House calculation more expensive than expected. Is this realistic?

That doesn’t fit together in my opinion: either

we brought in an architect (extremely competent)

or

The costs amount to approx. €830,000 according to the plan.
However, we expected ~€450,000.

because that can’t be reconciled, but neither can the budget and a three-family house.

The question is, on what basis did you expect 450,000 euros. The 830,000 euros are justified. Where does the other number come from?

So first of all, greetings from central Hesse as well [emoji16]

450,000 € for a 3-family household? That seems low to me at first glance as well. Is that just for the pure construction?

Of course, it is clear that one has dollar signs in their eyes and wants to build as cheaply as possible so that the return on rental is right.

I do not know the Landesbauordnung Hessen, but here in BW that would mean 6 parking spaces for cars, 6 covered parking spaces for bicycles, and a few additional areas plus stairwell, basement, and on top the apartments must be accessible barrier-free. In other words, equipped with an elevator. 300sqm x 2000 plus additional areas plus parking spaces plus architect fees and construction ancillary costs plus elevator. 830,000 quite possible.

From when does a children's playground have to be provided? Was it three residential units?