It looks weird that there's no foundation, it's just plopped on some grass right now. It's also odd how the building walls are flush with the cliff walls below. Like Yrr said you should extend it over the cliff a bit so you can add some shape to it.
EDIT: I'm going to reiterate my response to
@theatreTECHIE in more detail. Having such a large building without a proper foundation looks odd, and breaks game illusion. Simply changing the floor material to concrete would do the trick. Like I, and many others, said extending the building over the cliff a bit would give you more space to work with and break up the monotony of the cliff face. In addition, the building is pretty flat. Adding a simple trim, more brush details, and trying to break up the boxy shape would go miles. Moving on to a more detailed level, the main thing to remember per building is purpose, which many others have said. It's not vital to have a clear purpose, just make it seem like everything you add has a reason to be there.