What is a realistic construction cost / single-family house

Free disposable income after deducting all fixed costs as well as the average of other living expenses:
5500€

We wanted to set the budget at 600,000€ because I know our lifestyle ... a rate that is too high would restrict us too much.

That is somewhat contradictory. How much of the 5,500€ free disposable income per month goes towards the upscale lifestyle and how much remains for the property?

Theoretically, you could reach the million in costs with the salaries as well.

I also come from Lower Bavaria and don't know any civil engineer in their mid-20s who earns even close to that much. Congratulations. No one here has a brick facade either. At least, no property comes to mind right away. I am currently building as well. 2000€/m² is already difficult with larger construction companies. That is more likely a gas heating system with plastic windows and possibly decentralized ventilation. With 2200€/m² plus brick facade, you can currently get a good standard here.

Vacations and one-time expenses are not included in that - that's correct. It's also hard to calculate - since the size/extent of the vacation naturally depends on the financial situation. I would like to have a financial buffer of 2-3000€ after deducting the installment.

Vacations and one-time expenses are not included - that’s correct.
It’s also hard to calculate since the size/extent of the vacation obviously depends on the financial situation.
I would like to have a financial buffer of 2,000-3,000€ after deducting the installment.

There you go, 5,500€ freely available - buffer 2,500€ = maximum 3,000€ installment

That’s already a good basis to work with.

You don't have to push yourself to your limit. In 3 years, the first child will arrive and 4,000 net will become 1,800 net. Look at model homes. Visit bathroom and tile exhibitions, add up all the costs and set your personal standard. Be sure to take into account the additional construction costs, especially earthworks and slopes.

220k land
620k house
100k incidental costs incl garden
100k furniture and kitchen
=
1,040k

- Equity???

The installment should be around 3k