Construction project and building permit feasible?

I also consider the whole construct to be difficult and that it should at least wait a bit until there is some equity and it is clearer what/how is being earned. What does surprise me somewhat, however, is a net salary of €2200 as a nurse; from what I have heard, that is significantly above the normal income and usually only found in the area of ward management.

And the parental allowance fits only very limitedly with that, because €262 is indeed very little and does not correspond to the stated income.

I also see the whole construct as difficult and that it should at least wait a little until there is some equity and it becomes clearer what/how is earned. What puzzles me a bit, however, is a net salary of €2200 as a nurse; from what I have heard, that is significantly above the normal income and normally only found in the area of ward management.

And then the parental allowance hardly fits, because €262 is very little and is not in proportion to the stated income.

That is not above average. I know what I earned. And I know what my friends and colleagues earn – I work in the intensive care unit and had professional further training. I didn’t know that I now have to prove my salary with payslips. And I have already written at least five times that my wife is having the child, so she receives the parental allowance from HER salary. I am slowly starting to feel a bit humiliated... I have never read that anyone has doubted the salary of a fresh engineer...

Saturday a day trip to a model house town - the children will definitely find it funny

It's a nice pastime, many swear by it here in the forum as well. I was not a fan of it. In hindsight, I find it problematic that the model houses (the ones I have seen) have special features that are expensive (bay windows, dormers, mosaics, walk-in showers with seating, window seats, many square meters of living space, lift-and-slide doors, huge front doors, etc.). None of that has anything to do with a basic equipment.

Personally, I would now rather start an intensive search for a plot of land or divide the tasks (the woman looks at floor plans and general contractors; the man looks for land, visits building authorities in the region, asks neighbors).

Model homes help with dimensions - it becomes obvious when dollhouse furniture was used in the planning

Initial contacts are made. Every salesperson passes on knowledge alongside self-praise

You can coordinate your wishes and tastes. Even in the standard range of cheaper providers there is more than one tile and one color

One must not forget
Not every model home was created by the company shown inside
The consultants are more or less well-trained salespeople who want only one thing: your best, namely money

110 sqm for 5 people is more than tight.

It's a nice pastime, many also swear by it here in the forum. I was not a fan of it. In hindsight, I find it problematic that the show homes (that I saw) have special features that are expensive (bay windows, dormers, mosaics, and walk-in showers with seating, window seats, many square meters of living space, lift-and-slide doors, huge front doors, etc.). None of that has anything to do with a basic equipment level.

Personally, I would now rather start an intensive search for a plot of land or divide up the tasks (wife looks at floor plans and general contractors; husband looks for land, visits building authorities in the region, asks neighbors).

Yes, the special features in the show homes and partly also in the floor plans are quite annoying. I always lack a bit of imagination then. But at least once we visited the standard Bien and Zenker house. At least almost standard.

I have to do something. Yes, wife looks for plots of land and the other woman then the general contractors.

What you absolutely must also consider - you wrote

"Living on 129.5 sqm with deduction for sloping ceilings, so 170-180 sqm)"

and

"Our idea: 130 sqm living space with 4 rooms + living room + kitchen/bathroom and guest toilet with shower and utility room without basement"

Mentally subtract the sloping ceilings completely from your apartment. Now you don't need the hallway just once (ground floor) but twice (ground floor and upper floor). 5 to 8 sqm of the 130 in the house are gone. So far you have had a utility room outside the apartment. Another 7 to 10 sqm missing in the house floor plan. You might already have a guest toilet but now with an extra shower. You probably also currently have a basement room for storage, which is also extra sqm. That’s quickly 20 sqm missing, maybe even 30!

Compared to the apartment, effectively only 100 to 110 sqm remain in the house. Does the current apartment already have 4 bedrooms? If not, you also need space for the 4th bedroom... so hand on heart: would your apartment with 4 bedrooms, without sloping ceilings and with 20 to 30 sqm less "work"?

That would be a massive deterioration of the living situation with a 130 sqm house—except hopefully it won’t rain inside anymore.

Conclusion: Comparable would be 150 to 160 sqm in the house with the 130 sqm of the apartment. That in turn is roughly 50,000 more than for 130 sqm...