The cost to build an ADU in California typically ranges from about $150,000 to $400,000 or more, depending on the size, design, and site conditions. Garage conversions are usually the most affordable, while detached backyard ADUs cost more because they require a new foundation, utilities, and full construction.
Most ADUs are built for around $300 to $500 per square foot, though the final price can vary based on materials, permits, labor, and the complexity of the project.
Factors that affect the cost include the type of ADU, size of the unit, site preparation, utility connections, and the level of finishes. A builder can usually provide a more accurate estimate after reviewing your property and goals.