120 sqm house - are all costs considered?

What huge interest advantage would you have in 3 years? Does that compensate for the lost 40k?
Why not now and rather make special repayments?

For example, having to finance 150k less. So maybe 0.1-0.20% less interest and maybe only having to finance 500k. That’s almost 800€ less in monthly payments.
That’s how you have to calculate. The lost 40k levels out over a 30-year term – 4.5k less financing interest per year... (yes, it decreases during the term, but you still profit significantly). And of course, the healthier rate also compensates for the additional 40k.

The additional costs for the BP/piles calculated on top, the price is still significantly better than for many other plots - where you cannot live in the middle of greenery and still have a very good connection.

That’s what I meant by romantic idea. It’s rather the desire to build new than to look for something that means more financial comfort in the end due to a lower price.

The house is explicitly supposed to be small, since there are only two of us.

That is an important component that hasn’t been considered at all yet. You really don’t have to think “big” there. But to be honest, with this plot size I would go for a bungalow.

The price was from today

I assume that you only use the online calculators? It may be that the bank either does not accept a small and comparatively expensive house on such a plot, or that the installment is too high for them (although I don’t believe that). Just as a note in case you’re still reading.

We have about 25k equity. Since that is not really significant, I did not elaborate further.

25k equity, 6.5 HEK and then 3k installment? Children? Dual income? How many piles should there be? Diameter? Could then give you pretty accurate amounts

15 poles of 17 m each DM 300 I would say.

15 piles at 17 m DM 300 I would say.

17 meter diameter or 300 Deutsche Mark?