If you own the property and want the private investigator to go onto the property then yes they can go onto the private property with the property owners permission.
However if you hire someone to investigate and follow someone then no the private investigator cannot go onto the persons private property.
The private investigator must stay on public property such as a public sidewalk or public road and do the investigation from there.
They also cannot go onto the property to take any pictures but can take pictures from the road or sidewalk.
Private Investigators must follow the same laws as regular people do as they are too regular people but just doing investigations.