[hirsin]

They mean the apartment building hallways and elevators. As in, if you summed the square footage of all the apartments on a floor, you'd get a number greater than the actual dimensions of the floor, because the common areas are counted multiple times.

[close04]

That shouldn’t be the case. In some cases it would also be ridiculous, when the common areas are larger than the apartments themselves (corridors, elevators, technical rooms, etc.). They do include the proportional square footage corresponding to each apartment. But this is still very misleading because some buildings have a lot of common areas that can double the advertised size of the very small apartments.

[JoshTriplett]

Oh, I see. Yeah, that's false advertising. "Square footage" of a dwelling should only include your own private space, not any common areas.

[hirako2000]

Could include the size of the pavements fronting the building and the basement with those trash bin also are shared space.