I was curious how many houses on my server are being occupied by inactive players and FC's, so I started poking around Mist on Ultros with the following methodology:
If a house is attached to a FC or person who has been online since Heavensward launch, then I consider it active. If it's a FC, first I see if anyone is online, and if not then I check Lodestone to see if the FC has a weekly/monthly ranking. If they don't, it's an inactive property. If it's a single-owner house, then I check if that person is above level 50 on any class. If not, then it's an inactive property.
So far, I've counted 8 inactive properties in Ultros Mist Ward 1, which is a rate of 13% inactive on what seems to be our most popular Ward. I only expect that number to increase as I investigate other Wards.
Feel free to post your server's numbers here, but please don't post specific houses, as that may be a privacy issue.