Duplexes on A1 Family Dwelling in Austin

[Sal Zafar]

Hi BP - wanted to understand A1 and B2 differences with city of Austin or any other city. 

So, I've noticed, investors recently built very nice duplexes and when I check the county CAD it shows A1 Fam Dwelling. This makes me confuse, and get me thinking-- either one can build duplexes under A1 Fam dwelling because how city pass ALL the inspections without noticing it during the construction (specially city of Austin where they are really strict on enforcing construction codes). 
Or, there must be a loophole which allow constructing duplexes in certain way under A1 Fam dwelling which I don't know about. 

any thoughts? 

Thanks in advance.