So, having some time off a couple weeks ago, I looked into IDOCs and stats. Here's what I found...
45% of Somewhat Worn houses went to Fairly Worn.
54% of Fairly Worn houses went to Greatly Worn.
55% of Greatly Worn houses went to IDOC.
65% of IDOCs actually fell.
So, for every Somewhat Worn house you find, there is about a 9% chance it will fall.
* although there is a selection bias here, since I didn't track any houses I knew to be occupied by active players. So apply this number to houses you find where you don't know of the owner.