Calculateur de retour sur investissement ADU/Granny Flat

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).

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1. 1Step 1 — Enter total build cost including permits, plans, utility hookups, and construction
2. 2Step 2 — Enter expected monthly rent (use Zillow Rentals, Apartment List, or local rentals as comparables)
3. 3Step 3 — Enter monthly expenses (property tax + insurance + 1% maintenance + 5% vacancy + management if applicable)
4. 4Step 4 — Enter expected appreciation rate (3–5% typical US average)
5. 5Step 5 — Calculator computes annual net income: (Rent − Expenses) × 12
6. 6Step 6 — Cash-on-cash yield = Annual Net / Build Cost × 100
7. 7Step 7 — Projects 10-year total: income stream + appreciation = total return

• !Underestimating soft costs (permits, plans, utility hookups) — add 15–25% above construction estimates
• !Forgetting property tax increase in monthly expense calculation
• !Building luxury when market only supports mid-tier rent (oversized capital deployment)
• !Not factoring management cost when self-managing — your time is valuable
• !Optimistic vacancy assumptions — 95% occupancy is realistic, 100% is fantasy