Is it possible to determine the eligible costs based on a rough estimate?

That will not work for you. You have a fairly fixed budget, and the construction with an architect will probably fail because of that. The architect will only give you an approximate cost estimate. Either you have a really big buffer from the beginning (50k..70k) OR you find the miracle architect who can actually make it happen. But I don't really believe in that. Conclusion: When building with an architect, you should be financially somewhat relaxed and/or have real reserves. A 20k buffer might be okay when building with a general contractor, but with an architect, that's really not much.

This is about the architect's fee. And I find a 20k buffer very much. It's not about what the construction with the architect costs, but about how much I roughly have to pay for his work. And I find an estimate of 20k quite inaccurate. Because for >60k€ I would stay away from that architect if I look at the fees being mentioned here.

I am quite sure that no one here, not even you, has planned a 20k€ buffer for the architect's fee. Maybe 50-80k€ in the total costs, because you build more expensively due to additional services, but not because the architect suddenly costs 50% more.

If it is an architect who designs and supervises more than just one house per year, then the cost estimate should be accurate.

In our case, it was plus or minus €5,000 in the past; one should consider what demands one has oneself and what the architect is supposed to design.

It is still possible to spend a lot on many trades during the house construction afterwards.

The eligible costs are the construction costs excluding VAT and excluding structural engineers and the like.

Furthermore, the building services are also not fully included.

That means with total costs of €400k, I deduct 20% tax and another lump sum of €50k for things not accounted for, and then arrive at realistic fee costs of €30k with an 11% fee.

I'm curious what the two requested architects will answer.

Conclusion: When building with an architect, you should be financially somewhat relaxed and/or have real reserves. A 20k buffer might be okay when building with a general contractor.. but with an architect, that's really not much..

I would say that any construction project can go well or badly. We chose our architect because he stated from the beginning how much it would cost at most. And it actually ended up costing a bit less, even though we had changes and slight upgrades. He even planned an additional conservatory. Additionally, building with an architect was several tens of thousands of euros cheaper than the lowest offers from general contractors. Therefore, such statements should not be made categorically.