see below if you have time
I do not think your context concerns the payment for the use of common areas, but rather how much of these common areas may be used by the various renters. Payment for the proportional use of the common areas is probably calculated into the price per square meter of the rented floor space. Based on the typical German thinking about "Wohnfläche", some sort of living accommodation space is probably being referred to here but I do not think it matters at all whether we are talking about:
1. flats / apartments where the common area might be attic, laundry drying, cellar, garage or yard/garden space, or
2. shared flats/apartments where the common area might be kitchen, bathroom, or lounge/living room space, or
3. farming land where the common area might be barn, silo, shed or workers barracks space, or
4. industrial park space where the common areas might be display, sign, parking or storage space.
5. office building space where the common areas might be reception, display, waiting room or parking space.
The point remains that each renting / tenant party pays rent for as much useable floor area as stipulated in her/his/their rental contract. If one renter / tenant pays for three times as much useable floor area as another renter / tenant, then the first renter / tenant can also use three times as much of the common area. The number of people in a renter / tenant party is irrelevant. The landlord / owner / property management company does not care a hoot how many people are using the space rented by the various renters / tenants.
Example using living accommodation space in one building with a useable floor area of 200 square meters:
Renter / tenant number 1, a family of any number of persons with a good income, rents a large flat / apartment with a useable floor area of 150 square meters.
Renter / tenant number 2, a poor family of any number of persons, rents a flat / apartment with a useable floor area of 50 square meters at the same price per square meter.
There are no other renters / tenants or flats / apartments in the building.
The common area consisting of attic, laundry drying, cellar and garage space comprises 100 square meters.
The members of the renter / tenant number 1 family get to use 75 square meters of the common area.
The members of the renter / tenant number 2 family get to use 25 square meters of the common area.
It would not matter in the least how many people were in the above families, all that matters is the ratio of the rented space to the total space available for rent. It is also unimportant how much common area there is because the ratio does not change. If, in the example above, there were only a total 40 square meters of common area, renter / tenant number 1 would be allowed to use 30 square meters and the poor folks, renter / tenant number 2, would only get to use 10 square meters.
Sorry this answer is so long but there seemed to be some confusion about what was meant in your context
- HTH - Dan