I like your general building design, nice work dude.
Alright as your actually asking for tips to improve i will do my best to help.
First up i think the building needs more character, at the moment they actually look like squares, so maybe add pipes, sceatches, bad doors, rims.
The windows are really bright and distract the viewer from the rest of the building, you want them to hopefully focus on the entrance, so maybe make a sign above it?
Work on a green background or something, and start adding some lights, using white to make the shines and such, for example on the vent. Cross ways shines to make it look more interesting.
Also, make the centre of the building come out a bit further so its not so perfectly in posotion. Maybe add some stairs to it.
Little things like that help the artwork and it slowly builds up
Hope that helps a lil