So.. this person owns more then one house, or its supposed to be "You own a house"?
Well for some reason English decided to adopt the -s plural for "house" but leave the germanic vowel change on "mice" (Maus/Mäuse). It could just as well have been "mouses" and "hice". I feel like a lot of things in English are kind of funny if you actually look at them. It's such a "Mischsprache" at this point! :D