Considering building an ADU ? There are many different things to consider when you are building your ADU. FINANCING , most ADU's will run a minimum $75k for a Jr. ADU upto $450K for a 1200 sqft Detached ADU . Most people will need to finance the construction cost . California has a block grant of $40,000 to help you with pre-. construction cost including plans ,engineering, and permits for this portion at  https://www.calhfa.ca.gov/adu. Construction loans can be taken out for the rest of the construction cost but it will be at a higher rate for a 1 year term . Equity line of credits can be taken out rather than refinancing your low rate loan . Borrowing from your savings , CD's or retirement accounts can be another avenue to consider.
USE OF ADU . Are you planning on renting the ADU or do you plan to have a family member use it . This is especially important if its a family member privacy is less of a factor than if you plan on renting it out . You may consider the entryway to the ADU to be in a location that is more private and a fence to block off the entry and the location of the ADU to be further from the main house .
PARKING is usually required if you are more than 1/2 mile from public transit one car space per ADU.
UTILITIES for your ADU can be taken from the main house or can be separately metered . If its a family member usually sharing utilities would not be difficult , If you are renting the ADU out you can get higher rents if you share utilities , but your safer if you obtain separate meters . We usually always install separate electrical panels since the cost is not as much as the gas company and water company will charge for their installation of an additional gas meter and water meter . SCG will usually give you the gas meter but will charge you for the installation which can be from $3,000 to $5,000 per meter . Your water company may be much more as much as $15,000 to add a water meter to your property.

SIZE OF AN ADU. The size of an ADU will be important if you want to obtain higher rents and you have the room . A second story ADU can be added if your city will allow it and can be used to build an ADU up to 1200 sqft in most cities . Jr ADU's requires converting existing living space in your house to an ADU and is limited in size to 50 % of the size of your house . You can build an addition to your house and convert it to a Jr. ADU . An attached garage can also be converted to a Jr.ADU.