Two apartments can both be sold as "80 square metres" and one can have noticeably more usable space than the other. The reason is not fraud — it is that the number can refer to different things.
What is included
The net area is what you walk on: the rooms, the hallway, the bathrooms, the kitchen. The gross area also includes the thickness of the walls and, in some cases, calculations that include part of the building's common areas — the stairs, the floor corridors, the entrance.
Buildings in co-ownership have common areas that belong to all owners. When a new building is registered, the common areas are treated separately from the individual units. This means your share of them is real and has value, but it is not space where you can put furniture.
Balcony and terrace
Balconies and terraces are often counted with a coefficient, not in full. An apartment with a large terrace can show a high number of square metres without having more enclosed space than another. Always ask how it was calculated.
How to compare two offers
Ask for the floor plan, not just the number. The floor plan shows the dimensions of each room, and you immediately see whether the 80 metres are three usable rooms or two rooms and a long hallway.
Compare the price per square metre on the same basis. If one seller quotes net and the other gross, you are comparing two different things.
Questions worth asking
Is the number net or gross? How were the balcony and terrace counted? What is the ceiling height? Where do the load-bearing columns run — because they never move, unlike partition walls?
The answers to these four questions give you a far more accurate picture than any single number.