For the next bit of self built warhammer stuff, I made a watchtower. It has three levels, two interior one on top. Each one has space for 25 normal bases, and about 16 larger bases. Both the top level and the top interior level are removable. It is about 5"x5" on the base, and about 8" tall. Strictly speaking this is too big for a single unit, so to follow the rules for buildings you might want to make yours about 3-3.5" square at the base and about 7-8" tall. Also i tend to go overboard on buildings, again due to my architecture background, you do not have to put the interior levels in there, or cut the window and door holes. All you really need is a place to put a few models on top so it looks occupied.
To start as usual you need a base larger than the building you are going to build, another sheet of 1/16" is all you need. Then for the four walls 1/16" works just fine. If you arent cutting holes or making interior floors your job is pretty simple. Cut out four walls that are the same height, make two of the widths 1/8" shorter as the other two walls will go on the out side of these. Now when i made mine I imagined putting lots of detail and effort into it so I went beyond this simple level. I cut windows at about half an inch above the floor level, should be low enough for your models to 'see' out of. I made the top windows an inch tall and 3/4" wide, for the lower window they were only about 3/8" wide to make them more defensible. Of course the lowest level is raised more than an inch off the base so I guess the lower windows makes fighting monstrous infantry a level affair.
For the top level I used another 1/16" for base then added several layers of wooden dowels. the lower two were big 3/8", on the bottom layer i made it several layers deep to make a firing platform, then on the outside i added another 3/8" two deep, and then a 1/4" for the actual parapet wall. This should work to allow you to models on the firing step then deeper in .