Construction project and building permit feasible?

I also see the whole construct as difficult and that it should wait at least a little until some equity is available and it is clearer what/how is earned. What surprises 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 usually only found in the area of ward management. And the parental allowance only fits very limitedly, since €262 is very little and is not in proportion to the stated income.

You are welcome to research TVöD. In 2021, I am in level 4 + intensive allowance + shift allowance monthly. (+tax-free Nd allowances!!) I received 2150€ in Bv in 2016! Level advancement remains unaffected by EZ. And there is an additional monthly childcare allowance of, I think, 100€ per child. Nursing is unattractive because of staff shortages and the shifts.

What you absolutely must also consider – you wrote

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

and

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

Mentally subtract the sloping ceilings completely from your apartment. Now you don’t just need the hallway 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 the utility room outside the apartment. Another 7 to 10 sqm that are missing from the house floor plan. You might already have a guest WC but now with an extra shower. You probably currently also have a basement room for storage, which means some extra sqm as well. That quickly amounts to 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 will also need space for the 4th bedroom... so be honest: would your apartment with 4 bedrooms, without sloping ceilings and 20 to 30 sqm less still "work"?

That would be a massive deterioration in living situation with a 130 sqm house – except hopefully it won’t leak inside anymore.

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

Model homes help with dimensions - it becomes obvious when planning was done with dollhouse furniture

There are initial contacts. Every salesperson shares not only self-praise but also knowledge

You can coordinate your wishes and tastes. Even in the standard 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 that is 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.

Show homes help with dimensions – it becomes obvious when dollhouse furniture was used in planning.

There are initial contacts. Every salesperson shares knowledge alongside self-praise.

You can align your wishes and tastes. Even the standard of cheaper providers includes more than one tile and one color.

One must not forget:
Not every show home was built by the company inside it.
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.

What you absolutely have to consider – you wrote

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

and

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

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

Compared to the apartment, that leaves effectively only 100 to 110 sqm in the house. Does the current apartment already have 4 bedrooms? If not, you also need space for the 4th bedroom... so honestly: would your apartment with 4 bedrooms, no sloping ceilings, and 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 again is roughly 50,000 EUR more than for 130 sqm...

I don't like rooms, just square. And a nice living room.