First is a very basic factory/industrial building from Proxie Models:
This is four of the wall panels with a removable roof made from a "garage sale" sign. I also scraped off the huge doorknob. I considered doing internal floors, but I decided to spend my time doing multiple basic buildings, rather than a few with fancy interiors.
Second building will be a four story apartment, still in progress:
Here I've doubled up the Proxie wall panels to make a taller building. You can see the plastic strips I'm using to join the panels. With enough sprues, you could make a building of any height you want.
Last is an old HO scale model railroad house I've had sitting around half finished for a long time. I figure I might as well dust it off, and get it finished while I'm at it:
The main structure is together, I just need to glue in the separate windows and doors.