The local prices, which were confirmed to me again at the last construction fair (and within which range I have a concrete offer), are probably in the range of 1,200 - 1,300 EUR for the standard program (i.e. underfloor heating, no controlled residential ventilation, gas heating, including earthworks for ground class 3-5, natural stone window sills, reasonable simple electrical installation with network/SAT wiring, crank shutters, finished except for floor coverings in living and sleeping rooms as well as wall paint). The provider from whom I have the offer of almost exactly 1,250 EUR per sqm seems to be reputable according to several internet construction diaries and the price is realistic (i.e. no hidden costs, additional charges, etc.).
Extras of course cost extra, but I would not calculate those per sqm. A nicer staircase is a nicer staircase, completely independent of the house sqm. An air heat pump instead of a gas heating system, nicer sanitary fixtures, better interior doors, etc.
For 150 sqm I come to 188,000 EUR PLUS extras (controlled residential ventilation 10K, air heat pump at least a few k EUR, etc.).
Since most new build apartments here for families have 4 rooms and 90-100 sqm, I do wonder whether 150 sqm is a bit too wasteful.
And of course I mean it seriously, I neither want to live long-term as three people in a 2-room apartment nor do I want to let my binding land reservation expire. The problem may really be that I have no budget x and I do not necessarily want roof form y. In the end, I want a solution that puts price and performance into a reasonable relationship. Diminishing marginal utility, etc. (1 children's room is a must, 2 children's rooms are actually also a must, 3 children's rooms would be a nice gimmick, 7 children's rooms are too many - how much is the first and second children's room WORTH? How much is the 7th children's room WORTH? Little tip, construction costs are always the same, the VALUE for us and for most other families is totally different between the 1st children's room and the 7th children's room).