Cost for an offer

Getting a finished plan for 500 € sounds absolutely reasonable..

Be careful, you have to precisely examine what you get for the 500€ and in what form. The next general contractor will then say the plan is not sufficient and needs to be redone. If you generally like the company, then I would go for it. 500€ is already pushing my pain threshold. Regardless, even if it doesn’t work out with them, you might learn enough details to negotiate better with other companies.

If in a project of the size 500k€, for one thousandth of this amount you can gain a certain price certainty or more precisely, a significant increase in the accuracy of the price estimate, then this seems to me to be a quite reasonable investment. Additionally positive is the very good feeling during the conversation. The other general contractor just thinks something, as you might want, and in the end you actually have completely different wishes. And then his estimate is worthless. I think the actual calculation of the house price is already an extensive job that no company wants to do for a client who has no serious intentions. The 500€ is probably a "fun inquirer protection." The other company can only estimate very roughly to keep the effort low, and surprises are rather to be expected.

The general contractor wants to prevent you from having him plan the house or optimize the floor plan for you, and then you get competing offers and disappear. I think it's fair, as long as you really receive a reasonable plan. You can ask if you can record this as a contract in writing. Talk to the general contractor, share your concerns with him, try to find a solution. He has a good reputation and you feel comfortable with him. That is worth something. Other trades are now doing this partly as well. Heating engineers, for example. They plan and plan, make an offer, and the customer uses it as a shopping list and does it themselves. Those are also a few wasted hours.

a clear improvement in the accuracy of the price estimate,

Wait a moment. The accuracy of the calculation is the risk of the general contractor (GC) and not of the client. Of course, equipment wishes must be known. On that basis, a fixed price is then set. If a wall needs to be thicker afterward, that is a (financial) problem of the GC, not of the client. In this respect, it is questionable why the client should pay separately for this. I could even imagine that this is not entirely proper. But as already stated before, the GC can apparently afford it.

The question is whether a rough estimate like 2000/sqm is enough for you (for now) or if you need an exact figure for the window right now.

For a typical single-family house, whether a city villa or a gable roof house, you can generally get a good rough idea since the parameters are the same (WC, bathroom and kitchen, 3 bedrooms and living room, a staircase, a heating system). You can manage quite well with the sqm. Equipment prices come on top of that. The second general contractor doesn’t know that either. He is now drawing up the plan neatly for 500€ and will possibly come up with a slightly more accurate price, which will be similar to the first one’s... if they both build similarly. I’d say: a cheap provider will be cheaper than a high-priced company, no matter how they proceed. That also depends on the construction service description. Keep in mind that he now takes over your amateur sketch 1:1. Although he is now designing barrier-free, a lot could still change. The (fair) price mentally commits you to this design. After all, you have paid for it. Let the user community look at your sketch to see if there are any major errors. We have a special subforum for that.

Yeah, that will just result in a Frankenstein house again. With an amateur design handed over to the general contractor, who smooths out the worst mistakes and then builds it that way. For 500€ you only get a design or at most a submission plan. Most general contractors don't even make execution plans anyway. Just tell the general contractor what kind of room program you have in mind, what is important to you, and he should show you a few suitable standard designs. Is there even a plot of land for the house yet? If not, the whole thing is just a futile labor of love anyway, putting the cart before the horse doesn’t work when building a house.