The developer requires a down payment to start drawing

I had paid in advance for my draft from the general contractor in a planning contract, and it was similarly expensive. Afterwards, I had the draft calculated turnkey by several companies including this general contractor.

In my case, however, I then received an actual draft according to my wishes including several revisions and not the usual free general contractor copy-paste from old drafts. Whoever wants more performance must also be willing to pay for it. But of course, I would also hold the planner fully accountable then. And only do such a thing with companies with an excellent reputation...

Okay, that sounds good. I can then ultimately use this "trade" from the general contractor to approach other general contractors to see what the construction would cost there.

Okay, that sounds good. I can then ultimately use this "trade" from the general contractor to knock on other GCs' doors to find out what the construction would cost there

You write it yourself, you want to have the design including floor plans, additional stories, elevations and size adjustments, basically the complete planning, created by the general contractor. Preferably somewhere between "for free" and cheap. Then use it to get offers from the competition. The GC knows this type of customer (I've read somewhere about “floor plan moochers”) and resists by quoting costs right away. Before doing all the (pre)work and ending up being the fool at the end, I can understand that. Whether 4k€ is too much for that, no idea!? Ask an architect what they charge for it.

You write it yourself, you want the draft including floor plans, extension, views, and resizing, basically the complete planning, to be created by the general contractor. Preferably somewhere between "for free" and cheap. And then use it to get offers from the competition. The general contractor knows this type of customer (somewhere I once read something about floor plan freeloaders) and resists by naming costs right away. Before he has all the (pre)work and ends up being the fool, I can understand him. Whether 4k€ is too much for that, no idea!? Ask an architect what he charges for that.

As I described, I of course understand that costs are incurred here. However, in this amount, it is not comprehensible to me. I boldly claim that an architect who draws daily and knows everything inside out sits a maximum of 8 hours on such a project. That is already calculated generously. Hourly rate about 60 euros results in just under 500 euros for me. That there are freeloaders, no question! However, I think one should not demand unrealistic prices... At least this amount sounds unrealistic to me personally...

Sorry, but you are thinking too simply. If it is to be done properly, much more is involved than just drawing. The whole thing is supposed to be feasible on your property. Therefore, the development plan, the local regulations must be studied and the whole thing must be reconciled with your ideas (room size, layout, budget, etc.). Then there will be inquiries here and there and compromises found. That is just something that comes to mind for me as a layman. I do not see 8h as sufficient here. In my opinion, for 500€ you will get a quick initial draft as a sketch, but nothing reliable. According to Google, the average hourly rate of an employed architect was already 95€ in 2018. Net, of course.

I can then ultimately use this "trade" from the main contractor to approach other main contractors to find out what the construction would cost there

I would definitely have this confirmed in writing in a planning contract, that you are then allowed to build the design yourself, especially also with another company. Publishing or passing on the design is usually not permitted.

Since it is a larger version of the brochure house, we naturally expect additional costs.

I have not only found the exterior of the house that suits us, but also the complete floor plan of the corresponding house. Ultimately, this house is to be expanded from 2 to 2.5 stories and adapted to our maximum buildable area. Do you think 4,000 EUR is too much for that?!

That means the house you want is in his program, just smaller; and you don’t want it as a "town villa," but with an expanded pitched attic on top?
Only a few things can be changed "simply" in a construction proposal from the brochure (type design): moving non-load-bearing interior walls, extending along the ridge axis, increasing story or knee wall height, increasing roof pitch; and of course reversing the deactivation of the basement, then the stairs go downward instead of into a broom closet. All further changes require to varying degrees a pronounced redesign. Your description sounds more demanding than what the experienced housewife could do with ease, and yet unnecessary for the purpose of a price comparison. Moreover, the plans are only very limitedly suitable for a price comparison: the construction service description is crucial, you can build a house according to the same approved plans for 80 or 140 percent of the price (and conversely, a completely different looking one of the same size and equipment at the same price).
By the way: although not as a link, you could at least tell us the model name of the house you are so fond of.
Why is the house supposed to become significantly larger than its "original," do you have more children? — if the motive is rather to “treat yourself” to the largest possible portion of concrete gold: then I recommend a cure in the form of a beneficial infection with the philosophy of .