Simple single-story house without basement, 120 m2, cost estimation

So you want an estimate, no problem at all

small house, very simple equipment -> 1900 / m². That makes about 230,000 euros for you. Then the additional costs, about 20% of the construction costs. So all in all you are somewhere around 300,000 euros + a plot of land.

I emphasize: very simple equipment and a lot of own work!

Since I work in the construction industry, we have already developed a few designs.

Invisible designs do have the advantage that you can't complain - but also the disadvantage that it's then so hard to say anything about everything (including the costs). If I understood correctly, you only studied architecture and then specialized. Then I advise you to go to a colleague with design experience - otherwise you might easily end up building an approval-capable 'Uidashättenwirbessernochmalgeübt' house.