This is a bit tricky, but I’ll pass along what I think I know. I’ve worked more with non-residential codes here in MN.
The model code for California is the 2009 International Building Code (reference). The state then modifies portions of the code based on its agenda. Those amendments are public documents and can usually be downloaded from the state or count building department’s website. The main body of the code, however, is almost never freely available and you have to go to the physical office to view the publicly available copy (or buy your own for $140).
That said, since this is residential construction, there may be no legally specified height. Any code language would have to be “open” enough to allow sunken living rooms and the like, and would probably only require that stairs, if any, have a minimum tread depth and are all of equivalent height. Since you have one step of 1/2 inch, the code may be satisfied. However, if this were a public facility, or is otherwise covered by the Americans with Disabilities Act, the maximum allowable height difference is 1/4”, if left unmodified, or 1/2” if gradually transitioned. Anything greater becomes a “ramp” (see Section 303, here).
Bottom line: It’s sloppy workmanship, but it’s probably not a code violation.
