However, the problem is, that most of the time, from most any demolition contractor, is that you'll get a "no". Even if, truth be told, the thought never crossed their mind, nor would they have glanced twice, and are long gone by 5:30pm, etc... Because the moment they say "yes", they have images of legal ramifications, liability, etc.... Hence the easy answer is most always "no".
So while it's true that some city or contractor's Permission slip gives "peace", yet you'll find yourself precluded from most good demolition sites. We had this problem in our club, where those coming in with the goodies at each month's show & tell, were sometimes met with "How'd you get in there to hunt ? We asked and got told no , so ... how is it fair that you hunted there and we couldn't ?" ( for example at a public school's demolition project)