It is indeed confusing.
There are two measurements to caculate living space (Wohnfläche)The Brutto, which is what is calculated when a property is built, and this is what the architects use, then the Netto, which is the space taking into consideration the depth of the walls thereby resulting in the actual floor space. The difference between the brutto and netto can be 10-20%.
There are many
creative people out there when it comes to ads but in theory what is to be counted are heated areas. However, many owners and agents tend to add unheated areas to the calculation if they deem the rooms can be used. This is less of an issue with an apartment. A balcony should not be counted.
A floor plan with room sizes is useful to have but many older apartments don't have the plan and the owners/agents can't be bothered to measure it. Some will use this service (it's mostly for properties for sale) so if you see one of these, don't panic. They are very accurate.
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