New construction - Using the attic as living space - Increased costs

we have purchased a small urban plot (400m²) and are planning to build a single-family house.
Since the development plan allows it and we want to have as much of the garden as possible, we plan to build on 2.5 floors (2 full floors and 1 study in the attic).

Name (without link!) the development plan and show the plot. It is best to combine your previous three threads into a fourth one, starting it with the completed questionnaire followed by the links to the three original threads.

However, our architect has now clearly advised us to limit ourselves to only 2 full floors, as the costs in the attic would explode. Several construction companies, however, have a completely different opinion here. [...] In the end, we want to tender to various construction companies later and therefore take their opinions regarding the costs seriously and not just the architect’s.

Your architect’s view — I suspect rather "architects"? — is very likely both specifically correct as well as nonsense as a general rule. I had already given you the hint to my house construction roadmap, which you apparently have not followed yet. Therefore, I have two concerns: 1. that you are skimping on the architect’s fee (which is completely wrong, since every penny "saved" in the planning will cost you much more during construction), and 2. that you consider a floor plan set sufficient as a proper basis for tendering (which will make you an easy target). And at least for your simplistic calculation of a 67+67+16 house, the prophecy will come true on a biblical scale.

See you in your combined thread (and gladly also with me, see the location of the roadmap).

No, the construction companies do not know the floor plan because there is no fixed floor plan yet.

But that is not entirely unimportant for answering the question. We also had the idea to extend the staircase at the last minute. However, this would not have worked in terms of height without a dormer, and that was simply too expensive. It was also a bit too late to completely redesign the floor plan.