You will have to transfer the house.
It's a pain, which has been introduced to hit "professional" placers who needed to keep open a separate account for placing their loot and sale plots.
Now, with the delay after a house falls I see this becoming rather obsolete.
(Ok, people could still place for loot plots on IDOCs in several areas on several shards, but the quick place and sell mentality is not bringing enough profit any more on the most shards to keep extra accounts running.)
Besides that the current rules make it nearly impossible to help somebody with house placement, if the own account already has a house. Especially if the other person seems to be unable to hit the spot. In earlier times one would have placed himself and traded the house in such situations. And if you are in the time penalty for 7 days, you cannot even "showplace", since the valid plot does not stick long enough for the person you want to help to see where it is. The timer should only hit at the confirm placement in my opinion.
*Salute*
Olahorand