Good offer or wait for negative interest rates?

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


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

Math whiz?

It's a pity that you just missed the 80%.
10Y should currently go at around 0.7-0.8%, which calls the purpose of the funding products into question.

What do you mean by just missed? 20% of the 459K would be about 92K. However, equity is "only" 70K.
Or does it make sense to state more here? I still have just under 20K available, intended for unforeseen problems/emergencies...

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

400,000 construction costs *1.05 = 420,000€. So 20,000€ additional costs due to price increase.

400,000€ loan, 20-year fixed interest, 1,500€ installment

2% interest = remaining debt 154,000€
1% interest = remaining debt 90,000€

What do you mean by "knapp"?

I would still have just under 20K available,

That's what I mean by "knapp" 10K€ from the buffer included, 10K€ own contribution (garden) included in the calculation and there you have the 80% loan-to-value.

New building + granny flat including land: 460K

Sure?

Do you have some key data? How much is the land worth? The costs seem extremely low to me

400,000 construction costs *1.05 = 420,000€. So 20k€ additional costs due to price increase.

400,000€ loan, 20 years fixed interest, 1500€ rate

2% interest = remaining debt 154k€
1% interest = remaining debt 90k€

Yeah true. But if construction costs are higher and equity capital is available, it looks a bit different. With 100% financing without special repayment your calculation is correct....
(I wouldn't have thought it makes that much difference!)