Assessment of Financing New Construction 425k € / Overall Financial Situation

I roughly calculated the liabilities while reading.
You are at €3000 (I increased groceries and drugstore to €500!)
Remaining €3400 or €3800 if the wife works half days.
€20,000 equity goes towards the kitchen and the rest.
€400 for incidental consumption costs.
Remaining €1000-1400, for example, for financing and other things, such as new purchases or unforeseen expenses.

I would advise against it without a safety cushion.
Note: during probationary employment periods, I think you don't get financing.

Has it already been requested?

I read there is a double garage in the basement, so 160sqm living space and 220sqm in total.

I am 100% sure that you and your architect do not know what you are doing with the estimated amount. Do yourself and your life a favor and leave the nonsense at the poorly estimated scale.

Neither does the estimated amount for the house fit. It didn’t fit even before Corona or for example 4 years ago, nor does the income fit. Especially not with a child.

I also have absolutely no idea where the 425k loan amount comes from? For 160sqm + basement that sounds quite unrealistic. One might maybe still consider over 425k, but that will never be enough. And even 425k is already daring with the current net income, especially if children are coming. However, it has to be assessed individually. We really took on a lot of loan ourselves. But we are very frugal and modest people who can give up a lot for the house. We also have family support if something were to go terribly wrong. So in your place, I would first plan without a basement and with 140sqm. Then get a concrete turnkey offer to have a price orientation.

[*]New build, single-family house with basement due to hillside property, gable roof [*]Garages? Double garage in the basement [*]How big is the house? (living area / usable area) approx. 160sqm / 220 sqm

A hillside house is expensive. And with your salary, you are at the lower limit. The nice thing is that you already own the plot, so in that respect Istria is your equity. But overall, it is also "only" €70,000 that you have. 160sqm, double garage, basement... these are all dispensable things that are reserved for those who have saved well and have a good salary. No one is forced to build 2 floors plus basement. Do yourselves a favor and waive one floor: living spaces in the basement on the hillside side, double carport in front of the house, and everything is relaxed. I also point out that you want to use your equity for various things, that is wishful thinking.

What has not yet been mentioned is that TE and partner are obviously not married. This complicates the matter(s) in my opinion enormously – or not … depending on the attitude.

That really plays a rather minor role here and is not an insurmountable circumstance.