If you’re planning to build an ADU in San Jose or anywhere else in California, it must comply with the basic technicalities and design specifications as laid out by this state, in order to obtain the permits and approvals. For that, you need to submit a foolproof master plan. If it is an attached ADU, it can be no bigger than 1200 sq. ft. in size, with a ceiling height of 16’. Else, 50% of the size of the present residential unit, provided it is within 1200 sq. ft. it should have a separate entrance, even if it is attached to the main building. In case of a detached ADU, it should fully respect the setback rules of 4’ from the rear & side, 5’