(assuming your current house is a small)
1. Put 8m Gil onto your new character and buy an apt in same ward as your current house (so you can teleport slightly faster to the plot)
2. Plant them next to a small plot that has been sitting for a while (so it's not 3m, but 1.5-2m) log off character
3. Take your old character and self demolish your current house (if it's a highly desired plot like a beachfront then do this very late at night so no one steals it)
4. Log back onto your new character, buy the cheap plot in front of her for 1.5-2m gil. Build house estate.
5.Now, warp to your new character apartment (in same ward as the old house) , run up to the freshly demolished old house and buy it and choose relocation. The end