Is a house for a total volume of 400k still possible at all?

What do you advise him?

Act according to his possibilities and wishes and make compromises if they are incompatible. If both are too far apart, leave it.

I would increase the budget again by the 100,000 mentioned somewhere. Better to invest 500,000 well in a house where you feel comfortable than 400,000 for the absolute minimum, where you regret all your life that everything pinches.

Actually, building without a basement was not an option to start with. Now we will reconsider whether we actually want to take the opportunity.

If you don’t buy the plot, I’m interested ;)

You can also plan in 2 construction phases. That way your risk won’t be that high from the beginning and the house can grow with your space needs.

As a thought aid: 1 sqm of basement is bare more expensive than 1 sqm of living room with great flooring, electrical work, and nice walls.

As a thought aid: 1 sqm of basement is more expensive bare than 1 sqm of living room with great flooring, electrics and nice walls.

That's not quite true, we gave up the basement because of high groundwater. Above ground, we then built about 50% more of the lost basement area additionally, and in total it was hardly cheaper. Nevertheless, thanks to the above-ground compensatory area, I do not miss the basement at all now. The garden is just a bit smaller and the enlarged house looks noticeably bulkier. A compact above-ground floor plan also has its advantages.

I always like to refer to mental barriers here as well:
https://www.hausbau-forum.de/threads/leitfaden-zur-hausplanung-auf-dem-Grundstück.30891/#post-320882

That's not entirely true, we waived the basement because of high groundwater. Above ground, we then built about 50% of the lost basement area additionally, and overall it was hardly cheaper.

I don't understand what is "not entirely true" about that – it was cheaper for you too and probably better equipped.