Construction contract: Duration of construction

A specific completion date along with a penalty was also mentioned to me. Otherwise, I would not have signed.

You are all not completely wrong. Otus isn’t either ;)
His statement does not exclude that it might later possibly be fixed in writing or have to be proven.

Formulierung ist 1. "in der Regel". Das heisst für mich: Bei vergleichbaren Bauvorhaben der Firma,

No, basically for construction projects in general.

2. "Geplant ist Baubeginn für November/Dezember 2016". Das heisst für mich nicht: Geplant ist es, aber nicht festgeschrieben, im Zweifel haben wir Pech und es verschiebt sich, weil was den Plan durchkreuzt hat.

Yes, unfortunately you can’t see into the future.

If there is frost from November all the way into January, no earthworks can take place.
If your screed doesn’t want to dry, no one can lay tiles on it.

finde ich die Freiheit dieser Formulierung ziemlich groß. Klar sollte das Unternehmen auch ein großes Interesse haben den Bau zeitnah fertig zu stellen, um die Rate zu bekommen, da aber aktuell Hochkonjuktur auf dem Bau herrscht und das Unternehmen recht groß ist (baut um die 50 Häuser im Jahr), sehe ich da ein gewisses Risiko.

You are assuming the company is acting badly, but that is not the case. You cannot define or contractually guarantee the future.
Not even the construction company can. I will now assume the positive, that the company’s logistics are okay; insofar it also has disadvantages if construction is not continued quickly. In bad weather it’s got the short end of the stick and thus doesn’t get paid for construction phases.

For me, a concrete date for completion along with a penalty was also specified. Otherwise, I would not have signed.

there might be some fine print somewhere pointing that out, but those are just my five pence that I’m adding ;)

Of course, the company should also have a strong interest in completing the construction in a timely manner in order to receive the installment, but since there is currently a construction boom and the company is quite large (builds around 50 houses a year), I see a certain risk there.

That is how it should normally be. However, since the construction company usually advances the costs, the risk is limited if, for whatever reason, there is a delay.

there might have been some small print pointing that out somewhere, but those are just my five pence that I’m adding ;)

Of course, exceptions are weather and force majeure. But both must be proven. There is no small print in that. Nevertheless, there is a concrete date, and we have not been shown a bad weather day since construction started in March. The handover date was rather generously calculated. But still, there was a concrete date and not some completely vague assumptions.

Right, better two months later, but with a fixed date instead of wishy-washy and rushed and in the end just trouble.

It will take 3 months longer anyway than originally planned, but better that than hanging in the air forever. But it doesn't help at all if it takes even longer because of wwi...