House construction / Financing single-family house conceivable?

Regarding the house construction costs, I estimate a house for about €300,000 (I don’t know if that’s realistic). Large 130/140 m²

With a higher degree of personal contribution, that works out; without it, you will very likely exceed that amount including additional building costs, outdoor facilities/carport, and a buffer. However, I also don’t see half a million or more (all-inclusive) at the bottom of the calculation, unless you place value on particularly high-quality fittings. That would no longer fit with the available equity and the desired installment.

I would personally calculate the construction costs significantly higher; if it doesn't turn out that expensive, even better.
I would also include reserves for a new (used) car.

With your wife's earnings... Is there really not much more possible in the foreseeable future? Especially since no more children are planned?
€1,000 net for 40 hours/week seems to me personally, even in tax class 5, an outrage for a training profession.
What is that in gross?

The payment for the woman is a bad joke. That is just 10€ gross per hour. McDonald's, Aldi, Lidl, etc. pay significantly more for the simplest jobs.

I revise myself. House construction not representable

I revise myself. House construction not feasible

What is the reason for that?

I consider that to be Harakiri or I would lose the lightness in life there.