ADUs — Accessory Dwelling Units — are one of the most compelling ways to add value to a property. Done right, they can generate $1,500–$3,500/month in rental income, add significant equity, and provide flexible housing options. Done wrong, they're a half-million-dollar regret sitting in your backyard.
The key is knowing the real numbers before you start. Here's a complete cost breakdown for 2026.
The 6 Types of ADUs
Before diving into costs, it's important to understand what type of ADU you're building — because the cost variance between types is massive.
1. Detached ADU (New Construction)
A freestanding structure built on the same lot as the primary residence. The most flexible but also the most expensive. Typical size: 400–1,200 sq ft.
2. Attached ADU
Connected to the primary home but with a separate entrance. Shares one or more walls with the main structure. Lower foundation and roofline costs than detached.
3. Garage Conversion (ADU)
Converting an existing attached or detached garage into living space. You're leveraging existing structure — foundation, roof, walls — which significantly reduces costs.
4. Junior ADU (JADU)
A small unit (≤500 sq ft) created within the existing walls of the primary residence. Typically a converted bedroom with a kitchenette. Most affordable option.
5. Above-Garage ADU
An ADU built on top of an existing or new garage. Adds livable square footage without consuming additional lot coverage.
6. Basement Conversion
Converting an existing basement into a separate living unit. Costs vary enormously depending on ceiling height, egress windows, and existing condition.
Cost Ranges by ADU Type (2026)
These ranges reflect the national median. Coastal California, NYC, and Seattle will run 30–50% higher. Rural markets may run 15–25% lower.
- Detached ADU (new construction): $200,000 – $500,000+ ($300–$500/sq ft)
- Attached ADU: $150,000 – $350,000 ($250–$400/sq ft)
- Garage Conversion: $80,000 – $200,000 ($150–$300/sq ft)
- Junior ADU (JADU): $30,000 – $100,000
- Above-Garage ADU: $180,000 – $400,000
- Basement Conversion: $50,000 – $250,000
Hard Costs vs. Soft Costs
Every ADU project has two major cost buckets. Most people budget for hard costs and get blindsided by soft costs.
Hard Costs (Construction)
These are the physical construction expenses:
- Site work: Grading, drainage, utility connections ($15,000–$50,000)
- Foundation: Slab, crawlspace, or stem wall ($20,000–$60,000 for new construction)
- Framing: Wood framing, structural lumber ($25,000–$80,000)
- MEP (Mechanical, Electrical, Plumbing): New or extended service ($30,000–$90,000)
- Roofing and exterior: Siding, windows, roofing material ($20,000–$60,000)
- Interior finishes: Drywall, flooring, kitchen, bath, paint ($30,000–$100,000)
- General contractor markup: 15–25% of total hard costs
Soft Costs (Pre-Construction & Professional)
These are frequently underestimated but can add 15–25% to total project cost:
- Architecture and design: $8,000–$25,000 (5–10% of hard costs)
- Engineering (structural, civil, soils): $5,000–$20,000
- Permit fees: $5,000–$30,000+ depending on municipality
- Utility connection/impact fees: $10,000–$50,000+ (varies wildly by city)
- Survey: $2,000–$6,000
- Title and legal: $2,000–$5,000
- Financing costs: Construction loan fees, interest reserves
Real Talk: In Los Angeles, impact fees and permit costs alone regularly add $50,000–$80,000 to an ADU project. Know your jurisdiction before budgeting.
Financing Options for ADUs
How you finance the ADU dramatically affects your return on investment and cash flow during construction.
Cash
Simplest and cheapest long-term. No interest, no approval process. But ties up capital and limits your ability to do other investments simultaneously.
Cash-Out Refinance
If you have equity in the primary property, a cash-out refi can fund the ADU at mortgage rates (currently 6.5–7.5%). Works best when you can fund the full ADU cost out of equity without significantly increasing your monthly payment.
HELOC (Home Equity Line of Credit)
Flexible draw structure matches ADU construction milestones well. Variable rate (currently 8–9%). Best for phased projects or when you need flexibility.
ADU-Specific Construction Loans
Several lenders now offer ADU-specific products. These typically have a construction period (interest-only draws) that converts to a permanent loan on completion.
Renovation Loans (FHA 203k, Fannie Mae HomeStyle)
Government-backed renovation loans that roll construction costs into the mortgage. Require owner-occupancy and more paperwork, but offer competitive rates.
ROI Analysis: Does It Pencil?
The fundamental question: will the rental income justify the construction cost?
Example Scenario: Garage Conversion in a Mid-Market City
- Total ADU cost: $140,000
- Monthly rental income: $1,800/month
- Annual gross rent: $21,600
- Annual expenses (maintenance, vacancy, insurance, taxes): ~$4,000
- Annual net income: $17,600
- Cash-on-cash return (all cash): 12.6%
- Gross rent multiplier (GRM): 6.5
That's a solid return. Now with financing:
- HELOC at 8.5%: $140,000 × 8.5% = $11,900/year in interest
- Net cash flow after interest: $5,700/year
- Cash-on-cash (financed): ~4% if you contributed $140,000 to access equity — but property value likely increased by $140,000–$200,000
Break-Even Timeline
How long until the ADU pays for itself?
Simple formula: Total Cost ÷ Annual Net Income = Break-Even Years
$140,000 ÷ $17,600 = 7.95 years to break even on cash flow alone. With home value appreciation factored in, the effective break-even is typically 3–5 years in markets with 5%+ annual appreciation.
Key Variables That Make or Break ADU ROI
- Rental market strength: The rental rate you can charge is the most important variable. Know your market cold before committing.
- Soft costs in your jurisdiction: Impact fees can add $30,000–$80,000 in some cities, which completely changes the math.
- Construction cost control: Scope creep is the enemy. Lock in costs before breaking ground.
- Financing cost: Cash vs. HELOC vs. construction loan can swing annual cash flow by $8,000–$15,000.
- Property value uplift: In many markets, a finished ADU adds more to home value than it costs to build — making the equity play more compelling than the cash flow.
Before You Commit: The Pre-Construction Checklist
Before spending a dollar on design or permits, verify:
- Zoning: Is an ADU permitted on your lot? What are the setback, height, and size requirements?
- Utilities: Is your existing electrical panel adequate? What are the utility connection fees?
- Lot coverage: Does adding an ADU exceed your lot coverage maximum?
- HOA: Does your HOA allow ADUs? Some don't.
- Rental market: What do comparable units rent for within 0.5 miles? Talk to a local property manager.
- Construction costs: Get 3 contractor bids before finalizing your feasibility analysis.
The difference between an ADU that generates passive income for decades and one that becomes a financial burden almost always comes down to the quality of the pre-construction analysis. Run the numbers before you fall in love with the project.