When you are in the planning phase of your new granny flat, you should check with local and state building codes to make sure your granny flat is legal. For example, the state of California mandates that any mother-in-law unit be at least 150 square feet. For attached units, the maximum size is 50 percent of the existing house up to 1,200 square feet. For detached units, you can have the granny flat be up to 1,200 square feet even if the existing house is smaller than 2,400 square feet. Detached granny flats do not have to adhere to the 50% regulation that attached units have to. Make sure to check the regulations for your city and state.