Since we are currently also building a basement apartment which is not entirely dissimilar to what you are planning (approx. 155 sqm, 28 cm WU concrete exterior walls, 14 cm insulation outside as well as under BP, except in the garage area approx. 25 sqm, since outside the thermal envelope), here is what we have to invest:
Shell construction (prefabricated elements, incl. interior walls and 3 basement windows): 102 TEuro
Sanitary drinking/waste water: 7 TEuro
Underfloor heating (everything except garage and utility room): 9 TEuro
Controlled residential ventilation (only pipes, system itself separate): 7.6 TEuro
We are already at about 125 TEuro, and the costs for screed, floors, electrical installation, wall filling/painting, possibly suspended ceiling, staircase basement-ground floor as well as additional costs for excavation and possibly disposal compared to BP are not yet included.
The 150 TEuro are therefore in my opinion quite realistic, 200 TEuro on the other hand probably a bit too much – but this certainly also depends on the region (we are in a rather inexpensive area).
Wow, those are already quite steep prices you have there.
since the wall construction would be simpler, contains only a few small basement windows, the interior fittings are more spartan, no particularly elaborate technology would be installed (pipes not concealed, etc.), no bathrooms would drive up the average price per square meter, we had simply assumed that it might roughly add up.
But you are planning an expensive heating system in the basement... instead of 160 sqm in the main apartment, 150 plus basement replacement room and attic conversion – which, unlike the basement, is already present – saves huge amounts of money.