When to finalize the financing??

I'm just saying. A free right of withdrawal only exists if certain conditions are met. In all other cases, it will not be free.

Be careful and don't say no one warned you about pre-contracts.

There are no pre-contracts under German law. There are only contracts, and you have to abide by them.

So should a contract always be extended by a right of withdrawal due to failed financing?? Or what do you recommend?

Simply no condition regarding the topic of when a withdrawal is possible. So "free right of withdrawal" period, full stop, end, done. No restrictive condition like... if the property blah blah...

Such clauses do not exist in loan agreements. Once the right of withdrawal (14 days) has expired, the withdrawal is final.

That's how it is. At the latest 14 days after the bank receives the contract signed by you, the bank refinances itself. Roughly speaking, no non-acceptance penalty is incurred for the loan if the interest rate level has risen (perhaps just a processing fee). But if the interest rates fall, the bank will also charge you the compensation (interest loss).

Everything about commitment interest has already been said. Therefore, I would wait a little longer and only sign the loan agreement once the construction contract is signed. And that, as toxicmolotow already wrote, should be provided with an unconditional right of withdrawal (or at least with a free right of withdrawal if no financing is secured).