The contractor wants to issue an invoice for planning services

Again, if it had been an architect

Don’t discuss "what if." He is a contractor – if he wants remuneration, he MUST communicate it.

Did he really? Do you know if the general contractor said, I’ll create a draft for you free of charge?

According to the OP, he did. And that is exactly what I am reflecting on. Once again. He received a request to submit an offer. This offer, according to the OP, was made without stating a price. The offer was not accepted. Afterwards, he is now pissed off and wants to get paid. If he "forgot" to state a price for his service, that is simply his own bad luck. Nothing more, nothing less. If the case is exactly as the OP described, he is entitled to exactly €0.

He handed us the drafts, the description, a contract draft and an offer (as a lump sum price for the house including building application documents and structural proofs) in a personal meeting.

What kind of communication was there? On the last day of the handover?

It writes down so quickly now, but the whole process was tedious, required many inquiries, and took 9 months.

Yes, okay. But I wouldn’t judge it like that. A longer course can just as well be due to the “patience of the clients.”

We heard nothing more from him since September 2020

And taking a three-week spa cure should also be allowed. It was announced by him after all.
Dec 19, 9 months, + cure, Sept 20... that reads like a 9-month pregnancy ;)
I’m emphasizing this because the time frame is comprehensible and should not be included in a “judgment.”

That he might charge money for preparing his offer hadn’t occurred to us. As a client, you must be able to get and compare offers from companies.

True. But there are also oral or consensual contracts.
With an architectural office, for example: if you accept the service of planning, you have concluded a contract.
If I tell a carpenter, before his exact written offer for the kitchen is made, that he should already order the sample materials for the surfaces and give them to me, that could already be a contract commitment before I have even received the written offer.

Yes, he adjusted the draft and the building description. But in the end, both should have been correct for us to even be able to assess his offer.

I don’t know such a completely individual adjustment. I also don’t know of any adjustment of the building specification. The building specification is part of the work contract. It is usually legally reviewed so that no error can lead to a lawsuit. That’s why companies do not change the building specifications; they are extended by positions. That happens with a click, so it doesn’t take work. A calculation of a house is estimated by the sqm; you don’t have to move individual walls for that. The general contractor rarely builds extravagant things. But floor plan adjustments require work and cost money.

But then it has to be communicated to the customer beforehand, and also which price is being charged. Nothing like that was ever expressed to us.

You are right there. However, are you sure about that? See above.

If someone does not provide details about manufacturers, quality grades, etc. in their building specification, I don’t even know whether the offered price is overpriced or justified. Therefore, we wished for a concretization of the building description.

See above.

I would also not pay anything at first. However, that does not mean that you are in the right if many clients in a forum are outraged.

What does the fine print or similar in the documents say, which could give a clue about the status you are in?
There should also be basics, contractual conditions, etc.
The solution to the puzzle should actually be found there.

The general contractor must prove that a contract has been concluded, whether in writing or orally. Are there written documents/witnesses? Billing according to HOAI is probably also not possible.

Preliminary design is usually associated with costs. I am of the opinion that the proven effort must also be paid for.

I am of the opinion that the proven effort must also be paid.

But that's not the point. The point is that the general contractor did not say "The offer with planning services costs €1,000, which must be paid if we do not work together." He did not name a price and should therefore receive what he agreed with the client = nothing.

But that's not what it's about. It's about the fact that the general contractor didn't say "The offer with planning services costs €1,000, which must be paid if we do not work together." He didn't name a price and should therefore get what he agreed with the client = nothing.

You can see it that way, or you can see it differently. The absence of an explicit agreement does not automatically mean that the service is provided free of charge. If the general contractor now invoices €500 for preliminary planning because he assumed that the construction would be done with him and accordingly put work into it, that is legitimate. In case of doubt, a court will have to decide, unless one party finds it too foolish and gives in.