electrical or sewer upgrades to support the ADU, or the homeowner chooses premium finishes.
ROI Scenarios — When Does the ADU Pay for Itself?
The math depends on three variables: total project cost, monthly rent, and financing rate. Here are three representative Twin Falls scenarios.
Scenario A: Budget Garage Conversion
- Cost: $95,000
- Rent: $950/mo ($11,400/yr)
- Financing: HELOC at 8.5%
- Annual P+I: ~$10,800
- Cash-flow positive in Year 1. Property value increase $20k–$30k.
Scenario B: Mid-Range Detached
- Cost: $185,000
- Rent: $1,250/mo ($15,000/yr)
- Financing: Construction-to-perm at 9%
- Annual P+I (25-yr): ~$18,600
- Net cost $3,600/yr until equity build + appreciation flip it positive by Year 5.
Scenario C: Wood River Premium
- Cost: $280,000 (Hailey)
- Rent: $2,200/mo workforce housing
- Financing: Cash-out refi at 7.5%
- Annual P+I: ~$24,800
- $1,600/yr positive Year 1. Wood River Valley premium rents accelerate payback.
Scenarios assume long-term rental. Short-term rental (Airbnb) can run 2-3x the long-term rate but with higher vacancy, more management overhead, and jurisdiction-specific rules. We model both during the Feasibility Check.
Get Your Real Numbers
Start with a free Readiness Call, then book a Feasibility Check for a detailed budget range tailored to your property.