Exactly. Rule No. 1: Payment is made after something is finished - not before and not when it is almost finished. Especially not if the developer tries to be annoying.
With a property developer, you usually pay in 7 (if I remember correctly) installments, and these are each due after the completion of xy. This ensures that you do not pay more than what has been delivered. Theoretically, the developer can go bankrupt during construction and you need a new company to continue building. But does something like that happen with ETWs?
I know it like this: the developer (a "briefcase GmbH") goes "bankrupt" immediately after completion and then is out of any warranty and possible defect repairs. I wouldn't know what can be done about it. Except to report defects as early as possible (if he hasn't finished all the apartments yet, he cannot close the business).
Yes, that happens, and if you have already paid when a step was only partially completed, you end up with no money and no complete service. Therefore: pay only when the respective construction stage is fully completed. It is best not to make partial payments beforehand either, as that only reduces the motivation on the part of the developer.
In addition, 5% can be withheld directly today. This is intended by the legislator. If there are defects at the end and the developer goes bankrupt or does not want to fix them....
The 5% of the purchase price can be deducted directly from the first installment...