Like the title asks, what decides which house the next monarch is to be a part of? This is something that's always confused me about the British monarchy. Thanks!
It was from his father. Victoria was part of house Hanover, but Edward VII took the house of his German father, as was customary in those days (just like a last name). Up until this point there hadn't been any English Queens with issue - Mary I and Elizabeth I of house Tudor and Mary II of house Stuart had no children, while Anne of house Stuart had no surviving children. Elizabeth II also decided her children would keep her house name, which is why Charles is of house Windsor, rather than house Schleswig-Holstein-Sonderburg-Glücksburg (Prince Philip's house).