The Walsh House was retconned more times than Valerie's Dead Dad. Per my recollection, at the end of season 1 when they were going to move back to Minneapolis, it was mentioned that Jim's COMPANY owned the house, it was like, leased by his firm for them for some reason. Then when they moved to Hong Kong, it was retconned that they owned the house and were going to sell, and the gang threw a huge party to trash the joint because the seller planned to tear it down. But that fell through.
Then the gang just lived there for the rest of college, which at least makes a small amount of sense. P