Hey there, I’m a long follower but only playing since Steam EA release.
I have a similar problem, running R375 (x64) on Win8.1.
The predefined templates work, custom buildings and custom templates only sometimes.
It seems, when a custom building gets somewhat complicated, it doesn’t get build.
The problem doesn’t depend on simple shape foundations, unlike HamoPeche described above, i have tried two buildings with similar shape (both rectangular with little extension for the doorstep), one gets build (simple two stories, no stairs, roof), the other doesn’t (3 stories, custom roof, custom stairs).
Will provide screenshots if needed, but would run a test-world for that.
Additional Info: Happened since first day in game, saving and restart doesn’t help.
But, just to say: Great Game even now with the bugs!
EDIT: since I noticed there are also some problems about buildings with 3 or more stories around, I should mention that as long there are custom-built stairs (or anything else with slab tool, I guess) involved, the problem persists for 2 stories as well.