Currently(or the last time I checked) brick ownership is saved via a sort of stack system. All of the bricks in a stack inherit the ownership of the brick at the bottom of the stack. So if Blocklander A creates a house and a yard, and then Blocklander B build's a treehouse on Blocklander A's lawn. Then the ownership of Blocklander B's treehouse is lost when the build is saved. So when they return the next day to continue building, Blocklander B cannot continue building his treehouse without some sort of add-on, trust from Blocklander A, or a bit of tricky saving/loading. This can be a real problem with large community builds and any sort of events that use named bricks.
I'd like to see some sort of an option to save ALL ownership in the default game. Maybe a "save as > Commmunity Build" sort of feature.