Taken from a post made by Greywolfe on the UO.com forums
QUOTE
If you are doing an in-place conversion and keeping the old footprint, the process is 100% secure. If you are trying to increase the size of your house's footprint, however, it is necessary to actually pick up your house and replace it. That process is not secure, just as it is not secure at this time.
So basically if you plan on upping the size of your house better clean it out prior to AoS release so you are prepared or your neighbors might expand and dash your hopes of becoming a land baron.