Here are my experiences from building a house in 2014:
Costs per sqm: For us, it was SIGNIFICANTLY less than 2000 euros, if you base it on the pure house construction costs including planning (approximately 1500 euros). We built solidly according to the KfW70 standard. However, although we only used good branded products (Villeroy & Boch, high-quality country house doors, burglar-proof windows, etc.), we did not invest in luxury extravagances. Certainly, it also varies greatly regionally (we built in Niederrhein (NRW)).
Basement: We decided against it. We were told something about approximately 40,000 euros, and honestly - for what? We have a huge garage, garden shed, carport - that’s enough. We don’t need to store 3000 liters of heating oil or coal, nor do we need a space for an old kitchen that we don’t want to throw away. But of course, that is also a matter of philosophy - and if money were no object, we probably would have taken one. But for us, the price-performance ratio was completely off.
Interest rates: As was already evident from previous speakers - anyone who makes sure predictions here is either a charlatan or really has the insight that will probably make him/her a millionaire... I think one should calmly approach private planning, not let oneself be driven, and once everything is set, just see what the market offers.