For an estimate of additional construction costs, you can enter something like "What does a house really cost" on the largest search engine, then you will find a list at position 6. I found that while browsing around. After a longer search on the web, this was the list that seemed most plausible to me. However, it probably applies to the southern German area. For us, with 145m² KfW55EE with controlled residential ventilation including heat recovery, underfloor heating everywhere, fully basemented, a few additional skylights and lift-and-slide door on the ground floor, heated basement with underfloor heating but without a fireplace, without photovoltaic around 430K. Very affordable for solid construction but at the same time quite simple in standard. The same was offered to me for 500-600K also by a small regional general contractor. Prefabricated timber frame construction (Kampa, Streif Haus house etc. - however at KfW40) were around 450K. Additional construction costs about 90K on top.