Several troubling contract clauses

The builders and (Firmenname) may terminate this contract for important reasons without observing a notice period.

It is unproblematic as already stated by others.

or if there are disagreements during the construction phase (e.g. due to defects), the company could, citing some deliberately important reason (which is also not defined in the contract), withdraw from the contract and we would then be left with a half-finished house.

As already stated: this is not an important reason. "Important reason" is not arbitrary, but a legal term.

Until the house handover, the builders grant (Firmenname) a house right.

Make it a joint house right. The construction company needs freedom of action, you and, if applicable, your expert do as well.

We had the contract draft reviewed by a specialist lawyer before signing. At least 20 points were corrected and supplemented and then accepted by the contractor as such. If you are not an expert (neither are we), seek advice. Completely missing points to your advantage you do not see, and the contractor does not have to include them in the contract draft.

As already written: That is not an important reason. "Important reason" is not arbitrary, but a legal term.

That is correct. And somewhere in the contract it will also say that the contractor owes the client a house…

If you are not experts (neither are we), get advice.

Very good advice. We didn’t do that, and I would have had to take the blame for anything overlooked. Much was done with the craftsmen by handshake or informal agreement – which is not generally recommended but really great when it works.

At least 20 points were corrected and supplemented and subsequently accepted by the general contractor as such.

Since I do not consider you to be an overly anxious or otherwise difficult customer, this number seems too high to me and suggests the suspicion that the general contractor had his standard contract drafted many years ago by the neighbor's kid, just like the website. A regularly updated standard contract should basically fit without changes and then be sufficiently individually refined with only a handful of modifications.

Very good advice. We didn’t do that; any oversights would have been my responsibility. Much was handled with the craftsmen by handshake or on a verbal agreement – which is not generally recommended but really great when it works.

I love handshake agreements. For me, they remain the most popular way to conclude a contract. But only with people I know or who are recommended to me. Unfortunately, this unspoken contract has lost its dignity over the past few years... Nevertheless, for me, the classic handshake still stands for respect.