The size of your ADU will depend on the configuration of the existing property on your lot  , the size of your lot and the city requirements . The cities can dictate the size of your ADU but most cities will approve a detached ADU up to 1200 sqft in size , may be single story or two story depending on the city requirements . An attached ADU will be limited to 50 % of the size of your existing house Ie: if your house is 1800 sqft the attached ADU cannot be any larger than 900 sqft. A Jr. ADU is limited to no more than 500 sqft . This can be built converting and Attached garage or converting  rooms or bedroom in existing house .approximate-square-footage-allowed-