Prefabricated house or solid house BW

Yes, although the price increases are already factored in. The house is currently around €280,000 and ancillary construction costs are calculated rather conservatively when I compare my figures with those from the internet and the providers. I hope there are no BIG surprises.

Build first, then you'll see - I wouldn't go around with such numbers beforehand

Are there opinions and suggestions on which is the best or most efficient option for heating/ventilation? With KFW55, you probably can't avoid an air-water heat pump, but is controlled ventilation necessary or is decentralized enough? Is solar or photovoltaics a must? I am a bit clueless regarding heating because the prefab house manufacturers recommend very different things.

Suggestion: Geothermal energy as the most efficient heating. Why? Because we have it ourselves and only have about €55 monthly heating and hot water costs. Five people and 300 sqm heated area (= 250 sqm living space) in an approximately KFW 55 house. BUT: If I consider the higher investment and maybe compare it with gas, the total costs for 20 years might still be higher. Efficiency is therefore not everything. P.S. We of course also have controlled residential ventilation.