Whole bottom row, and 2nd to bottom row it is 5 tiles in.
Example:
X=Can't Build at any time; O=Can build if tiles allow
CODE
XXXXXOOOOOOOOOOOOOOOOOOOOOXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
[right][snapback]236328[/snapback][/right]
I think that in the left side there are more unbuildable tiles than in the right, but I'm not sure.