Gemara

42a (Mishnah): Locking the fence, fencing or breaching any amount in a wall is a Chazakah to acquire a gift or Hefker or to divide an inheritance.

53b (Rav Nachman): If Reuven built mansions on Hefker property (a convert owned it, and he died without heirs), and Shimon put doors on them, Shimon acquires them.

Without doors, a mansion is a mere arrangement of bricks.

54a (Shmuel): If one takes pieces of wood and grass from a field so it will be easier to plow, or leveled a field so it will be easier to plow, he acquires it.

57a (Mishnah): Raising chickens in a Chatzer is not a Chazakah. If one sets up a 10 Tefachim wall to enclose his animal, or for his oven or millstone, or brings chickens into a house, this is a Chazakah.

(Ula): Only a Chazakah that acquires Hefker property is a Chazakah in another's property.

Objection (Rav Sheshes): Plowing acquires Hefker, but not others' property! Eating Peros does not acquire Hefker, but it acquires others' property!

Gitin 77b: A man was about to die on Shabbos. Rava suggested that he give to his wife (on Shabbos), for a gift, the place where the Get is resting. She should close or open the door there to acquire it through Chazakah.

Rishonim

Rambam (Hilchos Mechirah 1:10): How is locking a Chazakah? The door to a house or Chatzer was open, and the buyer locked it and opened it. He did a usage that helps.

Magid Mishneh: Some say that 'No'al' means affixed a lock or erected doors, but one does not acquire through locking and closing the door. This is wrong. No'al refers to locking. The Gemara in Gitin proves this, and it is the Rambam's source that one must also open it.

Kesef Mishneh: The Tur (CM 192) did not understand why the Rambam requires opening it. The Ramban rejected the proof from Gitin, for the Mishnah says (only) No'al. The Gemara said 'lock and open' only because this is typical. I say that the Rambam agrees that locking alone acquires. The Gemara taught a bigger Chidush, that she may even open it again like it was before, and it is not evident that anything was done. Even so, she acquires.

Rambam (Hilchos Zechiyah 2:9): If Reuven built mansions on Hefker property, and Shimon put doors on them, Shimon acquires them. Reuven did not do anything to the land itself. He is like one who made a rockpile, who does not acquire. This fence does not help, for it is too wide and open at the ends. The form of the building is useful only once doors are set up.

Magid Mishneh: The Rashba says that the Ri mi'Gash says that Reuven did not dig foundations. Digging prepares the land for planting, like plowing. Even if he did not plant, he acquires. If Reuven made a place to keep animals or chickens, he acquires, like the Mishnah (57a).

Beis Yosef (CM 275 DH u'Mah she'Chosav u'Chevar, citing the Ran): The Re'em says that this is when Reuven did not dig foundations. It seems that the Rambam agrees, for he says that Reuven did not do anything to the land itself. The Ramban and Rosh say that digging foundations acquires only land destined to be plowed. We rule like those who disagree, for they are the majority and are greater (Chachamim).

Gra (22): We learn that plowing acquires from 54a and 57a.

Magid Mishneh: The Rambam did not specify whether Shimon acquires only the land, or also the building. I do not know how he acquires the building, unless we say that what one builds in Hefker is Hefker, since he did not make a Chazakah. The Ri mi'Gash says so.

Rosh (3:59): Rava said that a woman can close and open the door to acquire through Chazakah. We cannot say that he told her to affix a lock, for it was Shabbos. She acquires just through locking. A Mishnah teaches that bringing chickens into a house is a Chazakah. Ula taught that only a Chazakah that acquires Hefker property is a Chazakah in others' property. We challenged Ula, but not from the Mishnah. We must say that he brought chickens in and locked the door in front of them, and this acquires! However, perhaps this is considered eating Peros, and the Gemara challenged Ula from this.

Piplulei Charifta (3): One raises young chickens in a Chatzer. He need not lock in front of them, therefore, it is not a Chazakah. One must lock in front of big chickens. He brings them into the house, for one cannot lock in the Chatzer, for people from other houses must enter and leave.

Rosh (61): If Reuven built mansions on Hefker land, and Shimon put doors on them, Shimon acquires them. Even though Reuven dug when building the foundation, digging helps only for farmland. Even though the Rashbam explains that if one improved land for a fixed granary, he acquires, there is different, for he completed his intended improvement to the land. Here he dug the foundation to build a house, and he did not complete his intent, i.e. a house to live in.

Rosh (ibid. and Gitin 8:3): Some say that also opening a door acquires. It says that she should lock or open it. Normally, one opens a locked house only if it is his. I disagree. This is unlike breaching, which fixes the opening to enable entering and leaving. Merely opening to allow people to enter is not a Chazakah. It is unlike locking, which prevents people from entering. The word 'and open' is imprecise. Or, the door was locked, and she needed to open and close it. The Gemara said 'close and open' for people normally say it in this order. Alternatively, just locking does not prove that it was done for the sake of a Kinyan. Perhaps it was like she normally closes her husband's house, to guard the contents. Since she immediately opened it afterwards, this shows that it was for the sake of a Kinyan.

Nimukei Yosef (28b DH Linvei): Reuven receives like one who worked on another's field with permission. He has the upper hand. Surely, he intended to acquire the bricks through building them.

Poskim

Shulchan Aruch (CM 275:21): If Reuven built mansions on Hefker property, and Shimon put doors on them, Shimon acquires them. Reuven did not do anything to the land itself. He is like one who made a rockpile, who does not acquire. This fence does not help, for it is too wide and open at the ends. The form of the building is useful only once doors are set up.

Rema: Some say that the same applies if Reuven erected doors but did not lock them, and Shimon locked them.

Beis Yosef (DH u'Mah she'Chosav u'Chegon): The Rosh and Tur say that locking acquires, but just erecting doors does not. The Rambam connotes like the simple reading of the Gemara, that erecting alone acquires.

Rema: Some say that this is when Reuven did not own the wood and rocks, rather, they were Hefker and he intended to acquire them only through building, or he built through workers who did not intend to acquire. However, if Reuven acquired them, or they were his from the beginning, he can say 'I will take back my bricks', so Shimon's Chazakah is void, and the land is still Hefker. The first to acquire will acquire it. Some say that even if Reuven built with his own wood and rocks, Shimon acquires everything. Reuven does not acquire when his building does not help at all. If he made a building suitable for animals or chickens or he dug foundations for a building and built, he acquires the land through building. Some disagree and say that he does not acquire through digging foundations.

Tur: R. Yeshayah says that even if the wood and rocks were Hefker, Reuven acquired them through Hagbahah (lifting them), so Shimon must pay him for them. Since Reuven did not acquire land, he is like like one who worked on another's field without permission. The Ramah says that if the wood and rocks were Hefker, Reuven intended to acquire them only through the building, which he did not acquire. Therefore, Shimon acquired everything. If Reuven owned the bricks, or they were Hefker and he intended to acquire them, he did not acquire the land. Also Shimon did not, for Reuven can take back his bricks, so the door did not help at all. Therefore, if Reuven removed Shimon's doors and put up his own, he acquires. If not, Reuven takes back his bricks, Shimon takes his doors, and the land is still Hefker. The Rosh says that Shimon acquires only when Reuven built through workers who did not intend to acquire

SMA (27 and Gra 18): One who builds without permission can take back his wood and rocks. (Bava Metzia 101a explicitly says so.)

Gra (192:5): The Poskim hold that locking alone acquires. The Rambam requires to lock and open.