As Nixcomander indicated, you cannot activate the house transfer feature for a house if there is already a transfer of a house taking place. Transfers of houses are finalized after each SS so you would be limited to the next SS. The other restriction I have found is if you have placed a bid on another house. I discovered that the bids are account specific, so if you have just placed a bid on a new house and which to transfer a currently owned house, you will need to wait 7 days until the bid is finalized, or until you someone outbids you. Unfortunately, the restrictions seem account specific, rather than character specific, so you need to make sure no characters on the account in question has any house bids