House construction for €400,000 possible with a net income of €4,100?

Well, many people have already been desperate because suddenly €10,000 more went missing during excavation.

And in the neighboring village, someone had to add 30m³ of concrete because the ground was affected, which is also quickly €3,000–5,000 gone.

So it won’t work without a buffer. Of course, if I add a pipe here and some fancy stuff there, that’s my own business. The saying "I only build once" applies again.

I wouldn't generalize it like that. Those who calculate correctly end up with +/- 3% and that's fine. Using a buffer to "calculate" is nonsense per se. It would be better to call it "costs for foreseeable extra requests to be acquired".

Well, maybe a lot depends on the approach of the property developers. We were only given a few surcharges in advance and set our own layman (albeit very carefully researched) estimated prices for many things. Despite very generous rounding up (that's what I meant by buffer) they were often too low because the developer really made money on this.

We had few extra requests. Everyone has a few, because they hadn't considered this or that beforehand or simply misjudged?!

Sorry, if everyone can calculate so perfectly, then why do many have to refinance later or the outdoor facilities or garage get left out?!

I only wrote how I would do it. Calculating too tight and later missing 100 euros when you have kids is also crap! Better to plan a longer term and reduce term or rate if you end up needing 10,000 less!

Of course, one should not calculate in such a way that 10k additional costs mean failure

Sorry, if everyone can calculate so perfectly, why do many then have to finance afterwards or the outdoor facilities or garage are left by the wayside?!

Not everyone can. But you had previously implied that nobody can.

Before I would sign a contract, for example, I would have had all surcharges for special requests listed as options in the offer.

even if you are really great savers and already have a lot of money, I still find the house overall a bit expensive. the plot is basically a gift, but then such an expensive house? especially as an electrician you might know 1-2 other trades and can also do some things yourself. the electrical work is probably a "matter of honor" that you do it yourself?! does the basement really have to be there, for example? hardly anyone does that nowadays, except when the plot demands it because of a slope. basements are mainly worthwhile on expensive plots where you simply don’t have space on top. maybe you should first think more carefully about what you really need and, above all, what you don’t. by the way, a carport with a shed and paving underneath is nowadays hardly any cheaper than a proper brick garage. if you make a sturdy garage with a large storage room and including a windbreak to the terrace and other features, you will easily save €30,000 instead of with carport+basement. best to really read up and see what others have paid for it, get offers and specify exactly what you want. if you keep the price lower, you will be more flexible later or finished earlier. see the rate less as an evil or "I have to finish quickly" but as rent.

Before I would sign a contract, I would have had all surcharges for special requests listed as options in the offer, for example.

Yes, that was also our intention. However, the developer did not disclose them, and we were under quite a bit of time pressure. We were 15k over budget on everything (consisting of many underestimated surcharges, a few additional requests, a couple of unforeseeable items – typical mix, I think) but still just within the buffer.

Not everyone can. But you previously implied that no one can.

I didn't. I wrote "you will probably need them".