Construction costs are currently skyrocketing

And after acceptance, this is a case for warranty!

No idea... But I won’t accept a pitch-black roof with white stripes that you can see from 150m...

That’s nature. The general contractor can’t be blamed for that. And it won’t be the last time that something ends up on there.

How much does one have to pay nowadays for a 100sqm house (bungalow) without major special requests?

Building with current interest rates is something we cannot afford. But if we could afford it again, you can no longer get a plot of land. That's why we are currently trying to get a plot without a building obligation and build later, hopefully with better interest rates. We just have a plot in sight, the location is okay, but it is 1000sqm at 170€/sqm. Unfortunately, 170€ is not much anymore, municipalities already demand over 200€. Still, that's roughly 180k€ for the plot. Then there’s a shortage for the house. I'm even considering building a shell construction to reduce costs.

If house, additional costs, garden etc. are then 350k€ or more, it will probably become very tight again with just under 5k€ net income, I suspect.

An alternative currently would be a plot of land with 700 sqm for 155€/sqm from a municipality. However, there is a 5-year building obligation.

What exactly does building obligation mean? On the one hand, it is a matter of interpretation and is handled differently depending on the municipality.

For example, at the end of the 5-year period, must a completion notification be submitted, the move into the new house have taken place, or is a building application or the start of construction sufficient?

That is crucial. If the start of construction were enough, the building project could basically be postponed indefinitely. However, if it must be completed after 5 years, it becomes more difficult to circumvent the building obligation.

I saw a financing proposal for a single-family house somewhere in a forum yesterday and the bank's assessment of it.

Meanwhile, banks assume €920 ancillary costs for a normal single-family house. However, energy costs are already included in this.

But here too, a significant cost explosion. Of course, it can and will vary in individual cases and some optimization is possible. However, it is a cost block that can no longer be underestimated.

As has been expressed here several times, it is becoming increasingly difficult, especially in old age, to maintain a house. Therefore, one should already plan when building today to be as self-sufficient as possible and economical with regard to ancillary costs, since costs will continue to rise exponentially.
Maintaining a city villa with 250 sqm of living space will be a challenge for many in 20 years. And there are more large houses than small ones. Unfortunately often far too large and inflexible ([Einliegerwohnung] etc.).