Many co-op owners at the co-op board approval stage say to themselves, “Well, then we’ll just do what we want to do without the co-op board’s approval.”  But don’t even go there.  Some buildings, in fact, will only grant work men access to the building when the co-op board notifies them that a signed alteration agreement is in place.  If you go it alone then, you could find that your contractors show up for work, only to be bounced right back out that elegant revolving door.   Then again, even if what you’re going for is small, the second there’s plaster dust on the hall carpeting, or your neighbors hear a circular saw, both you and your renovation could be toast.