If you are looking for an 18x18 on Atlantic, you may not be able to place your own. I know that there are smaller size plots available on Atlantic, but 18x18 and larger are hard to come by and you may have to buy from a professional. Other shards have large plots available and some even have space in Luna available. My suggestion would be to place a smaller size of whatever you can find yourself and wait a bit to find a larger plot or a better location or have enough funds to buy. There is a house that has runes to IDOCs on Atlantic just outside the east entrance of Umbra. You might be able to place a house after one of those collapses. Good luck!