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ADU/Granny Flat ROIの包括的な教育ガイドを準備中です。ステップバイステップの解説、数式、実例、専門家のヒントをお届けしますので、もうしばらくお待ちください。
The ADU (Accessory Dwelling Unit) ROI Calculator projects financial return for building a granny flat, in-law suite, garage conversion, or detached backyard unit on existing residential property. Calculates annual rental income, cash-on-cash yield, payback period, and 10-year total return including appreciation. ADUs have surged in popularity since California's SB 9 / AB 68 legislation (2020+) and similar laws in Oregon, Washington, Vermont, and Massachusetts streamlined permitting and removed many local barriers. Typical California ADU economics: detached 600–1,000 sqft new construction costs $150,000–$300,000 ($250–400/sqft) depending on quality, location, and existing site conditions. Pre-fab/modular options ($100,000–$200,000) are growing market. Garage conversions are cheapest at $80,000–$160,000. Rental income depends on location: $1,500–2,500/month in mid-tier markets, $2,500–4,000+ in expensive coastal cities. Owner-occupied requirements vary by jurisdiction — California eliminated this in 2020, others require it. ROI calculation: Annual Net Income = (Monthly Rent − Monthly Expenses) × 12. Expenses include property tax increase (often 50–100% of normal rate on ADU portion), additional insurance, maintenance (1% of build cost annually), property management if used (8–12% of rent), and vacancy buffer (5–10%). Cash-on-cash yield = Annual Net / Total Build Cost × 100. Healthy ADU yields range 6–12%; below 5% suggests overbuilding for the rental market; above 12% suggests undervaluing your time and risk. 10-year total return includes both income stream and property appreciation. Most US markets appreciate 3–5% annually; ADU additions typically increase property value by 70–100% of build cost (you don't fully recoup at sale but the property value adds significantly). Calculator outputs 10-year cumulative income, 10-year appreciation contribution, and total return — useful for comparing ADU investment to alternative uses of capital (stock market, other real estate, paying down mortgage).
Annual Income = (Rent − Expenses) × 12; Cash Yield = Income / Build × 100; 10-Yr Total = Income × 10 + Build × ((1+a)^10 − 1)
- 1Step 1 — Enter total build cost including permits, plans, utility hookups, and construction
- 2Step 2 — Enter expected monthly rent (use Zillow Rentals, Apartment List, or local rentals as comparables)
- 3Step 3 — Enter monthly expenses (property tax + insurance + 1% maintenance + 5% vacancy + management if applicable)
- 4Step 4 — Enter expected appreciation rate (3–5% typical US average)
- 5Step 5 — Calculator computes annual net income: (Rent − Expenses) × 12
- 6Step 6 — Cash-on-cash yield = Annual Net / Build Cost × 100
- 7Step 7 — Projects 10-year total: income stream + appreciation = total return
Strong fundamentals — high yield, manageable payback period. 10-yr return: $180k income + $115k appreciation = $295k on $150k investment.
Moderate market with healthy returns
Expensive markets often have best ROI because rent multiplier (rent ÷ build cost) is highest.
Below 8% yield — consider whether capital might earn more elsewhere
Pre-construction financial planning
Comparing ADU investment to alternative uses of capital
Refinancing decision (cash-out for ADU vs other options)
Family financial planning (aging parents, adult children housing)
Bank loan justification documentation
Estate planning (rental income vs property sale)
Will I recoup the build cost when I sell?
Partially. ADUs typically increase property value by 70–100% of build cost — so a $200k ADU adds $140k–$200k to home value. You don't get full $200k back at sale, but the income stream during ownership plus appreciation makes total return positive. Don't build ADU expecting full cost recovery at sale; build for income + lifestyle (housing aging parents, adult children, rental income).
How do property taxes change with ADU?
Varies by jurisdiction. California Prop 13 keeps existing home assessment but new ADU is assessed at fair market value, taxed at standard rate (typically 1.1–1.3% of ADU value annually). Some jurisdictions reassess the entire property; others add only the ADU. Check your assessor's office for specifics before budgeting — surprise property tax bills are common.
What about owner-occupancy requirements?
Varies widely. California removed statewide owner-occupancy requirement in 2020 (some cities reinstated locally). Other states require owner to live in either main house or ADU. Important for investment strategy — owner-occupied requirement prevents pure investment property use. Verify with local zoning before committing.
Can I finance an ADU?
Several options: cash-out refinance of primary mortgage, home equity line of credit (HELOC), Renovation loan (Freddie Mac CHOICERenovation, Fannie Mae HomeStyle), or specialized ADU loans from CalHFA and some local programs. Construction loans then refinance to permanent are common. Expect 6–8% interest rates currently (2024). Plan for 6–18 month financing gap during build.
Detached new build vs garage conversion?
Garage conversion: $80k–$160k, faster (3–6 months), less rental value (often smaller, less private). New detached: $150k–$300k, slower (6–18 months), higher rental value, full kitchen and bath, more privacy. Detached new build typically yields better ROI despite higher cost — rental income premium more than compensates.
プロのヒント
Rent multiplier (annual rent ÷ build cost) above 0.10 = strong ROI; below 0.08 suggests overbuilding for the market. A $200k build needs $1,667+ monthly rent to hit the 10% threshold. Run this check before committing to detailed plans — adjust ADU size/finishes to align with market rent.