House planning from scratch - financial calculation, basement...

Yes, it is allowed to overflow. It is about a container after all.

"Forced emptying" into the container, yes. Exactly so that it does not "overflow" (spill out the top or back up when the nominal filling volume is reached).

Outdoor facilities/garden/garage is set at 25,000 euros

That’s gone so quickly, you can’t even say B yet. Especially since that also includes your infiltration.

We have currently set additional ancillary construction costs at 15,000 euros

Aha, and what about sewage, electricity, water, telecom, etc.?

Our plot is about 1.3 m below the current street level. It slopes slightly (about 50 cm over 20 m). Waterproofing of the basement against non-pressing groundwater is required

Is there a soil survey? Or is the house supposed to be below the street?

I see a 30-40K EUR shortfall in your calculation for a normal plot. With the -1.3 m situation, add another 15-30K on top.

Can only confirm what my predecessors said. We are currently at just over 500,000 for our ongoing house project (145 m2 with basement) (incl. good electrical equipment and tiles) and have only spent half of your planned amount for the plot of land... Due to the location and special features of your plot (lower to the street, cistern), you will probably have similar civil engineering costs to ours...

Aha, and what about sewage, electricity, water, telecom, etc.?

Depending on the area, 5-10 thousand €.