How do you say your house is our house in Spanish?

"De mi casa a su casa" (if you wish to be formal/polite); "De mi casa a tu casa" (if you're on more friendly terms)

De mi casa, a tu casa

nuestra casa es su casa.

The translation of the polite phrase is "Mi casa es su casa."

"Su casa es su casa". In Spanish, the posessive pronouns for "Your" and "their" are the same. To avoid confusion, you could say "La casa de usted es la casa de ellos".