Finance the property, construction starting in 2 years. How to finance?

The 700K€ are calculated with 100K€ equity.
I don’t understand the deduction of the equity in the calculation. It doesn’t change the total volume whether it’s cash in the account or sunk into the property.
Therefore, I would be at a remaining sum of 490K€ or 488K€ if I calculate the property.

We already live quite well furnished and also have a new, large kitchen in our apartment. I would have the latter redesigned by the local kitchen studio, so that with a few cabinets and new countertops everything would be done. Most furniture will go along, also all quite new and high-quality.

Exactly, then you are at €490k. Fits - I had misinterpreted it.

but then deduct the respective financing of the land. So with a €1,000 rate = €18,000 remaining.

211k * 5.65% makes €933 interest per month. But you will also have to pay off additional installments. Now think about how much money you save if you put in half of the 211k as equity now ;)

That is of course true :-).

Will the equity sunk into the property later be counted 1:1 as equity again by all banks?
I would then have to finance the later house 100% in order to have the still incurred ancillary construction costs (50k€?) available as cash.

Yes, it will be recognized 1:1. But you can also easily finance the construction phase costs, theoretically you don't need to hold anything except the buffer for unforeseen expenses. Whether that's 50k and whether you haven't saved it again by then is another question.

We bought the property in 2018 and financed it 100% (1.67% for 15 years), although the equity was available to pay for it completely. However, since we expected rising interest rates for the house construction, we wanted to keep the financing amount for the house build low...

Of course, it turned out differently than planned, the interest rates continued to fall, and we were tied to a "too high rate" for 15 years because we foolishly set the repayment to almost 4%. Nevertheless, at the beginning of 2022, we were still able to fix the construction loan at 1.3% for 15 years. It just got a bit tight with the overall burden due to the high land installment.

We were A) poorly advised, B) quite inexperienced with such situations, and C) naive. Accordingly, we made some expensive mistakes that did not have to happen. Of course, these then also tied us to the house construction project. That’s why we are seeing it through now.

I thought the advice of a fellow user a few posts earlier to consult you was very good. Definitely do that and don’t fool yourself into thinking that you have read enough information and opinions on the internet and can skip it now. It is good to get suggestions here, but discussing the entire project with an independent advisor is indispensable. Take this as good advice from someone who, as I said, was in the same situation in 2018 that you are currently in.