Construction costs net/gross? What counts towards living area?

Good day, we would like to have hopefully a simple question answered. For the construction costs often stated here in the forum of approximately €1800 plus additional construction costs and outdoor facilities+carport/garage - is this amount net or gross? If gross, it would thus be about €290,000 for a 160 sqm city villa (as a reference). Otherwise (if VAT is added) about €343,000. For the sqm of the calculation, are the total sqm of the house (including utility room, hallway, staircase) used or only the living area? What counts as living area (or what does not)? Thank you very much!

To my understanding, these are always gross values. For the calculation, I would include all square meters, including utility rooms, hallways, etc. After all, flooring, heating, electricity, sockets, etc. have to be installed everywhere. For the calculation of living space, there is even the Regulation for the Calculation of Living Space. In principle, all space is living space where you are not restricted by height, because (for example, sloping ceilings) then regulations apply.

Every QM will be used for calculation.