Let me start by saying, that the site-fencing is relatively new (30 or 40 yrs. old). I've been in this long enough, to remember when, back in the old days, there was nothing but a few orange cones, perhaps some yellow ticker tape, etc.... around old-town urban demolition. The problem is the litigical world we live in nowadays, and it has nothing to do with md'ing. Ie.: I doubt quite seriously any const. workers, or whomever's in charge, really cares less about some old coins you might find (in fact, most often, they're quite intrigued and think it's cool). But due to the lawsuit happy world we're in now, everythings got a fence around it. Even school yards, so no one can go jog the track or shoot hoops after school, etc....
I'm going to give you a real-world true example, and readers can "read between the lines" and determine what they want to:
There is a local high school here in my town, that was built in 1919. There was an inner grass quad. where.... for 80+ yrs, hundreds of students had sat to each their lunch. One day, the local newspaper ran a blurb about how the old highschool was going to undergo rennovations, rip out some buildings, put in new ones, re-configure, etc... I studied the plans, and found out that one part of the project was going to be ripping out the grass, and doing some sort of new landscaping. You can be I "smelled blood"
So, like you, I thought I'd better "do the right thing" and go "ask", because I feared there would be the obligatory fence put around it, once the project started.
As president of our local detecting club, we went about it totally professionally, sent off our proof of insurances, offered to donate relevent items to the city museum, blah blah blah. No one ever replied. Meanwhile, the project started. From the street view, I could see, each night, the progress, so I stepped up efforts to get a reply from the city, to allow our club members to get to hunt there. To no avail. Each time, they would say it was sent to this dept, and that dept, and under legal review, blah blah blah. Then one night, I could see from the street, that they were starting to rip out the grass. Despair sank in, as it appeared that this project would "come and go", and no one would get in on it. However, one night, I spotted someone detecting in there after the workers had cut out for the day! So I went in, and saw who it was: an acquantance who wasn't in our club, but whom I knew from the area. I asked him "how'd you get in here? Our club has pending permission, etc....". The guy just looked at me like I was from outer space. Turns out, he had just simply gone to the gate, which was held with bailing wire (ie.: a poor excuse for "fencing", but .... oh well), and helped himself. He couldn't what the problem was. He had been in there for multiple nights now, and no one cared less. Yup, in full view of passing cars on Main St. Even after school staff (in the un-affected bldg's nearby) would come by nighly to see what his latest finds were. In other words, no one cared.
He opened up his apron and showed me handfuls of silver coins, wheaties, etc...... That was all I needed to see. I grabbed my detector and joined him.
You can make of this story what you will, but I believe it speaks loudly to a lot of oldtown urban demolitions (sidewalk tearouts, etc...) where there's the obligatory cones, ribbons, or fencing. I truly believe that it's so that you can't slip and sue them. Does anyone really care, or notice, after 5pm? Probably not (until you ask that is).
Now in my example, it was for a public place, so I suppose the issue would be different for a private developer, on a private site.
. Fortunately we were able to work that site for about two or three weeks before we were asked to stop. In the early going all three of us were getting 4 to 7 silvers, a buffalo or two and about 30 or 40 wheats each time out. Even pulled out a couple of silver scout rings. I'm sure if I would have asked in the office they would have told me no.