Financing plan end terrace house

Increasing the monthly payment by 200 EUR is therefore more effective than a one-time special repayment of 2400 EUR at the end of the year.

Yes... incredible!
At 1% interest, that's equivalent to a case of beer per year..

I would be reluctant to refuse that, but I would prefer the flexibility.

At 1% interest, a value equivalent to a case of beer per year..

Indeed, and that only in the offer. :D (400,000 €, 1 €, 1,200 € instead of 1,000 € makes 12 €) If the flexibility (free repayment change) costs a significant interest premium, special repayments could even be cheaper.