The designation of zones began in Baltimore County, just as in much of country, in the mid-20th century. With the rapid expansion and building after WWII, it was found that regulations were need to prevent improper uses in the wrong place. Before that, responsible citizens generally understood what was not acceptable. One would not establish a service garage next-door to houses and, if someone did, personal pressure could usually get it removed. With the breakdown of such civility, jurisdictions found that uses had to be regulated.
The Baltimore County Zoning Regulations (BCZR) contain all the regulations pertaining to zones.