The name of the person complaining is no longer important if the building department has now found evidence of a violation. If you do end up in court you may seek the evidence in discovery and force them to produce it, however that really won't change your non-compliant status. Your time is better spent on reading the regulations and finding out if you truly are in violation or perhaps you have an exemption of some sort.
Only If and until you and I sign an Agreement for Legal Services, I am not your attorney. These answers are provided for informational and/or novelty purposes
I don't think it is possible to give a complete answer to your question with just the facts provided.
However, as a general matter it is normally possible to make a complaint about a building permit violation without giving your name or contact info, so there may be nothing to disclose. There are also a number of exceptions to public disclosure requirements. For example, if the person filing the complaint believes disclosure would endanger any person's life, physical safety, or property; or if non-disclosure is is essential to effective law enforcement or for the protection of any person's right to privacy (RCW 42.56.240).
It is not clear why the name of the person who filed the complaint is important, but if it is I would reccomend that you start by consulting with a lawyer, and discussing the value of seeking the name as well as the reason they are giving for withholding the name (which does not seem to be provided in the facts you gave).
The above answer is for background informational purposes only, is not meant as legal advice, and does not create an attorney-client relationship.