Once you have a design established with your architect/designer, it’s a great idea to discuss it with City staff so they can point out any issues before you prepare your application.
Depending on the city, you may be able to schedule an appointment to speak with a planner or walk in to the Planning Counter or Permit Center. For contact information, see the Local ADU Rules and Contacts.
This is also a good time to reach out to utility agencies (water, sewer, gas, etc.) to inquire about their infrastructural requirements and confirm connection and service fees.