No, a mortgage co-signor does NOT become part owner simply by virtue of co-signing, though he or she may have a legal claim against the owner for compensation if, at some point, the owner does not pay the mortrgage leaving the co-signor liable for it. It is possile to make a co-signor part owner, but you have to do that deliberately, and if you are interested in doing this, have a real estate attorney help you--it can be complicated. Co-singing another's mortgage is therefore a bad idea--you make yourself liable for the mortgage by doing so without necessarily getting any interest in or share of the house...just the possibility of suing the owner later, which possibility may be worthless if the owner has little or no money to pay.