Gemara

(Mishnah): One may cut a tree overhanging the Reshus ha'Rabim up to the height of someone riding on a camel;

R. Yehudah allows up to the height of a camel laden with flax or bundles of shoots;

R. Shimon permits cutting any tree to any height, for Taharah (lest the tree tower above Tum'as Mes and passersby, and make them Tamei).

60a (Mishnah): One may not extend ledges or balconies to a Reshus ha'Rabim. He may recess his house, and extend ledges up to the Reshus ha'Rabim.

If one buys a Chatzer with ledges or balconies extending to the Reshus ha'Rabim, he has a Chazakah (he may keep them).

23a: We claim for him that perhaps the area under them was really part of the Chatzer, or the public allowed the previous owner to build them.

60b (Rav Huna): If the ledges fell, he may rebuild them.

60a: Ploni had a ledge extending into the Reshus ha'Rabim. People complained that it was impeding them. R. Ami ordered him to remove it.

Ploni: You also have a ledge that juts out!

R. Ami: My ledge extends to an alley. The people pardon it. Yours extends to a Reshus ha'Rabim. How will you get everyone's pardon?!

R. Yanai had a tree whose foliage hung over a public domain. Also Almoni had such a tree. People complained that Almoni's tree was impeding them. That night, R. Yanai cut his foliage. The next day, he ordered Almoni to cut his foliage.

Originally, R. Yanai did not cut his tree, for he thought that people preferred the shade. When he heard that it impedes people, he cut it.

Question: If one recessed his house and did not extend, may he later extend?

Version #1 - Answer #1 (R. Yochanan): He may.

Answer #2 (Reish Lakish): He may not.

Version #2 (R. Yakov): All agree that he may extend ledges. They argue about whether he may return the walls (of his house) until the original border.

R. Yochanan forbids. This is like Rav Yehudah, who taught that one may not ruin a path in which the public was Machazik.

Reish Lakish permits. He holds that Rav Yehudah's law does not apply here, for there will remain enough room for people.

Rishonim

Rif and Rosh (32b and 3:77): Some permit extending a ledge into Reshus ha'Rabim above the height of a camel and its rider. We hold that if one recessed his house and did not extend, later he may extend ledges, but he may not return his walls to the original border. This is like Rav Yehudah, who forbids ruining a path in which the public was Machazik. If the ledges fell, he may rebuild them.

Rambam (Hilchos Nizkei Mamon 13:24): One may extend a ledge into Reshus ha'Rabim only if it is above the height of a camel and its rider.

Magid Mishneh: We learn from a tree overhanging Reshus ha'Rabim (27b).

Rambam (ibid.): This is if it does not darken Reshus ha'Rabim. He may recess his house and extend. If he recessed and did not extend, he may extend whenever he wants, but he may never return the walls to their initial place. One may not ruin a path in which the public was Machazik.

Rambam (25): If one bought a Chatzer with ledges or balconies extending to the Reshus ha'Rabim, it has its Chazakah. If it fell, he may rebuild it like it was.

Poskim

Shulchan Aruch (CM 417:2): One may extend a ledge into Reshus ha'Rabim only if it is above the height of a camel and its rider. This is if it does not darken Reshus ha'Rabim. He may recess his house and extend. If he recessed and did not extend, he may extend whenever he wants, but he may never return the walls to their initial place. One may not ruin a path in which the public was Machazik.

Beis Yosef (DH u'Mah she'Chosav Ela, citing the Rashba (3:165 and 176): If Levi opened windows to Reshus ha'Rabim, he merited. His neighbor across Reshus ha'Rabim may not extend a ledge to Reshus ha'Rabim within four Amos of them, or if it will damage Levi through Hezek Re'iyah (invasion of privacy). If there is a custom or government law about this, we follow it.

Beis Yosef (DH u'Mah she'Omar Aval): One may extend to a Mavoy if the Bnei Mavoy agree, like the Gemara says. R. Yerucham forbids if the Mavoy is Mefulash (open on both sides to Reshus ha'Rabim).

SMA (5): A Mavoy Mefulash is like Reshus ha'Rabim. Why did the Shulchan Aruch omit this?!

Beis Yosef (DH v'Chosav ha'Rivash): The Rivash (455) says that if water will drip from the ledge and damage people of the Mavoy, they can protest. Even someone outside the Mavoy can protest if it is below the height of a camel and its rider, or even if it is higher if it damages Reshus ha'Rabim in any way, e.g. it darkens it or deposits water in the middle. If it does not damage Reshus ha'Rabim, only neighbors can Me'akev if it harms them.

Rema: This is only if the Rabim walk there. If not, e.g. he built a platform, even if many pass their loads over it, he may return the walls.

Shulchan Aruch (3): If one bought a Chatzer with ledges or balconies extending to the Reshus ha'Rabim, it has its Chazakah. If it fell, he may rebuild it like it was.

Beis Yosef (DH v'Chosav Rabeinu): R. Yerucham says that if witnesses say that Reshus was like this from the beginning, we do not claim for the buyer.

Beis Yosef (DH Teshuvah): A case occurred in which Shimon claimed that Reuven rebuilt ledges bigger than they were initially. Witnesses testified not in front of Shimon that they were no bigger. If one person from Reshus ha'Rabim was there, the testimony was in front of the litigants, for all are partners in Reshus ha'Rabim. The witnesses need not testify in front of the entire world!

Rashba (2:292, cited in Beis Yosef DH v'Chosav ha'Rashba): Bava Basra 23a says that explicit pardon of the Rabim helps. However, one could say that one cannot appease the Rabim; and we assume that the seller recessed from Reshus ha'Rabim and build a ledge in his property. Nowadays, everyone makes ledges jutting out over Reshus ha'Rabim. No one refrains from this. Everyone needs to do so, pardons it and wants it. They permitted what is forbidden. The king authorized it. Even Stam, it is permitted, and all the more so if he authorized someone. The rulers may close roads and open new ones in place of them. The roads belong to them. It is as if they withdrew with intent to stick out ledges.

Gra (1): Silence of the Rabim does not help, like R. Ami said (about Reshus ha'Rabim) 'how will you get the pardon of everyone?!' However, explicit pardon of the seven leaders of the city, in the assembly of the residents helps, like Tosfos (23a DH Achulei) explains the Gemara there. R. Ami said that silence does not help.