Character mistake: In the start of the film, the sticker on Peter's head for his work misspells the word Bleecker Street on it.
Continuity mistake: Peter back flips off his moped to avoid the car racing at him. There were cop cars and other vehicles behind that car when Peter jumps, but when he lands only one cop car goes past in the lane next to him and the lane he's standing in is then clear of traffic as he talks to some kids.
Answer: While it's never mentioned in the film, the most likely explanation is that Harry, as a rich kid, grew tired of "slumming it" and decided to move back into his mansion.