Baba Basra 3:7-8

Baba Basra 3:7

A person may not install windows facing a jointly-owned courtyard. If a person bought a house in another courtyard, he may not open an entrance from it to the jointly-owned courtyard. If one built a second level to his house, he may not open an entrance from it to the jointly-owned courtyard. If he desires, he may build a room in his house or a second level that opens into his house (and the neighbors cannot object). One may not install a door facing a neighbor’s door or a window facing a neighbor’s window in a jointly-owned courtyard, If a door or window is small, he may not enlarge it, nor may he turn one door or window into two. One may install a door facing a door or a window facing a window in the public domain, as well as enlarge a small one, or turn one into two.

Baba Basra  3:8

A person is not permitted to dig under the public domain, wells, ditches or caves. Rabbi Eliezer permits one to dig under the public domain so long as it can still support a wagon full of stones. One may not make a projection or a balcony that extends into the public domain but if one likes, he may retreat onto his own property and build something that extends. If a person bought a courtyard that had pre-existing projections and balconies, they are covered by precedent (so the new owner cannot be compelled to remove them).