I read somewhere (maybe it was the "Possessives" lesson?) that some key phrases don't follow the normal form for possessives, and "casa mia" was the example. So instead of saying "la tua casa" you would say "casa tua"...I guess. :)
From how I have understood this, saying casa tua puts the emphasis on tua. I know we are used to putting possessives in a certain order, but when emphasizing homes, the order is different. Casa tua = your home. Casa mia= my home.