Nuff Sed
It might be illegal ONLY if you don't have whatever permission is needed from the owner and whoever regulates construction of "small shacks", if anyone. Not sure what "non-private/non-commercially owned" means. A commercial owner IS a private owner, since they're not a GOVERNMENT owner. So, maybe you meant "government owned"? Yes, there are thousands of shacks built upon government land legally. So, there's that.
?
Non-private? You mean it's government property? If it's not your (private) property, you have no right to build there.
Slickterp
It would be illegal because you do not own the land. Therefore, trespassing.
Maxi
Laws depend on where in the world you live............. unless the land belongs to you generally it is illegal to build anything as you don't own the land however even if you did own the land, you may need to seek building permission
princess pounder
You can probably get away with it for awhile till the owner finds out. Here in mi there are cabins in remote areas on federal or state forest land. But it is nearly wilderness conditions used only by hunters or hikers.