If it is, whats the first meaning that you get from it?
It is not correct. You can have a “large building” or perhaps a “medium-large building”. You cannot have an “appearance building” or a “medium-appearance building”
You can say “medium-large” since there is a clear, one-dimension scale of “size” from tiny to huge. You can be larger, or less large.
However, “appearance” is an abstract noun. There is no meaning to “more appearance” or “less appearance”, so a building cannot be “medium appearance”
You can say “nondescript building” or “a building with an average appearance” (but this is rather unclear) or “A building in a moderate state of repair”, for example.