Not so bad for your first architecture visualization.
Here some recommendations:
-Number 1, 3 and 4 are almost the same. Only post your favorite one

-Shift verticals.
-Do realistic grass, at the moment with octane the easiest way is to do in postproduction. Do the same with the tree in the foreground.
-Be carefull with the size and wraping of the texture. Textures seems to be to big in some cases.
-Add more details, I think some forms are not right in its proportion (the reailing for example).
-There are some good inspirations for architecture on Ronen Bekermans blog:
http://www.ronenbekerman.com/
-...
I hope it helps
Kind regards
Alain
Intel Pentium 2.8 GHz 2 Cores, 8 GB RAM, GeForce GTX Titan 8GB, Blender 2.72b, Win 7 64 Bit