Probably the furniture that makes the door openings almost blocked.
They all seem fine, the only thing I see is that the doors can't open inward.
Is it valid when there isn't an NPC in it?Yes, the house checker says that they are invalid housing but doesn't give any other information.
The only house that has an NPC assigned to it is the dye traders house. The others were assigned to houses but are not now.Is it valid when there isn't an NPC in it?
Once, I found out that if a house is the right size, it can be valid for one NPC or town pet, but then it becomes invalid so you can't have both an NPC and a town pet in the same house.