How much income do we need for our home loan?

How many years do you want to pay off at most (you have to be done by the statutory retirement age at the latest)?

Is this a joke or serious? I’m guessing a joke..?

As a rough guide:

House: 170 sqm * 2000€/sqm = 340,000€
Double garage: 30,000€
Basement: 50-70,000€
Additional building costs: 40,000€

That makes approx. 470,000€ (without outdoor facilities and kitchen).
For this amount, depending on the region and provider, you get an average standard. Sometimes better, sometimes worse.

Per 100,000€ loan, you should have at least approx. 1,000€ income available. That makes about 5,000€ per month for the above amount. Although I wouldn’t feel comfortable with that. In our financing, the ratio is better and otherwise we would not have built.

And if the car breaks down, you just get a new one for 3-5k. I don’t have to plan a savings rate for that. But that just happened to both of us recently because one car broke down 2 years ago and the other 1 year ago. So most likely no car purchase will come up in the next 5 years.

We don’t have any insurance except liability insurance for the car and private liability (totalling around 750 euros per year), which is 63 euros per year.

I am also within the estimate. Plus the outdoor facilities... that adds up comfortably to 500,000.

Yes. Exactly these, according to your calculation, 3-5,000€.
A total loss can very quickly be caused by oneself, so that something new has to be bought.
It may be that with your work on the house a car purchase is not urgently necessary, but employees usually rely on a reliable vehicle, so no clunker for a month’s salary.
Regarding the 3-5,000: you have to save that up besides a very high repayment, so it’s not something you get over quickly.

That will all have to change as well

With the good salary and no desire for children, a lot is possible. However, the bank wants reliable security, which self-employed beginners do not provide.
That’s why I would simply take the house number 500,000 and inquire at a bank. Then you can see for yourselves how your living expenses are calculated.
Apart from the numbers, I’ve never understood why one always has to have and want everything immediately after their training that others need 10/20 years for. If I were you, I wouldn’t put any pressure on yourselves at all. Having wishes can also be nice, developing and changing them. Take the next year for expert talks (bank, house builder, insurances), then see how it goes.