There are some limits on the size of an ADU, based on the size of your lot and existing house. First, an attached ADU cannot be bigger than 50% of the existing house. For example, if your existing house in 2,000 sq. ft., the attached ADU cannot exceed 1,000 sq. ft. in size. Second, there is a “baseline mansionization ordinance” that usually restricts the total square footage of all structures on a lot to 45% of the total lot size. For example, if your lot is 10,000 sq. ft., the total built area (existing house, ADU, garage, etc.) cannot exceed 4,500 sq. ft. In addition, most ADUs cannot exceed 1200 sq. ft. The Dept. of City Planning will need to verify the specifics related to your project.