I have hunted several sidewalk projects in the past. My approach is to get the OK from the construction forman. If anyone questions me, I tell them that I got permission from the forman. However, last fall I was hunting a location where I had permission to hunt and apparently someone called the police. The officer informed me of the complaint and apologetically suggested that I get permission from the City Administrator. By the time I got his permission, the project was completed.
In another town I got the business owners blessing as well as the Mayors. As a thank you, I and several other people who were detecting there presented the city a display of coins that were recovered. Because of that gesture, I was invited to the Rotary Club to give a talk on the hobby. It was a full house and many of the members invited me to detect their property.
I also know a construction owner who allowed me to hunt on a project he was contracted to complete.
I gave him several common date coins as a "thank you". He gave me his business card, and now calls me to let me know of any projects he is on and invites me to come and detect. Good luck!