This building is interesting. It looks very much like the real buildings you would see in most downtown areas around New York or San Diego. You've also gone for what appears to be a Second Empire Style skyscraper.
As you may be aware, Second Empire, originating in the mid-1800s is recognized through its taller, more governmental form, with steep mansard roofs, possibly a few gables, and clearly sectioned layers. You've got that down, it seems.
In recommendations to improve this, I would give you a few suggestions regarding the building itself, and then the roof separately. For the main building, you may want to work on using a few different but similar materials. You've got your off-white wall material through the entire 13 or so stories. I would break it up. Perhaps for that large middle layer, use a slightly different color, such as a beige or slightly darker grey. The purpose of this is to draw attention to different sections of the building, rather than allowing the viewer to see it as one large box.
Your window design appears to be all the exact same two 1x2 windows put together. Break it up some. Use some 2x1, or 3x2. Make it seem a bit more interesting. The style you're using is a bit more flamboyant, so let it breathe.
As for the roof, you've got the mansard down pretty well. I would take the angle down just a tad; let it angle a bit more than what you've got (currently, it appears to be around 80 degrees; take it to about 75). Perhaps give your dormers a bit more trim. And if I might recommend a trim for your mansard roof (only because the green wool doesn't have a lot of texture to it), I'd recommend stone brick.
To recap, I like the overall style. You've got that down really well. I'd recommend mostly getting rid of the monotony in certain areas, and taking that roof down a bit. Other than that, it's a nice office building, and I'd love to see an interior!