The structural construction company cancels one day after the excavation work began!

Now a letter from the company has arrived. They write (...) and that we do not have a legally binding contract anyway.

Depending on how the contractor's offer was phrased ("non-binding" according to § 145, alternative 2 of the Building Code, general terms and conditions, etc.), they are probably not entirely wrong:

No contract without express (written, see above regarding the offer) order confirmation!

This is not a cash business of daily necessity. The simplified legal chain for non-binding offers is:

[*]Contractor’s offer = invitation to submit an (own) offer by the client (Property. I want it (if everything fits) – maybe you want it too?) (lat.: invitatio ad offerendum).
[*]Order placed by client = offer from the client to the contractor to conclude (Yes, I want to!)
[*]Order confirmation = acceptance of the client’s offer by the contractor (I really do want to, and still do, with you!) = CONTRACT)

Now we do not know the wording of the specific offer (which is decisive). One could now look further into damages from pre-contractual obligations (contractor on site: "I can" – "Uh, unfortunately not"). But in practice, it is usually not worth the effort. Besides, without concrete damage, there is no concrete damages claim.

And the human evaluation: Freely promising first and then retracting – clearly – is not a brilliant performance by the contractor...

Never mind. Let it go. Next please!

Hello,

I would still cover myself and send the old shell builder something by registered mail so that he doesn't show up and want to start^^

In which industry are you actually? I ask because in my professional experience I have never encountered an employer who waived a written contract for such order amounts. On the contrary: usually lawyers first draft a contract... so it would be interesting to know in which industry such amounts are released verbally and with a handshake...

Chemical industry. Where have I or the OP written anything about it not being written? At our company and in others, of course a detailed tender is first created by the specialist department, then sourcing and then legal. Then incoming offers are thoroughly checked and possibly revised...

My point was only that accepting a binding offer constitutes a contract. But that is exactly what the OP did. Others brought up verbal agreements and handshakes. It was only about whether a contract exists without further confirmation from the employer to the contractor or not.