Build one area list for the whole building
Start with a single list covering everything: street frontage, entry, lobby, stairs, lifts, corridors, common washrooms, each tenancy, the loading area, bin room, car park and any external hardstand. Do not organise it by who pays, because that is the thing you are trying to establish. Against each area put the party who currently instructs cleaning there, based on the documents you can actually see rather than on assumption.
The result is usually revealing. Common washrooms and lobbies tend to be firmly covered. Rear entries, service corridors, bin areas, external glass at tenancy level and the strip of footpath outside are frequently claimed by nobody. Those are not exotic areas; they are simply the ones that fall between a landlord's specification written for the common property and a tenant's scope written for the demise.
