Here we'll talk about things that could cause damage to the public, which a person is not allowed to do. A person is not allowed to make a tunnel under the public domain, under a public street, for example, whether there are pits, ditches, or vaults. Any type of tunnel under the Reshut HaRabim (public domain), under the street, is worrisome because it could weaken the street above it, causing it to collapse and then cause damage to people above it. Rabbi Eliezer, though, allows the digging of a tunnel under the street as long as it's strong enough to hold a wagon that was heavy and filled with stones. If it was safe to travel over that part of the street, he allowed it, because then there's no concern that the street is going to collapse.
Rabbi Eliezer, though, allows the digging of a tunnel under the Reshut HaRabim. A person is not allowed to put poles or balconies that would stick out from a person's property over the Reshut HaRabim, because we're concerned that when somebody passes by, they're going to bump into it and injure themselves on the balcony or pole that's sticking out. Obviously, that's talking about a very low balcony, at the height of a person or somebody who's riding on a camel. A balcony that's very high, above that height, would not be a problem. So, if a person wants to put up a low balcony, the Mishnah says what he would have to do to make this work: he would have to set his house farther back into his own property and then put out a balcony or pole that would stick out but still be over his land. It doesn't go into the Reshut HaRabim. That's the only way he could work it out.
If somebody already bought a house and there were already poles and balconies in the courtyard that stuck out into the public domain, into the Reshut HaRabim, there's already a chazakah. He doesn't have to remove these poles or balconies. We assume that when the seller who previously lived there installed everything, had his house built, and put in this balcony and pole that came out, he did it in a permissible way over private land. Therefore, when he did it, it was okay. Maybe the layout of the streets changed later and that became a problem, but there's a chazakah. Since the status was like that already, he could keep these items. He would not have to tear them down.