The contractor wants to issue an invoice for planning services

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 cooperate.” He didn’t specify a price and should therefore get what he agreed with the client = nothing.

According to the Construction Contract Procedures Act, the market price can be applied if no explicit price has been agreed upon. This is basically the case. What exactly applies here, however, far exceeds my horizon.

What exactly applies here, however, far exceeds my horizon

In the specific case of this thread’s question, I also mean: how can there already be change requests when the contractor and the client are still in the harmless tea-dance phase (and why does the OP not yet see in the change requests an implied transition from chit-chat to a preliminary contract)? – With a construction service specification, it is clear: if, for example, wall tiles in the bathrooms are specified only up to shoulder height and I want them up to ceiling height, then this must still be changed in the offer before the decision. But if I say, paint the roof from a thirty-degree gable to a thirty-five-degree hip roof, otherwise I am undecided whether I like the construction proposal – then that is a design contract (also by email and if one immediately starts drafting without already having attached a price to the redesign). People must accept and come to terms with the fact that the tolerance for taste returns without charge, familiar from Zalando and the like, cannot be transferred to the construction business, and that a client is not just any “consumer” who takes the “right” as king customer to make the lackeys dance for his amusement and to pay nothing if he thumbs down and votes the candidate out before the finale.

The topic is apparently very controversial and the discussion shows me that the legal situation here does not seem to be black or white either.

As mentioned, we were not informed in advance that costs would be incurred. However, it was also not said that the further procedure would be free of charge. Nothing was said about it at all. Maybe it was naive of us to accept it that way – could be.
Our requested change to the building description was not about something like tiles or the roof pitch. It was about the fact that the building description did not even match our inquiry. We want to build a KFW 55 house and the building description says "... will be constructed according to the currently valid Energy Saving Ordinance." That simply does not fit. Therefore, we wished for an adjustment of the offer to what we inquired about. Otherwise, we would only get what is stated in the building description.

But honestly. Traffic light labels are to be printed on food packaging because consumers seemingly are not able to read an ingredient list or the nutritional information. The current TKG amendment provides that consumers must be given a one-page contract summary before concluding a contract because they apparently are not capable of reading a contract for an internet product.
But if a builder later remembers that his service actually did cost something, then is it okay to charge this afterwards without any announcement? In the worst case maybe several thousand euros? Most people only build once in their life and never deal with the topic again. And one is supposed to be so well versed in the law that one should be aware of this? The entrepreneur’s obligation to give a short notice to the interested parties would already be sufficient. "Please note that the requested service will be charged to you with xxx EUR." Period. End. Nothing more would be necessary. It’s a pity that private individuals are treated as less important here.

But well, you see, this topic annoys us! Let’s see how it goes.

I believe you, and you haven't even started building yet. I can already take that idea of a price guarantee away from you here and now, so don't even think about taking that term at face value.

Construction has its own rules and the client is at the bottom of the food chain. It's just the way it is. No one is smarter than the contractor and no one has more excuses ready :) , you just have to be able to deal with it and get involved enough to recognize nonsense.

But well, you see, this topic annoys us! Let's see how it continues.

I would like to give you a legal basis that supports you. Your case fits perfectly with the OLG/BGH rulings from 2000/2001 (just google them), which state that it is classic acquisition activity of the construction contractor. It is established here that the provision of planning services does not lead to an architect contract. Not even by implied conduct. The services of the construction contractor are solely to be seen as preparation for the final offer. For this, the construction contractor is not entitled to remuneration. Unless an explicit price is agreed upon.

That is very interesting. One wonders whether it will still be seen the same way after 20 years. Nevertheless, I would also have liked to see my questions answered by TE on the topic: what is stated in the documents?