Following up on this question, I came across an online quotation from the book Chevel Nachalato by Rabbi Yaakov Epstein, who tried to prove that this is not each resident’s individual obligation, but rather a joint obligation of all of them together, from the fact that a partnership with a gentile is exempt as a matter of basic law. For if the obligation rested on each partner individually, it should be obligatory even if one’s partner is a gentile.

In addition, I noticed that common practice is not to affix mezuzot on shared entrances in a building where most residents are secular. For example, I recently moved into a rental apartment where the previous tenant was a Haredi man, and I noticed that he had not taken care to put up mezuzot on the shared entranceways (for example, on the building’s two back doors). The same was true in a previous building where I lived.

And maybe one could also say that the obligation falls on the building committee as a kind of public representative, and not on each resident separately.

Even if the obligation falls on the collective, each individual still has to see to it that it is carried out. I wrote about this in an article, A Good Measure on the Torah portions of Nitzavim and Vayelekh, 5767:
https://drive.google.com/drive/folders/0BwJAdMjYRm7IY0xlc1dmYTMweVE

You wrote, “Each individual still has to see to it that it is carried out.” The question is how to define “seeing to it that it is carried out.” Do you mean doing everything yourself if otherwise it won’t happen? Or should we say that this means voting in favor of allocating resources for mezuzot on shared entrances in building committee meetings, but that there is no need to put them up yourself?

To illustrate the difficulty here, Maimonides writes:
“Therefore the gates of courtyards, the gates of alleyways, and the gates of provinces and towns are all obligated in mezuzah, since the houses obligated in mezuzah open into them.” Would we say here too that even one Jew living in a city/province of secular people would be obligated to put up mezuzot on all the city’s or province’s gates himself? That does not sound very reasonable.

Another point I thought of is that Jewish law distinguishes between a private mezuzah and a public mezuzah:
“A private mezuzah is checked twice in seven years, and a public mezuzah twice in a jubilee period” (Maimonides).
The question is whether we can say that mezuzot on shared entrances in a building have the status of a public mezuzah.

That is exactly what I was talking about when I hesitated whether my obligation to make sure there is a mezuzah means that I am obligated to buy it as well, even if others do not agree.
The distinction between partners and a public is not sharp-cut (and is not even agreed upon). Even according to the views that make such a distinction, it is not clear where the line is drawn. Does it depend on the number of partners, or on the nature of the partnership? I think that when decisions are made by voting and not everyone has veto power, that already starts to become a public. Because a partner who is sole owner over his share is not obligated to do something his fellow partners decided on. Of course, this too is not sharp-cut, and also depends on the agreement between them.

There are other commandments that are imposed on the public, such as building the Temple or settling the Land of Israel. And with those commandments, it seems to me that the individual has no obligation where most of the public is not interested in fulfilling them (correct me if I’m wrong). If so, why shouldn’t we say that the same applies to a public mezuzah?

I found further support for the idea that the individual’s obligation is more limited than the public’s obligation (according to the Rosh):
From the website Lemaaseh: There are two opinions regarding a poor person who does not go door to door. According to the Rosh, an individual is not obligated to provide him with all his needs; that obligation rests only on the public or on the charity administrators. According to Maimonides, however, even the individual is obligated to provide him with all his needs. The Shakh (sec. 250, s.k. 1) wrote that even according to Maimonides, the individual’s obligation to provide all his needs applies only when there is no public body present or when the public cannot afford it. In the case of a poor person who goes door to door, the individual is not obligated to give him all his needs and can suffice with a small gift.

Putting up a mezuzah on another entrance in order to benefit the public is not such a huge expense that an individual cannot bear it. The practical question points in a different direction: will the mezuzah survive on the shared entrance of the building, or will someone very quickly see it as “religionization” and tear it off?

In any case, you have merited and enabled us to merit, and may you participate in the housewarming with words of Torah.

With the blessing, “May your settling be good,” S. Z. Levinger

There is indeed a difference in scale, and also a difference between a collective and a partnership. I do not know exactly where the line is drawn, but building the Temple or an obligatory war is certainly beyond it.
Certainly the individual’s obligation is more limited, and as I wrote, the question is where the line is. The Shakh too speaks of a small gift (how small is “small”? That is precisely the individual’s obligation, which is not like the public’s obligation).
By the way, in my article on pilgrimage, which I linked to, I showed that the individual is obligated to contribute his share toward fulfillment of the public commandment. See there.

Following up on this question, would it be possible for me to declare my share in the common areas ownerless in order to exempt myself from the obligation of mezuzah?
I saw online that this also helps exempt one from the obligation to search for and destroy leaven in the shared areas.

I think in principle yes, if the declaration of ownerlessness is genuine. On Passover this is temporary ownerlessness (and afterward I reclaim it), so people do not care so much. By the way, even on Passover this usually is not done genuinely and therefore is ineffective, because if it were done genuinely there would be a risk that the neighbors would take ownership of your share, and after the holiday you would not be able to reclaim it.

So what is the proper practice on Passover regarding searching for and destroying leaven? To search the common areas of the building?

Yes.

Can one rely on the presumption that people do not leave leaven and food items in the common areas? Or alternatively, perhaps one can do a sale of leaven before the time for the search, so that all the leaven is already sold and there is nothing to search for?

In my opinion, no. Leaven can get there even without anyone leaving it there. The Talmud speaks about mice and other ways leaven can reach all sorts of places.

But in a building of secular people, some of the residents may bring leaven in during the days of the holiday, and maybe leaven is also thrown into the garbage room. What sense does it make to search for leaven in such a situation at all? Maybe one can rely on the sale of leaven for the common areas? Or perhaps on nullification of the leaven? Or maybe declare the common areas ownerless for seven days, aside from the right of passage? (Why did you understand that to be a declaration not made wholeheartedly?)

Oren,

Kudos that the move to a new apartment is occupying you so much on the halakhic side—where to affix the mezuzot in the apartment (in the other question), and on whom the obligation falls in the common areas.

By the way, from the video clip (there, there) it looks like the apartment is really very cute. So I bless you with the blessing “May your settling be good,” as we say in our neck of the woods, and may you merit “A pleasant dwelling broadens a person’s mind”!!!

By Torah law, nullification is enough. But rabbinically there is an obligation to search. Therefore it seems to me there is no avoiding the search, and if someone brings in his own leaven there during Passover, that is his leaven, and certainly the Torah does not place it in your domain so that you would violate the prohibition of possessing visible leaven. So there is no reason to be troubled by that. As for sale of leaven, I am doubtful whether it helps for leaven that has been thrown away and is fit for nothing, but as stated, there is no need to sell it.
I wrote that the declaration of ownerlessness is not serious, because presumably when you declare it ownerless you are not willing for one of the neighbors to acquire your share in the common area permanently. You want to reacquire it after the holiday. If you really do give it up—no problem. By the way, if you are renting, then the whole matter is not your problem, because I assume ownership in the common area is not significant from your standpoint. But it is not yours, and you cannot declare it ownerless. You can perhaps declare your rights in it ownerless, but then the landlord will acquire them, and he should be informed to take care of the leaven.

Following up on this question, is a mezuzah in a building considered a public mezuzah and therefore checked every 25 years, or is it considered a private mezuzah and therefore checked every 3.5 years?

It seems to me that a building is a partnership and not a public, and therefore it is like private domain.
I have now seen here that someone wrote as I did: https://www.yeshiva.org.il/ask/101410

Nowadays, is there any need to check mezuzot at all? Or because they are usually covered in plastic wrap, is there a presumption that they remain valid?

I do not know. I am not familiar with the reality on the ground (how much the plastic improves the situation). Still, according to Jewish law one should place it in two coverings in a bedroom, and I have not heard that they ruled not to check bedroom mezuzot.