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

I would go the route of a lawyer. If possible, I would of course have the substitute company build and check whether it is possible to get something out of the previous one. Worst case would actually be that you have already commissioned the new company and the first one suddenly remembers to fulfill their contract.

Worst case would actually be that you have already commissioned the new company and the first one remembers to still fulfill their contract.

In this sense, at least the aforementioned eMail

that the shell builder informed the civil engineering company by e-mail that he will not come next week and now has no time for our construction project anymore.

should end up in your own inbox. But a lawyer* won't hurt, and I would hold the canceler liable for at least his fees and the default interest for the duration of the delay.

*) for construction and contract law, an inheritance and divorce lawyer could make silly little mistakes there.

That's exactly how I will do it. I naturally have some witnesses now and am also happy to listen to the reasons for the cancellation. But realistically, it's simply that he got a better offer. Nevertheless, I would still like to have my additional costs reimbursed. Still waiting for the lawyer's response. There have been no further attempts at contact or correspondence from the side of the shell builder.

The problem is that you cannot unilaterally withdraw from all contracts. Now there could be problems if the contract still exists de jure, you have already concluded a new contract, and want to claim damages from the old company. And he now wants to fulfill the contract because of these damages. But the construct was conceived by me, but not impossible. And such things are known by the specialist lawyer.

Sad all around. You are just left stranded like that and the company claims there was never a contract ... and then you also have to worry whether that person might suddenly come up with such ideas after all. As a client, you seem to be the one getting the short end of the stick at first glance ...

Again: accepting a binding offer is a perfectly ordinary contract. I do this professionally almost every day and sometimes it involves eight-figure amounts. All details must simply be included in the offer.