Yes, in the state of California, your neighbor can report you for an illegal addition or renovation to your home. Building codes and regulations are enforced at the local level, typically by the city or county government. If someone believes that you have made an illegal addition or renovation to your home, they can report it to the appropriate local authority responsible for code enforcement.
Once a report is made, the local authority may investigate the matter to determine if the addition or renovation is indeed in violation of building codes or zoning regulations. If they find that you have made an illegal alteration to your home, they can take actions such as issuing a notice of violation, requiring you to obtain proper permits, or even demanding that you undo the unauthorized work.
It's important to comply with local building codes and regulations when making changes to your property to avoid potential legal consequences or penalties. If you are unsure about the requirements for a particular renovation or addition, it's best to consult with your local building department or a professional contractor who can provide guidance.