Years back I made a runebook which broke down the major housing areas using that with UOCartographer (UOAutomap is now more or less gone) I do a pretty systematic search.
Not only are people in and out of the shard or game a good bit but they are also upgrading their housing. Sometimes they are moving into larger digs or others are moving to a place with less spawn.
If, for instance, you saw a house which has gone into decay that is 15x15 and looks empty mark the spot and check back. Don't wait until the IDOC time. The owner may have upgraded and return to drop the house.
I dug my heels in years ago and refuse to pay for a spot. I was persistent and contiued to upgrade and now have 2 towers and a custom 18x18 for my 3 accounts.
Placement tools are blessed so there's no reason not to have one in the backpack of every character!
Good luck and never, ever, give up!