Unless your state statutes are explicitly clear that the building official can determine when an engineer is required I suggest you think again.
The licensing of engineers restrains trade and would be subject to the federal anti-trust laws except when the state legislature, not the city council or the building official, has clearly articulated a desire to restrain trade. The US Supreme Court made this clear. Thus if the local jurisdiction decided to regulate when an engineer was required they would be subject to anti-trust legislation.