How to build a massive single-family house most cheaply?

That alone is absolutely sufficient,

Just let the OP answer!

Thanks already for the opinions here! The assessments on the topic of "playing site manager yourself" are quite clear.. and they also make complete sense to me, but sometimes you just don’t want to fully admit it.

PS:
Better to do without parquet, an expensive designer kitchen, Q4 smoothed walls, expensive designer ceramics, built-in fittings, and whatever else, than to have 10m² too little in the end.

I totally agree with you here. We don’t need a pantry (well-planned kitchen with lots of storage space (e.g. tall cabinets) is sufficient in my opinion). A kids’ bathroom I find totally over the top, I wouldn’t even consider it. A guest bathroom with a shower on the ground floor is perfectly sufficient in my opinion. We also don’t need a walk-in closet in the bedroom. Essentially, we just want enough space for 5 people, reasonably large rooms for the kids, and possibly a small office if it still fits. Otherwise, the desk will be integrated into the bedroom, we’re rather pragmatic in that regard. We don’t need luxury ceramics in the bathrooms either.. we want everything more standard instead, and rather a bit more space.

The budget is around €540,000, including ancillary building costs and everything else that may be added. I know it could possibly be tight.. but still, I find it hard to get a really realistic price estimate. In my experience, it’s all very black-and-white, either "it doesn’t work at all, forget the project with this budget" or "sure it works, if you make compromises here and there"...

We’re not even at the concrete quotation stage yet, currently it’s more about whether we should even buy the plot or just drop the whole thing...?

Budget is approx. €540,000, including incidental construction costs and everything else that comes with it.

Including or excluding the plot? If the former, how much does the plot cost?

We don’t need a pantry (a cleverly planned kitchen with plenty of storage space (e.g. tall cabinets) is sufficient in my opinion).

What is above eye level (and even more so above head height) is not really needed. Do you want to get a ladder to grab a packet of pasta – or even be hit by cans?

currently it’s more about whether we should even buy the plot or just drop the whole thing...!?

Oh, the plot at three quarters of an hour away from your current residence (would it at least be more conveniently located for work and school places?) isn’t even yours yet?

Including or excluding land? If the former, what is the cost of the land.

Excluding land (and real estate transfer tax etc)

We are currently in the final phase before the start of construction and trying to do a lot ourselves. But this also involves coordinating earthworks, all house connections in the correct order, independent calculation of the heating system, electrics including KNX, and so on and so forth. We have the advantage that the husband understands the trades of electrical, sanitary, and heating, and therefore, with some help, he works everything out himself and is capable of it. I work in the building materials trade and can always ask someone. The crux is: For everything you are responsible for yourself (which it would be as a construction manager), you have to familiarize yourself so thoroughly that you know exactly what is supposed to happen. That starts with proper (meaning complete) tenders and ends with a functioning timeline. We both work full-time and our son is 6 years old. We have been completely at our limits for months – you can't just "learn on the go" how to build a house. Especially not with 45 minutes to the construction site... Find a suitable provider (there are many experts here who can assess that well) and go turnkey. If you have trained in a construction-related profession, you can contribute in that area. Or the classic floors/walls. With all the outdoor facilities, there will still be plenty to do for years, which you can easily do yourself...

Just my 2 cents :)

PS: We are building KfW40 close to passive house with 10 kWp photovoltaic and upscale features (which, through own contribution, can be compared in price to basic features with a general contractor). 145 sqm gross living area, visible roof truss, log beam. And we will come in under 500k including ancillary construction costs (hopefully :D )