Is financing possible from your perspective?

Children are planned in about 4-5 years.
We would definitely set the rate at around €1,800, with 3 free repayment changes possible
(It’s not exactly low now, but it should be possible and I thought it wouldn’t hurt to be forced to save a bit)

I wouldn’t do that. It’s commendable that you want to repay just over 3%, but with additional costs you’re at €2200-€2300 in housing burden. If you then realize the rate is too high, you’ve already wasted one of the 3 repayment adjustments. Take €1400-€1600 and top up the rest with the annual special repayment.

Children are planned in about 4-5 years.

Don't forget, if you are looking for a construction company today, you will probably lay the first stone in 2 years.
And the first payment to the bank is at the earliest in 3 years.

What is missing in the development of the property? Do you already have fixed costs there?

Overall, that can work. You can definitely save on the house; that is calculated very generously. You should definitely first determine all the costs for the [Erschließung], as that can potentially become really expensive.

Development costs are, in my opinion, among the most difficult to determine, as I don’t get proper information from anyone here :( Hopefully, however, they must not be too high, since we can connect sewage & water at the outbuilding.

Are you sure that you are allowed to do this and that it will be approved? That decision is not made by your neighbor but by the municipality.