ADUs are the hottest residential investment in 2025 California. Rental income: $1,800–$2,800/month; property value increase: 9.34% annually (2013–2023 FHFA data). New 2025 laws simplify permits: 60-day approval, reduced setbacks, pre-approved designs available. Typical ROI: 65–100% over 6–8 years. This guide covers costs ($100,000–$200,000+), LADBS/CALTECH permits, energy codes, and step-by-step approval for LA, Orange County, San Diego, and beyond.