Good offer or wait for negative interest rates?

Ask the bank for an update.

I did. They said nothing has changed. It's just a smaller savings bank... Mhh

They lie... in 4 weeks a lot has happened, just google the interest rate trend

The problem is that interest rates are falling and costs are rising. In the end, interest rates cannot fall as quickly and as much as costs are currently rising.

It is therefore always better to start building immediately.

That is simply not true. If you could reliably look into the future like that, you would probably already be a multimillionaire. For example, financing costs have dropped so much in the last 12 months that you should be happy to finance now and not a year ago.
15 years at 90% loan-to-value should currently be around 1.1-1.2%.
It's a pity that you barely missed the 80%.
10 years should currently be around 0.7-0.8%, which calls the purpose of the subsidy products into question.

Speak only about the current situation. Construction costs have risen by 5% over the past year, interest rates have only fallen by 1% (20 years). Even in absolute numbers, the winner was the builder a year ago.

The financing costs have dropped so much in the last 12 months, for example, that one can be glad to finance now and not a year ago.

Oh yes, I had to finance at the beginning of January

I'm only talking about the current situation. Construction costs have risen by 5% in the past year, interest rates have only dropped by 1% (20 years). Even in absolute numbers, the winner was the builder a year ago.

You can't calculate like that. If the interest rates were 2% before, and now 1%, then the interest costs have dropped by 50%!! Good deal for 5% higher construction costs.