Finance construction project, total costs: €395,000

Personally, I find the construction price quite low

But you don't have to understand that now.
The price per square meter is over €1800 and the additional costs are set extremely high at €65,000 (unless furniture and kitchen are hidden in there).
You might need to break down the additional costs once more.

Above all, I do not quite understand the saying in common parlance. If the interest rate is 6%, then supposedly no one would build anymore.

The annuity is still the interest plus the repayment, and the repayment portion is to be selected in relation to the interest rate in order to have paid off the loan within a certain period of time. For example, a 30-year-old might plan to do this by retirement age; at an interest rate of 2%, this would mean a repayment rate of around 2%, which accordingly requires an annuity of 2%. Note that this is the initial repayment based on the calculated term, which can be shortened at any time through special repayments and changes in the repayment rate.

I am not a fan of [Sondertilgung]. Only the bank benefits from it. There are dozens of publications showing that customers do not actually make [Sondertilgen] or do so irregularly in the first years. At this historical interest rate level, one should choose the repayment within the annuity as high as possible if possible.

I am not a fan of Sondertilgung. Only the bank benefits from it. There are numerous publications showing that customers do not actually make Sondertilgen in the first few years or do so irregularly. At this historical interest rate level, one should, if possible, choose the repayment within the annuity as high as possible.

Exactly, as high as possible, and 2% can be completely sufficient there.

I think that the repayment depends on the respective possibilities and initial situations. Not acquiring real estate ownership, but instead living in a rental property, should at least not be made dependent on the fact of only being able to repay 2% initially.

An individually developed financing model, characterized by flexibility and interest rate security, also considers a 2% repayment rate to be reasonable, if with this one achieves the debt freedom of the property by the time of retirement.

Above all, I don't quite understand the folk wisdom. If the interest rate is 6%, then supposedly no one should build anymore.

Of course, exceptions must be made to this rule with high interest rates :P. And naturally, every rule always has exceptions! Those who want can choose an initial repayment of 2%. However, this currently (low nominal interest rates) leads to long terms and accordingly high total interest.