Financing land and house construction

Well, with 7,700 net... Here, a rate of 2,000 for 5,000 EUR net is regularly considered borderline but still okay.

After deducting the rate, 4,000 EUR remain for living expenses for 5 people. That this works is beyond question. But you are indeed barely paying down the principal, which can become a big problem... Because you will hardly be able to make special repayments in the first 10 years.

And free yourselves from the thought of not wanting to live luxuriously... New furniture for the living room plus a new couch 10-15,000 EUR.
That quickly costs 50,000 for something luxurious...

PS:
And a kitchen for 20,000 including appliances? That is at best available from Nobilia?
We are currently looking for a built-in coffee machine... from 1,500 EUR...

Would be way too risky for me. How much remaining debt do you each have per loan at the end of the fixed interest period?

If I see it correctly, you basically don't repay anything until the end of the terms.

5 repayment-free years are really cheeky. They advertise how great they are, but in reality, these years are wasted and very expensive time.

But you hardly repay at all, that can become a big problem...

At the age of 30, that is basically not a real problem. At the Landesbank and the Commerzbank the terms run until retirement. Only the KfW I also see critically, unless provisions are made there for the time after 10 years.

To give an update here again, current offer from Commerzbank:
(1) €200,000 KFW 300 at 0.53%, 10 years fixed interest, 5 years interest-only
(2) €150,000 from IFB (Hamburg promotional bank) at 2%, 15 years fixed interest, 2.5% repayment, 1 year interest-only (IFB is subordinate)
(3) €440,000 Commerzbank at 3.31%, 15 years fixed interest, 1.25% repayment

This results in a total monthly payment of €2,830 after the interest-only period ends.

I find it positive that interest rates have dropped significantly again and that the rate is okay relative to your net income. However, I also see the problem that you are hardly repaying anything. Saving money and investing it elsewhere is a nice intention. But: in the first few years, there will still be various "small expenses" here and there that eat up the money. It is very difficult to consistently forgo these and save the money. In 90% of cases this doesn’t happen, and in the end the debt pile is large. I would strongly recommend putting a bit more money into the Commerzbank loan every month. A 3.31% interest saving, tax-free and guaranteed, is not bad, and by forced saving you also won’t be tempted to waste the money elsewhere. What absolutely makes no sense is to currently save money at 3.5–4% in hopes of being able to redeem the KfW loan. The general view right now is that the peak of interest rates has already been reached. It doesn’t have to be this way, but if that is the case, this is definitely the worse idea.

And a for €20,000 including appliances? That is at best possible at , right?

We paid 15k for a Schüller. So €20k should definitely still be possible today, of course without luxury items like a built-in coffee machine. But the question can be answered quite easily by the OP going to a kitchen manufacturer with the house design and discussing what their own ideas would cost.

5 repayment-free years are really cheeky. They advertise how great they are, but in reality these years are wasted and very expensive time.

I think it makes a big difference whether you choose 5 years without repayment to consume or to save the money and invest it more profitably (our case). At 0.53% interest, I’m damned if I’m going to pay off more than the absolute minimum. We calculate from the start with a €3000 monthly installment and put aside everything we don’t currently need for the installment, in order to reduce the remaining debt after 10 years. (of course, this also requires appropriate discipline)