How much does it cost to build an ADU?

In general, it is helpful to avoid having a fixed budget total in your head as you explore your options. Cost per square foot is a good way to estimate, though this too can range — a very rough placeholder for you to use is $450-$600 per square foot for construction (“hard costs”) and design and fees (“soft costs”), depending on your design and the materials you chose.

The San Mateo County ADU Calculator is a great place to start when developing a budget. It provides a rough estimate of costs and income and will help you understand how choices can impact your budget over time.