Prices can vary surprisingly by region, where do you want to build? Nevertheless, given the described key data, I also consider the 310 TEU to be quite high. Provided the design is not too complicated, e.g. recesses and projections, cantilevered floors, balconies, etc., in my experience, rather 240-270 TEU are appropriate (excluding interior painting work, floor coverings, and garage) – for a living area of about 150 m2 with a basement. However, the mentioned price range only applies if the list of special features is not too long. For a single prefabricated garage, around 6,000 euros will be added – depending on size and equipment.
The air heat pump (air-water heat pump) usually replaces the gas heating and sometimes also a solar system, so the additional costs from this equipment mainly arise from the necessary underfloor heating, but not to a significant extent. With increased wall thickness or improved insulation and the air heat pump, substantial expenses for the KfW-70 standard are already included, although additional insulation in the basement is required for the KfW-70 standard.
The mentioned own contributions of tiling, planning up to the building application, and interior/exterior painting work actually do not bring savings at the hoped-for level. Planning costs do not play a big role for the general contractor anyway, especially since he assumes he will have to carry out further planning in any case.
I advise caution with the own contribution "exterior painting work" if it concerns the exterior plaster – because this involves a system structure. In the event of damage, it can be difficult to clearly determine responsibilities.