Uncertain due to financing

crazy people here the question is whether there will be any further increase in value. 200 euros in the sticks is already a lot of money

Well - you have to consider the surroundings. Just across the Moselle is Luxembourg. Prices there are significantly higher again. A colleague built in a Luxembourg border town (a small town - possibly known for its strip of gas stations) and paid 550 euros/sqm there. (Here I also shake my head again.) In Luxembourg City, land prices are unaffordable for the average person. Accordingly, Luxembourgers look for "more affordable" alternatives and then build "cheaply" in Germany. Only that this spoils the prices for the average German citizen or possibly brings an increase in value if you own property.

Tomorrow I will get the exact numbers, then I will get back to you again

Exactly in this direction goes the consideration. After 20 years, my wife's life insurance will also mature. We have 2 children, although the little one would prefer a horse to a house

We want to build on the Hessian Bergstraße. Centrally located between Mannheim and Frankfurt. Very good highway and train connections.
We currently have a loan amount of about €260,000 with 1% repayment and 2.4% interest over 15 years at one bank and 2 x €50,000 at the KFW (0.85% and 1.35%). This results in a monthly burden of €1065.00 + additional costs.

Addendum: This is the first draft. Surely, the repayment can still be increased by a few decimal points. With the 1% repayment, we would have to make special repayments of about €5,500 per year to be finished after 25 years, which is unrealistic. I am still unsure....

That's exactly the direction the consideration is going. After 20 years, my wife's life insurance will also mature. We have 2 children, although the little one would prefer a horse to a house

We want to build on the Hessian Bergstraße. Centrally located between Mannheim and Frankfurt. Very good highway and rail connections.
We now have a loan amount of approximately €260,000 with 1% repayment and 2.4% interest over 15 years at a bank and 2 x €50,000 at the KfW (0.85% and 1.35%). That results in a monthly burden of €1065.00 + ancillary costs.

Addendum: This is the first draft. Surely the repayment rate can still be increased by a few decimal points.
At 1% repayment, we would have to make about €5,500 in special repayments per year to be finished after 25 years, which is utopian.

I am still uncertain....

Hmm - if I understand you correctly, you want to use your wife’s life insurance after 20 years as a special repayment?
Then you should calculate the €260,000 with a fixed term of 20 years.

Personally, I am of the opinion that you should not use the KfW 153 30/5/10 and KfW 124 35/5/10 (in which case you would be over 75 years old) variants. Because here, the fixed term expires after 10 years and, due to the low repayment rate, you have a high remaining renewal amount after 10 years. Although the fixed term also expires after 10 years in the 20/3/10 variant, the "risk sum" is not as high due to higher repayments.
You can and should also shorten the repayment-free period. It is nice to have less burden at the beginning, but it hits hard in the end.

From the remaining balances after 20 years, you then deduct the guaranteed amount of the life insurance (if you want to use it for special repayment) and then recalculate any annual special repayments.

So here in southern Hesse, 20-30 year old houses go for between €330,000 and €380,000. Comparable to our plan. The advantage with us is that the plot comes from the city and is therefore cheaper. Although the exact price of the plot is not yet determined. Plots from private sellers or agents can be completely forgotten here.

Again, it's not about taking the loan into retirement, that would be madness. Instead of pleasing the landlord, we want to live in a nice house and please the bank. After about 20 years, sell the house again, maybe even break even, and then see what is possible with life insurance, possibly inheritance. To break even, we would have to sell the house for about €250,000. That is currently well below market value. Whether we use the life insurance for repayment or not remains to be seen.

These are all still plans at the moment.