We are currently building for approximately €330,000 including everything ([Grundstück], house, incidental costs, outdoor facilities, garage, purchase incidental costs), but excluding kitchen and furniture. We are building 160 m² (2 full floors) + large double garage on a 632 m² plot. I would say with "medium equipment," that is underfloor heating, air-to-water heat pump, controlled residential ventilation, two walk-in showers, electric shutters everywhere or partially electric venetian blinds, etc... With some own contributions (drywall, floors, painting, outdoor facilities, otherwise everything by companies). In RLP, rural region, so no idea if it is comparable to Lower Saxony. But depending on the region, building in this price range is quite possible!