The devil is in the details here.
As can also be read in the initial post, depending on the applicable state building code, there are building types that can be constructed without a permit within defined boundary conditions. In BB, this also seems to apply to small garages, whereas in NRW, for example, these generally require a permit.
: There are always special commentaries on the respective state building code, which your lawyer should have or be able to find. These also explain in a way that is somewhat understandable even for laypeople what is meant in the law.
Basically, however, even buildings that do not require a permit must comply with the above-mentioned conditions. Whether this is actually the case with the garages you see in your neighborhood is another matter. It is possible that there are steel garages that were constructed correctly, for example with sufficient distance to the boundary or to the next building. It is also possible that steel garages were erected with an additional/special permit/building application because certain points of the permit exemption were _not_ met but could be rectified through a formal building application—and it is possible that there are steel garages erected without a building application but nevertheless violating the conditions of the permit exemption.
In the last case, this does not create a legal entitlement for you— and or any other lawyer will also explain this to you— to make the same mistake.
In my opinion, your contact person here is your architect, as he must be familiar with the construction of such garages in BB—but based on your description, I believe the caseworker is correct.
Best regards Dirk Grafe