If your project is indoors (an interior remodel, water heater replacement, electrical work, etc.), a good place to start is with Building Safety to determine whether or not you will need a building permit. Most indoor projects do not require Planning & Zoning review, but many will require a building permit.
If your project is outdoors (a home addition, casita, shed, ramada, outdoor kitchen, etc.), start with Planning & Zoning. Planning & Zoning will be able to tell you whether what you want will be allowed, and if so what the required setbacks will be. Planning & Zoning will refer you to Building Safety to determine whether a building permit will also be required.
If you are looking to start a home-based business, start with Planning & Zoning to determine whether your business idea will be allowed to operate as a home-based business.