He is eventually able to move out thanks to getting his Assistant To The Traveling Secretary gig with the New York Yankees, but his time spent with Frank (RIP Jerry Stiller) and Estelle Costanza is most certainly a nightmare, as they are deeply unpleasant people.
George Costanza is certainly no stranger to making the last resort move of moving back in with his parents.
In fact, it’s a huge part of his “arc” in Season 5 of Seinfeld, as he is out of work and out of money after the deal with NBC to develop a sitcom falls through at the end of Season 4.