Surveying Services - Cost Differences

The land registry office had no idea and immediately referred me to the building authority.

What now? They should be able to say whether the boundaries have been determined and what the quality of the coordinates is. Even if the only statement is that this information is only provided upon formal request and for a fee.
VVBauPrüfVO:
When delivering the property map/cadastral map, the cadastral authority also states upon request whether the facts mentioned in paragraph 3 No. 1 and 2 are present.
Direct complaint to the head of the office - boom, head off.

In a conversation with a city building officer, I was told that it is usual to submit an official site plan, even if it is not strictly necessary.

I already feared something like that. "We’ve always done it that way." Smart moves are not only made in Cologne. Very annoying.
Direct complaint to the mayor - boom, head off.

Order the official site plan from the publicly appointed surveyor (costs €2100 according to the fee schedule) and commission the rest with the non-publicly appointed surveyor.

I cannot recommend this, as synergy effects cannot be utilized then. Unfortunately, it is not the case that the services can be commissioned individually at the stated prices. The procurement and evaluation of the cadastral documents, for example, are necessary for each service but only once in a package.

Either the non-publicly appointed surveyor knows what he is doing and is sure that no official site plan is needed. Or you commission the publicly appointed surveyor engineer to prepare a site plan for the building application. Since he can probably access the cadastral documents online if he is local, he can find out in 10 minutes whether an official one is necessary. However, regarding building burdens, you have to be sure yourself, as these cannot yet be retrieved digitally in many municipalities.

P.S.: It makes more sense to find the surveyor you trust than the cheapest one.

Whether it is wise to complain to the head of the office / mayor when you want to submit a building application sometime AFTER that. It will certainly be veeeerrry closely scrutinized and will take quiiiite a long time. That can definitely be predicted already.

I have feared something like that. "Hamma imma so jemacht". Smart people are not only found in Cologne. Very annoying.

However, this coincides with my experiences as well as those from my extended circle of acquaintances. Out of 20+ construction projects, there wasn't a single one that didn't require an official site plan - in very different municipalities across the country.

Did they all explicitly investigate whether they actually needed an official one?
I rather believe that 99.9% just get the official one done. Either because they want to be on the safe side or because they don't even know that the non-official option exists..

The answers from the building authority suggest that they very rarely deal with the question there.
Good for all publicly appointed surveyors.

Yes and no, they each—of course "only" at the building authority—asked, and always received a similar response as above. Some building authorities also have information leaflets that state this, independently of any regulations.

The building authority naturally only has its own perspective, and honestly, the few extra euros get lost in the background noise of a construction project anyway. However, I will ask my contact at the land registry office to see if one could get a different answer than "We don’t know." from them.

and honestly, the few extra euros also get lost in the background noise of a construction project.

Exactly. You plan the approximate costs under ancillary construction costs, ask 2-3 surveying offices, and are happy that the offers stay within the budget. Very few people then investigate whether it might be cheaper.