House construction planning for a single-family house of approximately 170-180m²

Strange recommendation. Personally, I much prefer the tutti kompletto indication as a rough guideline (because all of this is just rough).

I’m happy to explain it to you. The 2500 includes constant basic costs and standard. Cellar costs vary depending on the soil, as do ancillary building costs. Equipment and garden are also very variable. It doesn’t make sense to calculate with a flat rate. It is also not common.

This is a detailed and explained statement!

I just wanted to comment first that 3,500 EUR / sqm is not entirely implausible. In my case, the price includes everything except the outdoor facilities (furniture and lighting of course not included) - included are incidental construction costs as well as the following equipment: Viessmann heat pump, Zehnder ventilation system, Schüco aluminum windows, parquet and tiles (120 EUR/sqm), sanitary (20k per bathroom). The whole thing is in monolithic construction with aerated concrete in 42 cm blocks (due to KFW 55).

Since I have not yet decided between a prefab house provider and an architect, I wanted to ask you whether my construction project is feasible from your point of view with a budget of max. €450k?

I know this problem. My opinion or current experience: The cause is high demand coupled with high profit margins that are being exploited as a result. Then there is raw material shortage and constantly increasing bureaucratic efforts (regulations, expert reports, etc.). Unfortunately, there are only a few architects and construction managers who manage to slow down suppliers and trades in margin skimming. They exist, but they are hard to find. Building a house is no fun at the moment. My first cost estimate by an architect for a similarly sized house was €600k. I asked him if he was aware that I already own the land. Such prices are simply ridiculous.

I just wanted to comment first that 3,500 EUR / sqm is not entirely implausible.
In my case, the price includes everything except the outdoor facilities (furniture and lighting of course not either) – included are ancillary building costs as well as the following equipment: Viessmann heat pump, Zehnder ventilation system, Schüco aluminum windows, parquet and tiles (120 EUR/sqm), sanitary (20k per bathroom). All this in monolithic construction with aerated concrete in 42 cm blocks (due to KfW 55).

But one has to say that this is far, extremely far, ultra far beyond standard and has nothing to do with upscale anymore. Tiles/parquet for 120 euros per sqm, respect. Schüco aluminum windows are no bargain either...