Special repayments or investing in the market? Alternatives?

I would, in your place, build up a „play money pool“ that is not exclusively based on security and invest it. In doing so, do not jeopardize the ongoing financing and only use surpluses.

What exactly do you mean by "do not jeopardize the ongoing financing"? We have a 15-year fixed interest rate. We unquestionably pay our regular installments. But the amount of special repayments made naturally determines how much money remains outstanding as a loan at the end of the 15-year period—either for follow-up financing or to repay directly—depending on how the market performs, that wouldn’t even be unthinkable. If I only exhaust the special repayments, I still end up with about 100k loan amount at the end of the period that we would have to refinance.

What exactly do you mean by "not jeopardizing the ongoing financing"?

By that I mean that you stick to the planned financing behavior and now happily use the unexpectedly remaining money elsewhere. If an increase in finances was anticipated in your financing concept, then of course it makes sense to take this into account and withdraw a little less "play money". That is what I mean by "not jeopardizing".

The calculation is simple. If you think you can beat the construction loan interest rate with a secure investment after taxes, then save. If it gets too stressful, then make special repayments.

We have decided on ETFs. Our mortgage has a term of 20 years and the remaining debt is still somewhat manageable, so a stock market downturn at that time wouldn't be too bad. This means we pay off 2% of the mortgage and put the rest into the MSCI World for 20 years.

One should overall deal with the asset situation. There are very safe asset classes, up to (perhaps) highly risky ones. You consider how much percentage you want from each risk class and act consistently accordingly. For me, that meant that we contributed a six-figure equity into the property and financed it with >2% initial repayment. That means a very significant part of our assets is already tied up in the house, and with every monthly installment it increases (four digits). Due to the low interest rate level and the long interest rate lock-in of 20 years, I currently see no need to make additional special repayments. There is already enough money tied up in the "lump sum", in other asset classes rather less. Therefore, instead we are expanding the equity share, if you like also with ETFs. A small portion remains in cash (not excessively much!). I consider that very legitimate and balanced. We can sleep very well. Wanting to get rid of the loan in a rush may bring peace of mind, but economically it is not. On the other hand, when I see average earners who want to reduce their repayment to a minimum in order to invest in ETFs instead... that would be clearly too risky for me. Repayment substitute products are not so new, and mostly that went wrong, as long as they invest in another risk class. One is welcome to learn from the past.

Thank you all, it seems some of you handle it similarly to how we are considering.

Have you ever used an independent consultant and do you have any tips on how to find one?