Construction financing proposal, opinion?

That smells like refinancing...

If your property is located in a new development area, then even the real estate transfer tax is not calculated correctly! You only pay real estate transfer tax on the "bare" land price without development costs, without construction costs (= proportional charge to the municipal utilities).

You need to deal with the subject matter in advance, an offer from a general contractor alone is not enough, so that you do not have to provide additional funding afterwards! It is usually the costs around it that make additional funding necessary. If you misestimate by 5k-10k€, the consequences are relatively minor, this can still be absorbed by the family. Probably absorbing 50k€ as in your case is then already difficult to almost impossible for almost everyone here.

I also advise caution: You are approaching this too naively. Both with the contractor and with the financing.

Take your time first and read up on the subject. This forum offers a good opportunity to do so.

Building a house should not be a rushed decision.

How big should the house be?

For rough cost planning, you can assume 2000 euros per inhabited square meter. Depending on the building location and the extent of your own work, this could be reduced to 1800/sqm.
In addition, there are the plot + additional costs (I also count house connections etc. among the additional costs) + possibly garage or similar.

A building savings contract that is not allocated at the end of the fixed interest period cannot be recommended, nothing is repaid and sparse interest is received in the building savings contract, while the entire loan amount remains at risk after 10 years (except for a possible partial allocation).

It is not really possible to evaluate the offer from the construction company, as it is not clear which costs are covered by the construction costs. Possibly the wrong thread for the evaluation of construction costs and actually you do not ask for an evaluation or question it here with us.

Regardless of the fact that this may not belong here, 15,000 euros for external facilities may be okay, but ancillary construction costs cannot be integrated there and should then be reflected in the construction costs.

Yes, the figures are from him. Just a silly question, why does he calculate everything so low? We have already informed ourselves about the [Hausanschlüsse] with the responsible parties, they are approximately €7000.

Yes, the figures are from him. Silly question, but why does he calculate everything so low? We had already inquired about the house connections with the responsible parties, they are around €7000.

Maybe it would first be useful to find out where he is even building, otherwise the construction prices cannot be properly evaluated or analyzed.