I'm not sure, but it seems like the requirements for valid housing have changed in Terraria 1.2.
I usually build a big central chamber like in the image below, but since the 1.2 patch this setup doesn't work. The query tool give the "This is not valid housing" message.
The room is enclosed by bricks and platforms. It has a door, chair, table, background walls and light sources. This was usually enough for valid housing.
Have the requirements for valid housing changed in Terraria 1.2? If so, what are the new requirements? Or am I just doing it wrong?
For those that don't find any answer in the "duplicate" question: Rooms have a max size. The exact maximum is elusive since there are a lot of variables. This forum post has a nice guide about house size.