So, now I also have time to answer the further questions:
1. What is the interest rate on the large building savings contract?
0.5 %
2. Are the ancillary construction costs of your property also including the purchase of the kitchen?
That is already factored in, yes.
3. How high is your wife's additional minor income (I know - it is included in the 2,200)? Only if you want to say - what does she do there (childcare, cleaning, etc.)?
400 € in the online sector
4. Have you already started anything (initiated land purchase, started construction planning, etc.) or how did you come up with your figures ("Land and house (161 m², with KfW70, without basement), currently amounting including painting and flooring work and special equipment, ancillary construction costs, outdoor facilities to 347,000)"? Maybe you have a breakdown of the planned costs, so that, for example, a building expert can check if something is missing.
We are currently interested in a plot of land and are currently only waiting for concrete numbers for the development of the plot, as this has so far been factored in at around 35,000 € (rear plot). Further ancillary construction costs, such as construction water/construction electricity, surveying, building permit, soil replacement (soil report is already available), have already been taken into account. The figures for the house costs are based on a concrete offer from the construction company.
5. Can you get a subsidy from somewhere else (parents, etc.) that increases your equity a bit?
Unfortunately no.
6. Does capital still have to be considered for new purchases of furniture?
No, this is not necessary.
7. Is there also such a nice child salary increase for your wife, as described recently in another thread?
I also receive additional payments if there are children. For the first and second child respectively 95 €, from the third about 290 €. (Compared to what children cost, however, rather negligible)
Daycare would only have to be paid between the ages of 1-3 years, as it is (currently) free afterwards. To what extent we can do without this through home office and possible telework (working from home 1-2 days would possibly be possible for childcare) would have to be checked. However, this should not be a decisive point for the basic affordability.
Regarding the 'large' building savings contract, different repayment rates are possible, but it must be at least 800 €.
Many thanks to you all for the previous answers and suggestions. :)