A man may not open his windows onto a jointly owned courtyard. [If] he buys a house in another courtyard he may not open it onto a jointly owned courtyard. [If] he builds an attic above his house, he may not open it onto a jointly owned courtyard. However, if he wishes, he may build a room within his house, or build an attic above his house, and open it into his house.  A man may not open onto a jointly owned courtyard a doorway opposite another doorway or a window opposite another window. [If] it was small, he may not enlarge it; [if there was] one, he may not make it two. However, he may open onto the public domain a doorway opposite another doorway and a window opposite another window. [If] it was small, he may enlarge it; [if] there was one, he may make it two.

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.