Financing commitment at the current time

Our setup is a bit complicated, so it is understandable that the bank is hesitant. Here are the numbers so that we can get a well-founded assessment from you:

House construction + land purchase including all additional costs: 800k

Debt load old house: 320k
Market value old house: 420k
Min. equity after house sale: 100k

Additional cash assets: 50k

Financing amount: 700k

1 income + child benefit: 6.4k
Monthly burden new house loan: 3k
Monthly expenses according to household budget book: 2.5k

According to FiBe a realistic undertaking – what do you think?

Forget it!
Sorry if this sounds harsh, but you are not doing yourselves any favors with this.
When we built our house with costs of 730k and around 200k equity from the old house with 4 children, 2 incomes (1 civil servant) and a total income of about 7,600€, we were already at the limit. But unlike you, on the right side of the limit.

What you are planning would be irresponsible...

Just saw your numbers... Wow.. So a high loan amount with the income and 6 people... That is quite risky... It can happen at any time that the sole earner, for example due to an accident (doesn't have to be anything really serious), ends up on sick pay. Then the income with the high payment is no longer enough and you would have to dip into savings immediately. That would probably also quickly be the case with repairs in the house or a new washing machine, etc. How much savings would still be in the account in the calculation? In my opinion, the flat rates at the bank are not too high. A while ago, we first inquired about what kind of purchase budget we would have from the bank's perspective... A possible payment was assumed there, at which I immediately said "no, that's definitely too much risk for me"

A risk assessment is always a very personal matter, we are more the experimenters and as long as no one gets hurt, everything else is manageable. Money is for us a means to an end and as long as we have food, everything is fine. I will definitely work again, at the latest next year when the youngest is in kindergarten, there are thousands of jobs, I don’t know if any of you work in nursing – [uns wird der rote Teppich ausgerollt]. And sure, shift work, but no problem at all with a partner who is 100% in home office. Again, that was not even the question.

...
But the tiler charges extra for that ;)

I'm really quite embarrassed about the hexagonal bathtub, but "for the sake of keeping peace within the marriage" ;-) it just had to be. For cost reasons, precisely because of the tiler, I chose it to be square rather than round. Fortunately, the bathroom upstairs will most likely only be used by my husband and me, so no one will see it. :)

Too complicated to list here, as due to many renovation measures in recent years there were always refinancing and small change... when renovating the old house we would have to summarize anew anyway. We have been rather mediocrely advised in recent years

That is already the basis to even work with numbers.

Is space the only reason for a new build?
Is your current garden big enough?
Then you could build a huge garden house in lightweight timber frame construction in EL for 2 children's rooms + WC.
That is probably still cheaper despite water, sewage, electricity and proper insulation and heating.
Should cost around 60k in EL.
Maybe such manageable 30, 35 m2 more living space would already be enough for you?

Given your framework conditions, you should really look beyond the horizon to the left and right.
Does your husband perhaps have experience in construction himself and a network?

The hexagonal bathtub is really very embarrassing for me, but "for the sake of dear internal marital peace" ;-) it had to be. For cost reasons, exactly because of the tiler, I then chose it not "round" but angular. Fortunately, the bathroom upstairs will most likely only be used by me and my husband, so no one sees it. :-)

It gets really expensive with freestanding or with whirlpool function. Nevertheless, having a bathtub at all is actually already a luxury.