For instance, there's a floor and there are four flats on it. Two of them are enclosed with an extra room where both flats can leave their shoes or save supply meters. What do you call this room?
The above describes a communal area.
Communal areas are those parts of a house, block of flats, street or estate which tenants have a right to use in common with other tenants and for which a landlord is responsible.
Fire safety In communal areas of our blocks of flats
Safety in communal areas
Typically, a space that is shared between tenants is a common area (or common room). This is often used in the sense of a shared recreational space, but it can also be a generic term for any room or area that is communal in some sense. (It will often be used on blueprints without a designated purpose, aside from the fact that multiple parties can make use of it.)
However, specific instances may be referred to differently—for example, a shared toilet or communal storage. I have not heard of a word or phrase for a space specific to a utility meter—or to shoes . . .