(A zecher lechurban here refers to the part of a house that is intentionally left unfinished to commemorate the Temple's destruction.)
In an apartment building, does there have to be a zecher lechurban in each apartment, or can a space in the lobby exempt the dwelling spaces inside? Perhaps a spot in a single apartment suffices for the entire building?
I know that leaving a zecher is a mitzvah for the builder (or building), not the resident (i heard this in a shiur recently), so a person who buys a house without a spot does not have to remove a tile to make one. However, the case of an apartment building with several living spaces is not clear to me: Is the entire complex considered as one, or do we view each apartment separately?