One may not dig a hole under the public domain —- pits, ditches, or vaults. R' Eliezer permits [it] if it is [strong] enough for a wagon loaded with stones to ride [over it].  One may not extend protrusions or ledges into the public domain. However, if he wishes, he may recess [his wall] inside his own property and extend [them]. [If] he bought a courtyard in which there are protrusions or ledges, they retain their status.