In general, it is helpful to avoid having a fixed budget total in your head as you explore your options.The cost to build an ADU typically ranges from $30,000 for a simple interior conversion JADU, to $400,000+ for a large detached ADU with high-end finishes on a hillside lot. Cost per square foot is a good way to estimate, though this too can range — a very rough placeholder for you to use is $300 per square foot for construction (“hard costs”) and design and fees (“soft costs”), depending on your design and the materials you chose.