The Xover House is the house next door to Joel and Phil.
In The NYE, there was a New Years' party there. Joel temporarily moved there in Project 2. Waterman, Pal, Roy-Bot & Mr. Dillo also temporarily moved into and completely remodeled the Xover House with a screwdriver (guided by Roy-Bot's heart) in Accursed Meeting.
Although Xover House has only had two such occurrences, "Xover" probably means "X-Over," or "Crossover".