Investing in an ADU in Marin County is generally considered worthwhile due to significant property value increase (20-30%), high rental income potential (thousands monthly), tax benefits, and addressing the housing shortage, with recent fee waivers and streamlined processes making it more accessible, though costs are high and planning requires navigating specific local rules. It offers a strong return through rental income and appreciation but requires careful budgeting, design, and understanding complex local zoning, especially considering high construction costs ($200k-$300k+).